GLASS AND CERAMIC MATERIALS

Study Support

Jozef Vlček

Ostrava 2015

VYSOKÁ ŠKOLA BÁŇSKÁ – TECHNICKÁ UNIVERZITA OSTRAVA

FAKULTA METALURGIE A MATERIÁLOVÉHO INŽENÝRSTVÍ

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Title: Glass and ceramics materials

Code: 635-3003/01

Author: Jozef Vlček

Edition: first, 2015

Number of pages: 50

Academic materials for the Metallurgy Engineering study programme at the Faculty of

Metallurgy and Materials Engineering.

Proofreading has not been performed.

Execution: VŠB - Technical University of Ostrava

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CONTENT

1. INTRODUCTION ......................................................................................... 6

1.1 Glass ............................................................................................................................ 6

1.2 Transformation of glass ............................................................................................... 8

1.3 Stabilisation of glass .................................................................................................... 9

2. CHARACTERISTICS OF MELTED GLASS ........................................ 11

2.1 Viscosity .................................................................................................................... 11

2.2 Crystalisation ability ................................................................................................. 13

2.3 Surface stress and density .......................................................................................... 15

2.4 Thermal characteristics .............................................................................................. 16

3. MELTING OF GLASS ............................................................................... 18

3.1 Material for the production of glass .......................................................................... 19

3.2 Chemical reactions during melting of the glass ......................................................... 21

3.3 Melting of solid materials in the molten glass ...... Chyba! Záložka není definována.

3.4 Fining and solubility of gases .................................................................................... 24

3.5 Flowing, mixing of glass metal and its homogenisation ........................................... 26

3.6 Evaporation of volatile components .......................................................................... 27

4. PRODUCTION OF GLASS ....................................................................... 29

4.1 Preparation of batch ................................................................................................... 29

4.2 Glass furnaces ............................................................................................................ 31

4.3 Forming of glass ........................................................................................................ 32

4.4 Cooling of glass ......................................................................................................... 38

5. CHARACTERISTICS OF GLASS ........................................................... 41

5.1 Mechanical characteristics ......................................................................................... 41

5.2 Restistance of glass against changes of temperature ................................................. 44

5.3 Optical characteristics ................................................................................................ 45

LITERATURE ................................................................................................... 50

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INSTRUCTIONS FOR STUDYING

Glass and ceramic materials

You have received study material which includes integrated scriptum for extramural studies

that includes instructions for studying. This material is for the subject Glass and ceramic

materials for the 2nd

semester of Heat engineering and ceramic materials.

Prerequisites

There are no prerequisites set for the studies of this subject.

Aim of subject and outputs of studies

The aim of this subject is to give theoretical knowledge to the students from areas of glass,

ceramic materials and glass-ceramics, to emphasize the difference between glass and crystal

structure. To apprise students with the technology of glass production and traditional ceramic

products, information about rules of use of these materials will be provided as well.

What the student should be able after studying this subject:

to be able to characterize differences between glass and crystal structure

to master the technology of production of glass and traditional ceramic materials

to be able to make changes in material composition and changes in the technology of

production of glass and ceramic materials with purpose of aimed modification of

parameters of these materials

to be able to draft applications of ceramic materials depending on the demands of

technical profession

Who is this subject designated for

The subject is set into the master’s degree studies of the field Heat engineering and ceramic

materials of the study program Metallurgical engineering, but it can be studied also by a

person who is interested and comes from any other field, if he, or she fulfils required

prerequisites.

Source for studying is divided into parts, chapters, which correspond to logical division of

studied material, but are not so broad. Estimated period for studying of chapter can be

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different that is why are the long chapters divided into numbered sub-chapters and further

described structure corresponds to them.

We recommend this process during studies of each chapter:

You should understand this text, look for further information in recommended literature and

study stated problematic.

Way of communication with educators:

There will be tutorial of the subject during the semester, in a scope, which are students

familiarized with in detail and with the content of the subject and about the ways of

communication with educator. Educator solves questions of students individually or in case of

broader interest, there is set consultation with a group of students.

During normal education the students of extramural studies are given semestral projects,

which are necessary for getting credit. Student is able to solve semestral project thanks to his

knowledge, with which he is familiarized during normal studies. In case of need, consultation

is possible as well. Student gives solved project to educator personally, or electronically.

Student receives the evaluation of his project in electronic form within fortnight.

Educator: doc. Ing. Jozef Vlček, Ph.D.

jozef.vlcek@vsb.cz

tel.:596991523

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1. INTRODUCTION

Time for studying: 1 hour

Aim After studying of this article you will be able to

define glass state

define transformation and stabilisation of glass

Definition

It is not easy to define glass materials. There is whole range of types of glass, which are

produced in different ways; they have different chemical composition and different

parameters. It is possible to define traditional glass as solid inorganic material which was

produced by solidification of molten glass.

Stated definition is very simplifying and in present time it does not correspondent to

knowledge that was made in the area of glass. It is a fact that most of glass contains basic

glass-producing silica oxide, but at the same time there is also range of glass with organic

base. It is necessary to state that it is possible to produce glass also by different methods

besides cooling of melt.

Common sign of all types of glass is their irregular setting in longer distance (multiples of

size of elemental building parts) and glass embodies transformational behaviour (glass

transit).

1.1 Glass

Glass is produced in most cases by cooling of molten glass of suitable chemical composition.

Besides melting it is also possible to produce glass in other ways, which are much more

complicated and less regular. It is possible to present further non-traditional ways of

production of glass

:

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o Condensation of steam

o Diversion of crystals into unformed form in a mechanical way

o Dehydration

o Sintering of gels

o Irradiation with fast neutrons

We consider following materials as glass-producing (by fast cooling from the melt):

o Elements: S, Se, Te, P

o Oxides B2O3, SiO2, GaO2, P2O3, As2O3

o Borates and silicates Na2BO7, Na2Si2O5

o Other reactants BeF2, AlF3, ZnCl2, KHSO4

o Further – sulphides, selenides, tellurides and some nitrates and carbonates

o Glass can be prepared from different materials by other techniques than

cooling of melt.

Inorganic glass is generally not stoichiometric compounds; they represent more difficult

systems with variable ratio of materials (often basic materials which are glass-producing and

further infusion of non-glass making materials).

Most common types of glass:

- Oxide

- Silica

- Borate-silica

- Fluoride

- Phosphoric

- Chalcogenide (S-Se-Te)

Further typical attribute of glass, besides amorphous structure, is its transformation.

Amorphous structure shows regular setting only in short distance (regularity is only in setting

of neighbouring atoms, distance circa 10 -8

m). Glass does not show regular setting on longer

distance. Basic glass-producing oxide is SiO2, its crystal and amorphous setting can be seen

on figure 1.

In case of real glass there is besides the basic glass making oxide (SiO2) also non glass

making part (e.g. CaO, BaO, Na2O, K2O) so called modifier. Modifier, because its cations

come into net making oxide and deform it. Modifiers work as agents of depolymerisation and

further change characteristics of glass (e.g. viscosity).

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Fig. 1 Demonstration of structure of crystal SiO2, amorphous SiO2 a sodium silicate glass

(Hlaváč, J.: Základy technologie silikátů. SNTL Praha, 516 s., 1988)

1.2 Transformation of glass

It is possible to describe the transformation of glass (glass change) with graph of dependence

enthalpy of volume of matter on temperature. According to figure 2 – material, which during

cooling (from the liquid state) produces crystal structure, greatly changes, during constant

temperature, its volume (or other physical value dependent on temperature). The volume

changes during further declination of temperature by other relation, than during higher

temperature than the temperature of melting.

Material, which produces glass structure, at the temperature of melting, does not show rapid

change of relation of volume on temperature. Below the temperature, which is the temperature

of melting, given system becomes, meta-stabile, system is in the form of supercooled liquid.

Only with further declination of temperature the material transforms from formative into solid

state. Curve of relation of change of volume on temperature has large change of further

relation in this moment. Matter shows glass structure from this moment. It is a state

thermodynamically unstable. Temperature, by which supercooled melt changes into glass

state is called temperature of transformation. Transforming temperature is not constant, its

value changes with the rate of cooling. With growing rate of cooling rises value of

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temperature of transformation. For glass making system it is possible to define transforming

interval of temperatures, in which, related to the rate of cooling, the matter shows solid state.

(SHELBY, J. E. Introduction on Glass Science and Technology. Cambridge: The Royal Society of Chemistry,

2005. 291 s. ISBN 978-0-85404-639-3)

Fig. 2 Transformation of glass

Existence of transformational interval is very important. If thermodynamically unstable glass

develops from meta-stabile supercooled melt in broader interval of temperatures,

characteristics of glass will be dependent on the rate of cooling, this means on temperature

history of glass. According to figure 2 slow cooled glass shows higher density than fast

cooled glass.

Temperature of transformation is different for different types of glass, for most types of glass

applies that transformation comes to place by viscosity of glass close to the value 1013

dPa.s.

1.3 Stabilisation of glass

Thermodynamically unstable glass, which was produced by supercooling of molten glass,

after warming on temperature, which belongs to the transformational interval, has the

tendency to change into meta-stabile supercooled liquid. Stated action is labelled as

stabilisation of glass.

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Further it is possible to define fictional temperature. It is temperature, at which the

characteristics of particular glass are same as characteristics which respond to the meta-stabile

state.

Summarization of terms

Glass

Transformation of glass

Stabilisation of glass

Questions

1. Typical attributes of glass.

2. Present glass making materials.

3. Kinds of glass.

4. Explain term: transformation of glass.

5. Explain term: stabilisation of glass.

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2. CHARACTERISTICS OF MELTED GLASS

Time for studying: 4 hours

Aim After studying of this article you will be able to

Know characteristics of bath

Definition

Main way of production of glass is controlled cooling of melted material mixture in the way,

which prevents crystallization and secures transformation of supercooled liquid into a glass

state. For successful mastering of these requirements it is necessary to know characteristics of

melted glass. Melted material for glass production is in technical practice labelled as molten

glass (bath).

It is necessary to define these characteristics of melted glass for glass making:

- Viscosity

- Crystallization ability

- Surface stress

- Density

- Thermal attributes

2.1 Viscosity

Viscosity is key attribute of bath. For glass production it is necessary to know the relation of

viscosity to temperature. It is characteristic for glass production that technological operations

of its processing are directed by viscosity, which is shown by bath. For example upper

annealing temperature is defined by viscosity 1013

dPa.s. Temperature at which the stated

value of viscosity is reached will be different for different types of glass; it is related to its

chemical composition.

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From physical point of view is the viscosity defined as mechanical parameter of matter, which

defines ratio between tangent stress and change of rate related to the distance of two

neighbouring layers of flowing liquid.

dvA

dyF

.

. (1)

F is force acting in parallel with monitored levels of liquid (N)

dy - distance of layers (m)

A - area of layers (m2)

dv - rate of layers (m.s-1

)

Glass is thermodynamically unstable matter, which has autonomic tendency for

stabilisation/change into state of unstable supercooled liquid; it is possible to monitor

viscosity of glass also at the laboratory temperature, however in very limited extent (at large

unnoticeable).

Realistic (noticeable) viscose flow comes by the temperatures slightly above 500° C. Stated

temperature is only informative; the temperature varies for different types of glass. Single

phase crystal systems lose their mechanical characteristics rapidly, glass in the opposite only

gradually.

Referential viscosity points:

- Temperature of melting: log = 2,0 dPa.s

- Temperature of processing: log = 4,0 dPa.s

- Point of flowing: log = 3,0 dPa.s

- Point of softening (according to Littleton): log = 7,65 dPa.s

- Upper annealing temperature: log = 13,0 dPa.s

- Lower annealing temperature: log = 14,5 dPa.s

- Transformational temperature: log 13,3 dPa.s

Viscosity of glass depends especially on its composition. Effect of each component depends

on its overall composition. The same oxide lowers viscosity, and it increases it by other type

of glass. In the area of transformational temperature is the viscosity function not only thermal,

but chronological as well. Viscosity changes its value, which belongs to meta-stabile state.

On fig. 3 is shown dependence of viscosity on temperature. There are shown ranges of

viscosity on the figure, for which there is recommended technological operation during

making of glass. Viscosity at lower temperature is shown in dots.

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The reason is stabilising ability of glass, where the viscosity becomes function of time. From

real point of view this fact does not have meaning. It is possible to state, for better

understanding that value of viscosity of glass for forming is similar to viscosity of honey at

room temperature. Sodium calcium glass reaches stated viscosity (104 dPa.s) at the

temperature between 1015 – 1045 °C.

Transfers by glass making happen at different temperatures based on type of glass, however

by comparable viscosities.

(http://www.brand.de/en/tech-info/materials-glass/mechanical-resistance/)

Fig. 3 Dependence of viscosity on temperature

2.2 Crystallisation ability

It is possible to create crystallisation in glass by warming. Final state can be the change of

glass into polycrystalline system. Exception is glass from oxides B2O3 and spars K2O.

Al2O3.6SiO2

Industrial glass crystallizes after longer thermal exposition in area of temperatures where its

viscosity reaches values of forming – this is 103 to 10

8 dPa.s. Forming must be done so

quickly and crystallisation so slowly that it does not happen under these circumstances.

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Crystals are visual defects and under influence of different thermal expandability cause stress.

Presence of glacial phase can cause autogenous break. Phase diagrams help to value possible

crystallisation of glass.

For crystallisation is important:

- Temperature of melting (liquids on more phase systems)

- Rate of nucleation

- Rate of growth of crystals

Crystallisation of glass is conditioned by the creation of glacial nucleus, which further grows

towards molten glass. Both actions, creation of glacial nucleus as well as the growth of

crystals are dependent on temperature. Maximum rate of both actions are not present by the

same temperature, more in fig. 4. In most cases is the maximal rate of creation of nucleus

recorded by lower temperature than maximal rate of growth of crystal. This is the reason why

it is easier to supercool the melt and prepare glass, when during warming of glass structure

there is realistic risk of its crystallisation.

Growth of crystals in glass making systems is very limited, that is why crystallisation usually

does not happen even in case when there are crystal nucleuses in the matter. Major influence

on crystallisation of glass has the ability of growth of crystal. If the molten glass has in the

area of maximum rate of growth of crystals high value of viscosity, the ability of growth of

crystals is limited. High value of viscosity lowers the mobility of structural units of matter.

If emerging crystal has same composition as surrounding glass, then the rate of its growth is

constant and is not dependent on time. If its composition is different from surrounding bath,

than rate is function of time. Growth of crystals slows down in time due to needed transport of

demanded particles by the mechanism of diffusion.

Some glass crystallizes towards the surface. This effect is used in practice when the purposely

created crystal surface layer gives material better characteristics than is given by glass itself.

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Fig. 4 Relation of rate of nucleation and crystallisation on temperature (SHELBY, J. E. Introduction on Glass Science and Technology. Cambridge: The Royal Society of Chemistry,

2005. 291 s. ISBN 978-0-85404-639-3)

2.3 Surface stress and density

Surface stress is force upon unit of length which has to be executed to expand the surface.

Action that is done during production of new surface is surface energy.

Multiple component arrangement has usually lower value of surface stress than single

component arrangements. Matters with low surface stress concentrate on the surface of liquid.

Matters with higher surface stress concentrate in the volume of liquid and so its influence on

the final surface stress lowers rapidly.

Values of surface stress of silica glass are 200 – 360 mN.m-1

and with growing temperature

decrease a little. Surface stress of bath is dependent on temperature only in minor, compared

to relation on presence of surface active matters.

Surface stress has meaning from the point of view of glass making due to these reasons:

- Evaluation of soaking of solid phases with melt glass

- Corrosion of heat resisting materials

- Fining of bath

Surface stress is basic presumption for possibility of glass forming by the method of drawing.

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Density

Has meaning by autogenous convection of bath brought up by the difference inside the melt

which helps homogenisation. Density falls with rising temperature, while more intensive

decrease is experienced after passing Tg. Reality is obvious from fig. 2 where relation of

volume to temperature by given temperature radically changes. Density of glass depends,

besides on temperature, also on its thermal history. In relation with stated graph, fast cooled

glass shows different density than glass cooled slower. The density of glass (bath) is most

significantly influenced by its chemical composition. Lower value of density than is set for

glass of given chemical composition can indicate presence of gas in the glass.

Glass is further valued based on the value of its molar volume (volume filled with 1 mole of

material), which is defined from the relation:

ii

m

nMV

. (2)

M is molar weight of component i (g.mol-1

)

n - molar fraction of the component i (1)

- density (g.cm-1

)

2.4 Thermal characteristics

It is valuable to know thermal characteristics of bath. It especially means thermal capacity and

thermal conductivity.

Process of glass making by the method of cooling of material mixture is practically

independent on the pressure. That is why measurable thermal capacity by constant pressure

has its meaning. Its value is significant for thermally balancing calculations of bath melting.

Glass production is highly demanding energetic process. Decisive parameter for preparation

of bath is its ability to share heat. Initial warming of material component is limited by its

ability to share heat by the process of leading. At higher temperatures the importance of

radiation grows. It was found that by temperatures above 385 ° C both mechanisms

participate in the process of heat sharing with the same share. At the temperature above

1200 ° C 90% of heat is transferred by radiation. In case of flowing melt is the process of heat

sharing further influenced by conduction.

Next major parameter of melt as well as glass is its thermal expandability. Thermal

expandability is change of dimensions of body during change of temperature throughout

constant pressure. The real coefficient of thermal length expandability is defined by the

relation:

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dT

dl

l.

1

0

(3)

real correspondent of length thermal expandability (K-1

)

l0 - original length of body by the temperature T1 (m)

dl - change of length of body by the change of temperature for dT(m)

dT - change of temperature of body (°C)

Besides length thermal expandability there is also set volume of thermal expandability ().

In this case all in previously stated relation is value of length changed instead of value of

volume. Glass is isotopic material that is why it is possible for practical consideration to use

relation 3.=.

Summarisation of terms

Viscosity

Crystallization ability

Surface stress

Density

Thermal attributes

Questions

1. Explain the term viscosity of molten glass, draw viscid curve.

2. Define relative viscid points.

3. Glacial ability of molten glass.

4. Explain the term of surface stress.

5. Density of melt.

6. Basic thermal characteristics of bath.

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3. MELTING OF GLASS

Time for studying: 6 hours

Aim After studying of this article you will be able to

Define material for glass production

Know chemical reactions proceeding during production of glass

Know processes of interchange of material from solid form into liquid

form

Know basis of actions needed for de-gassing and homogenisation of bath

Definition

Glass is produced in many different ways. It was stated previously that the most common

method of glass production is controlled cooling of melted material mixture. Before the melt

is prepared to be formed into a particular product, there are many actions that need to be

taken.

First there has to be set material for glass production, so called glass root. We take into

consideration the request for chemical composition and granulometry. Then there is the

process of heating. Before melting of each material part of mixture there are chemical

reactions. It is necessary to dispose the bath from gas, to homogenise it and adjust its

temperature so the bath shows requested viscosity, which allows non-problematic forming of

glass.

It is necessary to state that the glass production by the method of melting is considered as

waste-free technology. While during metallurgical processes of metal production there is

significant volume of waste dross, glass production does not generate similar type of waste

product. Part of produced glass is whole material mixture, besides gases that were eliminated

from bath during glass production.

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3.1 Material for the production of glass

Wide spectre of materials is used during production of glass. Constitution of materials comes

from the demands on the chemical constitution of produced glass. Synthetic materials with

high purity are used for production of special glass, or natural materials for production of

common glass. Overview of materials used for production of glass is in fig. 5. Inconsiderable

material part for the glass production is waste glass.

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Fig. 5 Materials for glass production

(CARTER, C., B., NORTON, M., G. Ceramic Materials Science and Engineering. New York: Springer, 2007.

716 s. ISBN 0-387-4620-8)

Materials for glass production can be divided into five groups:

- Glass producing

- Suitable for melting

- Modifier of characteristics

- Colouring additives

- Fining

Same type of material can have different role by different types of glass.

Major part of glass is glass producing material. These materials make basis of glass structure.

It can be one or more parts. Type of glass producing material in most cases is defined by the

name of prepared glass. For example SiO2 – silica glass, if SiO2 is in combination with major

share of B2O3 it is borosilicate glass.

Basic glass producing ingredients by commonly produced glass are SiO2, B2O3, P2O5. Further

glass producing ingredients are GeO2, Bi2O3, As2O3, Sb2O3, TeO2, Al2O3, Ga2O3, V2O5. These

ingredients besides GeO2, do not produce glass structure themselves, their glass producing

ability shows in combination with other oxides. S, Se and Te are used as glass producing

components by chalcogenide glass.

Most commonly used SiO2 as basic glass producing oxide allows easy transformation from

supercooled melt into glass structure. Further advantage of this material is its availability. On

other hand industrially produced glass (container, flat) do not contain only silica oxide. Its

own application into material mixture for glass production requires high temperature

processing. Temperature of processing is around 2000 ° C. Because of requirement for

decreasing of this temperature into material mixture for production of common glass there is

added to SiO2 melt, for example Na2O or PbO, which decreases temperature of processing

under 1600 °C.

Melt lowers temperature of melting, on other hand it can degrade characteristics of glass. For

example stated content Na2O in silica glass lowers its chemical endurance. There are added

components modifying its characteristics to the material for glass production because of this

reason, (for example Al2O3).

Colourfulness of glass is altered by colouring additives. Their content is not important from

the point of view of quantity. Colouring additives are often present in material against the will

of producers of glass. These are polluting components of dominant material. For example

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presence of oxides of steel in silica sand. Additive which has to restrain colouring effect from

other substances is called decolorant.

Finers are added to material for glass production because of easier de-gassing of bath and

better homogenisation. Their content is up to 1%. As finers are used sulphates (silica, calcic,

baric´.

3.2 Chemical reactions during melting of the glass

The most common glass, so called calcium silicate glass (material Na2CO3-CaCO3-SiO2) is

produced below the temperature of 1600° C. Before the glass root is melted, there are many

chemical reactions already in solid state.

-

Fig. 6 Equal phase diagram SiO2-Na2O

(Hlaváč, J.: Základy technologie silikátů. SNTL Praha, 516 s., 1988)

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Silica oxide

- Is brought in the form of silica sand as relatively gritty fraction (circa 1 mm).

It melts at the temperature of 1726 °C. Bath excels by its high viscosity which requires in

the case of pure SiO2 rise of the temperature up to 2000° C for adequate processing

(homogenisation, fining). From stated reason the mixture contains molten glass. During

warming SiO2 comes through modifying changes, which in case of glass production do

not have meaning.

Calcium carbonate

- Its dissociation happens from the temperature of 600 ° C. It is strongly endothermal

reaction which is shown by the used energy needed for melting of material.

Sodium carbonate (soda)

- It begins to melt before dissolving at the temperature of 852° C. Soda majorly joins

ongoing chemical reactions.

Reaction SiO2 with Na2CO3

4SiO2 + Na2CO3 = Na2SiO3 + 3 SiO2 + CO2 (4)

Reaction happens already in solid state. Na2SiO3 melts at the temperature of 1088 ° C.

Importance of reaction is in the fact that SiO2 integrates into its process; product of reaction

has lower temperature of melting than SiO2. In molten glass which comes up with SiO2 which

had not been involved in reaction yet melts gradually. As a result of stated actions the

temperature of melting of glass and decreases energetic demandingness of its production.

Further in the scheme Na2O – SiO2 molten glass comes up at the temperature of 790 ° C.

Emerging of molten glass speeds up the process of chemical reaction.

Reaction of SiO2 with CaCO3

- Because of the presence of stated components 2CaO.SiO2 is produced and by further

increase of temperature compounds rich in SiO2

Reaction of Na2CO3 with CaCO3

- Already under the temperature of 600 ° C there is double carbonate Na2Ca(CO3)2,

which melts by the temperature of 813 ° C (lower than Na2CO3).

23

In the case of common glass is the process of reactions in solid state completed by the

temperatures below 1000° C. Occurred heterogeneous system contains rests of SiO2 and in the

case of further ingredients with high temperature of melting and limited solvability. Dominant

phase is molten glass which is rich in volume of gas phase.

Course of chemical reactions can be sped up by dosing of fine grained material. Precaution

can be contra-productive as a result of occurring of air bubbles of smaller diameter, which

further surface from the bath hardly. It is advantageous that period of reactions in solid state

proceeds in maximum level influenced by slower pre-warming of glass root. Production of

low melting products is supported and process of melting is shortened. Process of melting can

be slowed significantly by moistening of the material (4-8% H2O).

It is possible to use fining substance for example Na2SO4 to increase the intensity of the

process. This melts by the temperature of 884° C without decomposition. With SiO2 reacts up

to temperature of 1200 ° C according to the equation:

nSiO2 + Na2SO4 → Na2O.nSiO2 + 2SO2 + 1/2O2 (5)

Reaction is slow (and by relatively higher temperature), that is why there is a risk that Na2SO4

is unmixed under the influence of lower density onto the surface of melt. Sulphate molten

glass is produced and further homogenisation is difficult. Segregation is prevented by addition

of reduction agent (for example powder coal). By the temperature of 600° C there is reaction:

nSiO2 + 2 Na2SO4 + C → 2Na2O.nSiO2 + 2SO2 + CO2 (6)

Addition of soda sulphate and its previously described reactions are related to the degassing

of bath. This problematic is described in further part of text.

Beginning process of melting of glass root has to be seen in wider circumstances. Reactions,

which were described before, take place at the same time. It is not clearly process of reactions

in solid state. During reactions a part of matter melts with the rising temperature. Temperature

of melting is exceeded. At the same time are some particles in arisen molten glass, parallelly

with the process of mentioned reactions, melted. Creation of prime molten glass is welcomed,

at the same time the molten glass must not oppose to the process of reactions. Material

mixture allows creation of molten glass based on the ratio of substances, which can be close

to the composition of eutectic points. For example in system of calcium silicate glass with

content of soda and potassic oxides molten glass is created by the temperature around 775 °C.

3.3 Melting of solid materials in the molten glass

Solving of solid rests is the leading action by the temperatures above 1100 ° C. SiO2, Al2O3,

ZrO2, mullite are preserved in solid state for the longest time. Limiting factor for dissolving is

24

high viscosity, which prevents convection and further lower diffusion coefficient. Driving

force of dissolving is concentrating gradient in the direction from un-dissolved grain towards

the molten glass.

Dissolving of SiO2 takes 90% of time of melting even if it reacted at 80% of SiO2 in previous

stage.

With the high content of SiO2 seems as major part the process of dissolving of this material

substance. Speed of dissolving of SiO2 depends on:

- Diffuse coefficient SiO2 in the molten glass,

- Concentration gradient on the interface of SiO2 – molten glass

- Contact area of SiO2 – molten glass.

Use of theoretical basics of process of dissolving by melting of real systems is difficult.

Particular grains have different shape and size, composition of molten glass gradually changes

with the rise of the temperature (the reason are reactions and forthcoming process of

dissolving). Definitely time needed for dissolving of particles is equivalent to the starting size

of particle and concentration gradient.

Flotation can prevent the process of dissolving of solid particles, their elutriation onto the

surface of the melt, where particles become insoluble. Flotation is supported by degassing of

material. Washed up bubbles of gas bring solid material to the surface level. Flotation can be

lowered by surface active matters (e.g. Na2SO4). These decrease surface stress and so increase

wettability of surfaces of undissolved solid matters. In case of addition of sodium sulphate it

is necessary to protect its segregation onto the surface level of the bath, as it was stated

previously.

Process of melting shortens presence of water in the batch. Water is often bound to material

from which is the glass root made, it was already stated that material mixture can be dosed

dehumidified. Presence of water is expressed by the decrease of viscosity of molten glass of

oxides, what has positive influence on dissolving as well as on surfacing of bubbles of gas.

Positively is also expressed increasing of partial pressure of water steam above the batch.

3.4 Fining and solubility of gases:

Bath contains significant share of gases. Gases come from the material (space between grainy

material is filled with gases) and gases also emerge as a result of process of chemical

reactions among substances of material mixture and also gases can be produced by reaction of

bath with fireproof lining. Even after complete transformation of solid matters into liquid

state, molten glass contains major share of gases. Gases must gradually surface onto the

surface level. At the same time the rise of gas towards the surface must not cause flotation of

undissolved particles. Process of degassing of bath is called fining and it happens after the

25

transformation of solid matters into the liquid state. Further part of gases is dissolved in the

bath.

Most commonly represented in bath is CO2, SO2, N2, NOx and H2O. Degassing of bath is

supported by so called fining (As2O3, Sb2O3 together with alcalic nitrate – suitable for crystal,

Na2SO4 – for packaging glass and NaCl – for glass with resistance against sudden changes of

temperature and so on).

Speed of surfacing of bubbles is defined by Stokes’ law:

18

..2

g

dv (7)

v is speed of surfacing of bubbles of gas (m.s-1

)

d - diameter of bubble (m)

– weight acceleration (m.s-2

)

g - difference between density of gas and molten glass (kg.m-3

)

- viscosity of the molten glass (Pa.s)

From stated relation is clear that for speeding up of the process of purifying it is necessary to

secure conditions so the bubbles of gas have as large diameter as possible. That is why dosage

of fine grained material from the point of view of generating small bubbles of gas by

processing reactions seems as disadvantage, even though the reaction surface is increasing,

which on other hand speeds up process of for example dissolution. Further the process of

surfacing of gas bubbles is sped up by the rising of temperature, by which it comes to

decrease of density of value of viscosity. Validity of Stokes’ law is in practical conditions

influenced by many different influences, for example process of chemical reactions happening

in the bath, dissolubility of gas in the molten glass etc. Decisively factors of size of bubble

and viscosity influence degassing in the sense of stated commentary.

For the process of fining is valid:

- In the beginning phase of fining the connection of bubbles (coalescence) is usual

- Size of bubbles in molten glass without fining agents by constant temperature almost

does not change

- Size of bubbles in the molten glass with fining agents rises with the growth of

temperature by the mechanism of diffusion of gas from oversaturated molten glass.

Principle of purifying is based on the addition of ingredient, which reacts in the bath during

the production of large gas bubbles. Bubbles take smaller scattered bubbles of gas during

surfacing.

26

Bubbles CO2 in molten glass with fining agents change their composition. O2 but also SO2 +

O2 diffuse into them mainly.

Bubbles become smaller by the decrease of temperature in the molten glass with fining agents

during process of dissolving of gas content in the molten glass. Dissolving of gases in the

molten glass becomes practical mainly in the case of small bubbles.

Stated notes can be cleared on the basis of function of effect of fining agentsAs2O3 a Sb2O3,

which are added to the glass root in the amount of 0,1 to 1%. Process of realized processes is

difficult, and it can be described by further commentary. Arsenic and antimony fining agents

react in the bath with potassium nitrate (also intentionally added into glass root) after the

reaction:

4KNO3 + 2As2O3 → 2K2O + 2As2O5 + 4NO + O2 (8)

Gases are produced by reaction (NO, O2) they form relatively large bubbles, which surface

and at the same time take smaller bubbles which are scattered in the molten glass. This

mechanism degasses the bath. Also at the same time with further growth of temperature of

bath is pentavalent form of arsenic becomes less stabile than trivalent form and there is

reaction:

As2O5 → As2O3 + O2 (9)

Emerging oxygen is formed into bubbles. For stated reaction is considerable fact that its

balance is dependent on the temperature. By the decrease of temperature stated reaction

begins to act in opposite direction. Oxide present in the molten glass is used for emerging of

pentavalent form of arsenic. Same mechanism of process of chemical reaction is used for the

fining agent As2O3 as well as for fining agent Sb2O3

From the reasons of negative influence of arsenic oxide on the human health (matter

suspected of carcinogenic effect) is this fining agent subsidised by ceric oxide which has

similar fining effect, and the formula is:

4CeO2 → 2Ce2O3 + O2 (10)

Other important fining agent is soda sulphate. By his presence there is previously stated

reaction (5). Other fining agents are halides.

3.5 Flowing, mixing of glass metal and its homogenisation

For securing of qualitative production of glass it is necessary to secure homogeneity of glass

from the point of view of chemical composition as well as in the sense of distribution of

27

temperature in its volume. By melting in glass furnace there is always autonomic flowing, and

its importance grows in continually working furnaces. Flowing of molten glass is positive

from the point of view of transport of matter and temperature, negative it is from the point of

view of wearing down of inwalls of glass furnaces. Flowing of molten glass is caused by the

taking of bath and further by the emergence of autonomic convection. It emerges as a result of

gradient of temperature and density. Flowing of bath has laminar character.

From the point of view of good homogenization of bath important element is size of grains of

material components. There is a rule that the less the material is present in glass root the lower

its granulometry has to be.

Flowing of bath and by this also its homogenisation can be successfully intensified by mixing

of bath with the help of agitating element, which is realised especially in the case of

production of optical glass. Further means of homogenisation of molten glass is its bubbling.

This is realized in the place at maximum temperature through pressure inert gas. Large

attention by the production of the glass is also given to monitoring of equability of flowing of

the whole volume of bath.

3.6 Evaporation of volatile components

Evaporation of volatile components during melting of the bath is a negative effect. There is

uncontrollable change of chemical composition. Vapours of volatile components can further

influence durability of fire-resistant inwall in disfavorable way.

Among volatile components are alkaline oxides PbO, B2O3, ZnO, P2O5, fluorides,

compositions of selenium. Speed of evaporation of volatile components depends on partial

pressure of volatile components above the surface of the bath. Evaporation of volatile

components during glass melting can cause change in composition of bath for tenths even

units of percents. During formation of glass root it is necessary to dose larger scale of those

components which evaporate easily, this larger scale compensates evaporation.

Summarisation of terms

Material for the production of glass

Melting of glass

Fining of bath

Flowing and homogenisation of bath

Evaporation of volatile components

28

Questions

1. Material for the production of glass.

2. Describe chemical reactions during melting of glass.

3. Melting of solid matters in the molten glass.

4. Explain the principle of fining.

5. Types of flowing in the glass furnace.

6. Homogenisation of bath.

29

4. PRODUCTION OF GLASS

Time for studying : 4 hours

Aim After studying of this article you will be able to

Differentiate basic types of glass furnaces

Master techniques of glass forming

Know fundamentals of glass cooling

Definition

Production of glass is composed from these operations:

- preparation of batch

- melting process

- forming

- cooling

- finishing operation

4.1 Preparation of batch

Natural and synthetic material is used in production of glass. Important material component

during production of glass is recycled glass.

Basic material for glass production is SiO2. Its source is silica sand. Sand is supplied for glass

industry in different types of quality, its granulometry (often 0,1 to 1 mm) and purity are

distinguished. Washing of sand gets rid of argillaceous components. Further monitored

components are oxides, which can cause unwanted colouring (Fe2O3, Cr2O3, TiO2).

30

Next glass material is CaO and MgO. Source of stated oxides is limestone and dolomite.

Production of a lot of glass cannot happen without alkaline components. Main source is

sodium (Na2CO3). Alkali can also be brought into material by feldspathic materials. In this

case is the material also upgraded with component Al2O3. Next alkaline material is calcined

potash or hydrate K2CO3.1,5H2O. These last matters are hygroscopic (they absorb airy

humidity).

Tri-hydrogen boric acid is among boric material (H3BO3) and borax (Na2B4O7.10H2O),

eventually borax dehydrated into penta-hydrate. Barium is in the form of BaCO3 or BaSO4.

Lead is brought in the forms of Pb3O4, PbO or in the form of Pb(NO3)2. Alumina is brought as

part of mentioned feldspars or as supporting component of sands and limestone. In case of

demands for higher content of alumina the material is enriched by argillaceous hydroxide or

by kaolin.

Besides mentioned substances further substances can also be brought into the glass root in

minority substitution, which have specific effects (colouring or fining effect, oxidizing or

reduction agents). These are for example phosphates, fluorides. In glass industry is used slag

from blast furnace, its trade mark is Calumite. Mentioned material brings especially SiO2,

CaO, Al2O3, MgO a MnO.

Particular material composition of glass root depends on the type of produced glass. Figure 7

shows composition of different types of glass.

Fig. 7 Composition of chosen types of glass

(CARTER, C., B., NORTON, M., G. Ceramic Materials Science and Engineering. New York: Springer, 2007.

716 s. ISBN 0-387-4620-8)

31

Glass root is produced from mixture of material of suitable granulometry. There is a general

rule that the less material is represented in the root, the more this material should be fine

grinded. This reduces mutual distance among grains of this material. Constitution of glass

root is determined by calculation. Basic are stoichiometric calculations, which guarantee that

share of input material, which partially dissolves during melting process by the emergence of

gas products, secure required chemical composition of produced glass. Important element

during production of glass root is homogenisation of material. During industrial glass

production is the granulometric adjustment, transport, homogenisation and dosing fully

automated. Usually is the material before dosing into the furnace aggregate moistened (3 – 4

hm%). This for example prevents pulverization of material and also decreases ability of its

segregation and speeds up the process of change into liquid condition. Ground material can be

granulated, as bonding agent can be used water glass, sodium hydroxide, or calcium

hydroxide (content of gasses among grains is decreased, transport and dosage is simplified).

During melting is the bonding effect impaired and further reactions proceed in normal way.

4.2 Glass furnaces

There are two basic types of furnaces, pot and tank furnaces.

Pot furnaces are designated for low volume production of glass, for example art glass, where

is prevalent high share of hand production, furnaces work periodically. Pot furnace has low

use of thermal energy, and its advantage is that it allows producing of various sorts of glass.

Pot furnace is furnace space of arch design, in which are a few glass pots. Space of furnace is

heated by gas, Furnace space can be technically divided in a manner that secures establishing

of specific regime (various temperature or various atmosphere) in each limited parts of

furnace. Glass pot is filled by root and further there is process of melting including fining and

homogenisation. In this phase is the temperature of bath at its highest. Homogenisation is

supported by bubbling with the help of submersible nozzle, or there is mechanical mixing.

Produced bath is taken from the pot usually manually through openings in the furnace.

Particular pots which are in the furnace can be in different phases of glass production at the

same time. Content of the pots is around 100 to 1000 kg of bath; there are 4 to 12 pots in the

furnace.

Tank furnaces allow non-stop functioning. The name is derived from its shape. We can easily

say that the glass root is put into the furnace on one end and on the other end there is bat

ready for the process of forming. Tank furnaces allow high productivity during the production

of glass. Production of glass is in this case fully automated.

Tank furnace allows better use of thermal energy. Generators for use of so called waste heat

are part of furnace. Burnt gas led from the furnace space shows high enthalpy that is why

thermal exchanger for heating is part of the furnace, for example combustion air.

32

Described precautions significantly increase energetic efficiency of process of glass

production. Batch and then bath is warmed by burnt gas, source of thermal energy is burnt

natural gas above the surface of the bath (material). Warming of batch and melting process

can be supported by electrical reheating. Along the length of furnace are spaces in which there

are different phases of cycle of glass production. Valid is again that the highest temperature is

in the area of fining and homogenisation, temperature of bath before flowing into the forming

machine decreases on the value which secures demanded viscosity. Performance of tank

furnace is in the case of container glass or glass sheet on the level of hundreds of tons of glass

a day, meanwhile the performance of furnace for the production of utility or technical glass is

on the level of units to tens of tons of produced glass each 24 hours.

4.3 Forming of glass

Various forming techniques are used for production of glass. Viscid flow of bath is used for

the forming of the glass. Decisive parameter for successful progression of forming process is

right value of viscosity, which is different for various forming techniques. Lower in the table

are mentioned these recommended values of viscosity for different forming techniques.

Table 1 Usual values of required viscosity of glass needed for its forming

Glass Beginning of forming End of forming

log (dPa.s)

Cast of flat glass 2,8 4,0

Packaging glass 3,0 – 3,2 5,5 – 6,0

Drawing of tubes 5,0

Drawing of flat glass 4,0

Manual production 3,4 8

(Hlaváč, J.: Základy technologie silikátů. SNTL Praha, 516 s., 1988)

Forming must proceed under conditions which expel possibility of crystallization of glass.

Glass is formed freely or into forms, which are made from metal (grey cast-iron, alloy steel).

There is not usually problem of gluing bath onto the forms because of the reason that during

forming there is between form and bath gas layer. Manually produced glass can be formed

into wooden forms.

The glass was formed by blowing in the past. Dosage of glass was dipped on caxal bel cone.

Bath was formed by blowing into the bel cone. Final form was secured by the use of form,

into which the end of the bel cone with glass was put or with the help of other forming

instruments. Process was used for all kinds of forms, for example for forming of packaging or

flat glass. In case of production of flat glass there was formed a cylinder by blowing, which

33

was then cut and heated on temperature which allowed its flattening. Principles of mentioned

forming are used up until now by manually produced decorative or utility glass.

Forming of container glass

Forming of container glass is in present realized exclusively by machines. Drop of melted

bath is taken from the bottom of glass tank with the help of drop dispenser. Bath flows

through incoming canal into forming machine. Forming machine works in so called press-

and-blow, or twice blown way.

During press-and-blow process is from a drop of bath pressed basic shape of bottle. In further

step is this semi-product set into final form and resulting form is reached by blowing it out. In

the case of twice blown process, action of prime press is, in difference to previous way,

subsidized by blowing. Next in the final forming, the eventual form of the product is blown.

Formed glass in the process of production cycle further continues into cooling furnace, and

then is the bottle ready for shipping.

Fig. 8 Scheme illustration of production by press-and-blow and twice blown way

(http://www.eurotherm.com/industries/glass/container-glass/)

34

Typical composition of container glass: 73% SiO2; 11% CaO; 14% Na2O; 2% Al2O3. Glass

also contains minority share of MgO, K2O a SO3. Glass used for storage of chemicals and

pharmaceuticals can be based on borosilicates.

Forming of sheet glass

Production of sheet glass has gone through intensive evolution. Sheet glass is presently

produced on continually working furnace by drawing or by so called Float.

In the case of drawing (Fourcault glass process), is the bath led into drawing machine. There

is drawn continuous belt of glass in the vertical way from the surface. Bath is drawn from the

surface through so called extruder, which secures stabile width of glass belt. Glass is then

drawn with the help of rotating cylinders through drawing shaft.

Glass is in the process of drawing

gradually cooled. It is technology

which presents from today’s point

of view older way of production of

sheet glass. Glass shows many

optical faults (imperfect flatness of

the surface.)

Further option is the production of

glass by casting when bath flows

between rotating cylinders from

which is the glass drawn in

horizontal level. Process is suitable

for thick-wall glass and especially in

cases when is glass strengthened

with wire insert.

Fig. 9 Schematic illustration of the production of

glass by drawing

(http://upload.wikimedia.org/wikipedia/commons/c/c4/Fourcault_process_for_flat_glass_for

ming.svg)

35

Process of production of glass in so called Float way represents nowadays the most common

way of production of sheet glass, production by mentioned way reaches on worldwide level of

90%. Technology allows high productivity of production and produced glass shows very good

quality parameters. Float technology was developed by the company Pilkington in Great

Britain in 1959. Principle of production of glass in Float way is based on the fact that glass at

temperature of 1000 ° C flows through rotating rollers on the surface of molten tin

(temperature of melting of tin is 232°C). Melt of metal has almost ideally flat surface, this

characteristic is also in produced glass. Production processes in protective atmosphere

(mixture of gases 90% N2 and 10% H2), so oxidation of molten metal does not happen. Upper

level is smoothened by influence of surface stress; favourable physical ratios are secured by

the influence of seeding from installed heating bodies. Glass is then gradually cooled and

comes out of space of tin bath by the temperature of circa 600° C. Movement of glass is

provided by rotating rollers. In this part of production process is the viscosity of glass so high

that it does not come to damage of smooth surface of the product. Mentioned way is usually

used during production of glass of width of 2 to 20 mm.

Fig. 10 Scheme illustration of production of glass in Float way

(http://www.wisedude.com/science_engineering/glass.htm)

36

Forming of glass tubes

Way of production of glass tubes depends on the required parameters of final product.

Danner process is suitable

for production of thin

wall tube glass. Bath

flows on rotating bel cone

into which is the air led

in. Cavus form of glass is

preserved by blowing into

the glass, which is drawn

from the bel cone

Fig. 11 Scheme illustration of Danner process for glass

production

(http://upload.wikimedia.org/wikipedia/commons/4/4a/Danner_process_for_tube_glas

s_forming.svg)

37

Other universal way of

production of tube glass (thin

and wide walled) is so called

Vello system. In this case the

bath flows from the opening put

in the bottom of glass tank. Bath

flows onto the form, into which

is compressed air led again.

Construction of form predestines

form of final tube.

Forming of glass fibres

Process of production of glass fibres is determined by their measurements and purpose of use.

There are fibres, so called endless, clip for production of filtration fabric or chemically bound

isolation mats, further isolation padding and microfibers.

Endless glass fibres are produced in furnace made of platinum alloy. Platinum furnace has in

its lower part, small openings, through which each fibre (fibres are drawn) flows out. Fibres

then come together into stream. Furnace is heated electrically.

Fig. 12 Scheme illustration of production of tubes in

Vello system

(http://upload.wikimedia.org/wikipedia/commons/1/1e/Vello_process_for_tube_glass_forming.svg)

38

Fibres for chemically bound isolation mats are produced by blowing of bath. Bath falls onto

spinning roll, which has many fibre producing openings on surface. Bath, which is casted

aside from the spinning roll, gains needed oblong form of fibre.

Fibres of small diameter, so called microfibers are produced by defibering of primal fibre,

which is drawn from platinum furnace. Secondary defibering comes by operating of stream of

hot gases.

Fig. 13 Production of glass fibres with spinning wheels

http://www.ilo.org/oshenc/part-xiii/glass-pottery-and-related-materials/item/925-

glass-ceramics-and-related-materials

4.4 Cooling of glass

It is necessary to solve stress which is produced inside of the produced body during the

production of glass. Process during which internal stress is eliminated is called cooling.

Relaxation of glass (relaxation of stress) happens during viscose flow in cooling interval of

temperatures, which is limited by upper and lower annealing temperature. Values of cooling

temperature can be different for different types of glasses of different chemical compositions.

39

Upper and lower annealing temperature is set exactly at the values of viscosity 1013

and 1014,5

dPa.s.

Temporary stress

There is difference between temporary and permanent stress. Existence of temporary stress in

glass is experienced in case of occurrence of thermal gradient and in the case that the

temperature falls beyond the lower annealing temperature. These conditions correspond to the

state when the glass behaves as solid and flexile body. In case of thermal gradient, each levels

between warm and cold side of the glass have tendency to gain size corresponding to the

value of coefficient of thermal expandability, which is different for different temperatures.

Characteristic of glass (below the lower annealing temperature) prevents free expansion or

contraction of single layers of glass that is why there is stress in glass. During both-sided

cooling of glass sheet there is on the surface draw stress and inside there is pressure stress.

Warmer layers in direction from surface have effect in drawing, as a result of thermal

expandability, on layers in the direction towards the surface of cooled body. During warming

is share of stress after cut of body opposite. When the temperatures of body after cut equal,

each layer has same temperature and same value of coefficient of thermal expandability,

previously mentioned stress disappears. From mentioned reason is this described form of

stress called temporary.

Temporary stress limits durability of glass against sudden changes of temperature. Break is in

the case of glass spread from the surface towards the centre of the body. Glass is from this

point of view less durable against sudden fall of temperature, where there is draw stress on the

surface, which is approximately lower of one degree than stress.

Permanent stress

Permanent stress is caused in different way. Permanent stress is caused in the glass, which is

uncontrollably cooled. In state where the glass is formable (it subjects to viscose flow) stress

is compensated by the flow of the matter. During the decreasing of temperature below the

value, when the glass does not subject to the viscose flow, stress begins. There is pressure

stress on the surface and inside there is draw stress. Cause of the share of stress is the fact that

inner layers of glass shrink also in the moment, when surface temperature does not change.

Permanent stress happens in the case when the glass is solidified, this means when the

temperature falls from the values exceeding transformational temperature to lower

temperatures. At these temperature is the matter changed from supercooled molten glass into

solid matter, which is not able to compensate stress which is already present. Permanent stress

can cause destruction of product already during the process of its cooling, or contributes to its

permanent decreasing of strength parameters of product.

40

Cooling of glass

Temporary and permanent stress does not happen in the product if the speed of change of its

temperature gets closer to infinite low temperatures. The requirement is unrealizable. Process

of forming of glass is subjected to the efforts of getting the product of desired forms and

measurements. It is not possible to simultaneously develop precautions forbidding upcoming

of permanent stress. That is why there is controlled cooling of glass following the process of

forming. Cooling of glass consists of phases:

- Warming of product

- Persistence on upper annealing temperature

- Slow cooling in the range of viscosity 1013

to 1014

dPa.s

- Final cooling to normal temperature

Usually the product after finalizing of process of forming has temperature lower than the

temperature corresponding to the state of viscosity flow. From this reason is the first phase of

cooling of the glass its warming. Speed of warming must respect limits coming from the

emergence of temporary stress.

Consecutive persistence on upper annealing temperature (viscosity 1013

dPa.s) processes

during the time, before the temperature gradient disappears from the product. Prescribed

temperature allows viscose flow of matter, in process of persistence permanent stress

disappears.

Further it is possible to cool the product. First phase of cooling by the viscosity of 1013

to

1014,5

dPa.s processes slowly, so repeating emergence of permanent stress is prevented. Then

the speed of cooling can be increased. Limiting factor for the speed of cooling is in this phase

again the risk of emergence of destruction by the influence of temporary stress.

In the case that viscosity curve for given type of glass is not known, it is possible to take as

transformational temperature (Tg), temperature of endurance during cooling. It can be

obtained from the dilatation curve, increased for 5 to 10°C.

Table 2 Cooling temperature of chosen types of glass

Type of glass Cooling temperature °C (log=13,0 to

14,5)

Glass Fourcault 521 – 483

Colourless container glass 542 – 505

Sodium potassium crystal 500 – 460

Lead crystal 453 – 416

Simax 582 - 540

(Hlaváč, J.: Základy technologie silikátů. SNTL Praha, 516 s., 1988)

41

Toughening of glass

Emerging of permanent stress can be used positively during the production of toughened

glass. Emerging permanent stress which displays during cooling of the product causes surface

stress on its surface. Extent of the surface pressure stress can be supported by quick cooling of

the glass from the temperature close to its softening (viscosity according to Littleton: log =

7,65 dPa.s). Precaution, when strong stress of the product emerges in controlled way on the

surface. Stress helps the toughening of the product, because the break of glass emerges from

the surface of the product by the draw stress. Break of toughened glass emerges after

overcoming of stress, which came by its controlled supercooling by the flow of gas or by the

dipping into the cooling bath. Efficiency of toughening of glass increases with increasing

speed of cooling, by the fulfilment of condition that starting temperature of cooling is higher

than temperature of softening. Increasing of temperature above the stated level does not

increase the level of toughening of the glass.

Toughened glass falls apart during destruction into many not-sharp fractions. Toughening by

the method of supercooling of surface of the glass can increase the toughness of the surface

up to six times. Toughened glass is used in means of transport. This glass cannot be cut,

drilled, or mechanically processed in any other way. In the case that the surface treatment is

eroded, inner draw stress causes destruction of glass. Toughened glass does not resist working

of higher temperatures (circa above 200° C), where there can come to relaxation of surface

pressure stress and glass decreases its toughness.

Summarisation of terms

Batch into the glass furnaces

Melting furnaces for the production of glass

Forming of glass

Cooling of glass

42

Questions

1. Preparation of batch for glass production.

2. Types of melting furnaces for glass production.

3. Methods of glass forming.

4. Forming of container glass.

5. Forming of glass sheet.

6. Forming of glass fibres.

7. Principle of glass cooling.

8. Occurrence of stress in the glass.

9. Principle of toughening of glass.

43

5. CHARACTERISTICS OF GLASS

Time for studying : 6 hours

Aim After studying of this article you will be able to

Know mechanical characteristics

Know thermal characteristics

Know optical characteristics

Definition

In previous part of text was large attention given to the characteristics of melted bath. Hard

glass has totally different character from the bath that is why attention in next part is given to

the characteristics of glass in solid state.

5.1 Mechanical characteristics

Glass is relatively fragile material, which in comparison to metal, but also to most ceramic

materials, shows mechanical attributes of lower use value. For evaluation of glass there is

important solidity in drawing or solidity in bending, because draw forces are usual cause of its

break.

Relation of stress to deformation, in case of glass, shows during laboratory temperature

almost ideal linear progress, which corresponds to Hook’s law.

.E (11)

is mechanical stress (MPa)

E - Young’s module of flexibility (MPa)

- linear deformation (1)

44

In case of thin walled glass materials there is markedly expressed dependence of solidity of

material on its crosscut. Glass fibres have majorly higher toughness in drawing than for

example massive glass. In case of microfibers this difference is far more noticeable

(toughness in drawing of microfibers is on the level of thousands MPa, while massive glass

has toughness below 100 MPa).

It is approved that break of glass is influenced by the surface defects, whereas destructive is

especially draw stress. Toughness of glass majorly decreases with the occurrence of surface

defects which occur during production as well as during use of the product. Toughening is

easily provable by the evaluation of kind of cutting of the glass. Indistinctive surface scratch

causes its easy division. Drawn fibres have surface with lower number of defects than glass of

different type that is why it shows higher values of solidity.

Mentioned notes can be used during increasing of toughness of glass. Solidity can be

increased by thermal toughening (chapter cooling of glass), but also by smoothening of

surface. Surface can be smoothed with higher temperature (viscose flow) or mechanical

polishing.

It is also possible to modify chemical composition of surface layer with the aim of increasing

of solidity of glass. Chemical solidifying of glass happens for example as effect of dipping of

glass in to the lithium liquid. Ionic exchange happens, ions Li+

are transferred into the glass at

the expense of ions Na+ and K

+. Surface layer emerges with lower value of coefficient of

thermal expandability than layers towards centre of product have. By this is the surface layer

during cooling of glass less shrunk than layers below it, expected pressure stress emerges on

the surface of product. Described ionic exchange happens above lower annealing temperature.

There are techniques which happen on similar principle by lower temperature, for example

enrichment of surface layer by ion K+ at the expense of Na

+. By this way it is possible to

increase the solidity of glass in draw for up to one level against the state when ionic exchange

is not realized.

Next toughening method is spreading of protective surface layer. Layer is put on the glass

which prevents emergence of mechanical damage. Layer is applied on hot surface after

finishing of forming. Limit for protective layer can be possible change of optical

characteristics of glass.

Next mechanical parameter is solidity of glass. Solidity of glass is according to Vickers on the

level 2 -8 GPa up to 11 GPA pro nitride glass. Silica glass has higher solidity than lead or

boric glass.

5.2 Resistance of glass against changes of temperature

Resistance of materials against changes of temperature is limited especially by the coefficient

of thermal expandability. Problem was partially mentioned in connection with existence of

45

temporary stress. It is possible to defined stress which occurs in the material during the

change of temperature by the relation:

1

.. TE (12)

is stress from change of temperature (MPa)

E - Young’s modulus of flexibility (MPa)

- right coefficient of thermal expandability (K-1

)

- Poisson’s number (1)

T - difference of temperature in the body

Value of Young’s modulus of flexibility does not make sense to decrease on behalf of

decreasing of stress occurring as a result of change of temperature. Young’s modulus of

flexibility influences also solidity of material in the same way, more in Young’s law.

In case of request for material with good resistance of materials against changes of

temperatures it is necessary to apply material with low coefficient of thermal expansibility.

From this point of view is silica glass advantageous, its value of is on the level of 6,7.10-

7 K

-1, while this value by sodium potassium crystal reaches almost 100.10

-7 K

-1.

Coefficient of thermal expandability has other meaning for glass. During monitoring of

progress of curve of its dependence on temperature it is possible to set the temperature of

transformation (transformational point makes on the curve the point of break).

5.3 Optical characteristics

Glass has wide use in optical industry that is why its optical characteristics are more and more

valuable. Basic optical characteristic of glass is refractive index.

sin

sinn (13)

n is refractive index (1)

- angle of impact of light

- angle of refraction of light

Refractive index is dependent on wavelength of light. There are contractually set refractive

indexes for certain wavelengths. For example nD represents refractive index in yellow spectre

of visible radiance (= 589,3nm), nd is refractive index at the wavelength (=587,6 nm.).

46

Values of refractive index are further dependent on chemical composition of glass, by silicate

glasses the values are about 1,5-1,9; in case of special glass they can exceed the value of 2,2.

High value of refractive index gives glass higher shine. Value is increased by PbO and BaO.

Next significant optical characteristic is transmitting T (light transmittance). It is ratio of

luminous molten glass on the incoming body to luminous molten glass from the outcoming

body. Part of the luminous molten glass reflects from the surface to the surroundings, part of

molten glass is absorbed in the body and part is coming out of the body.

0

T (14)

T is transmittance (1)

0 - luminous molten glass falling onto the body

- luminous molten glass coming out of the body

There is also inner transmittance (Ti), it is ratio of luminous molten glass from the body out

coming to the luminous molten glass into the body penetrated (after reflection).

Ability of material to absorb radiance is called absorbance:

:

iTA log (15)

A is absorbance (1)

Ti - inner transmittance

47

Fig. 14 Spectrum of electromagnetic radiance

(http://www.mun.ca/biology/scarr/Electromagnetic_Spectrum.html)

It is interesting to monitor rate of transmittance of glass for the spectre of light and infrared

radiance. Common glass shows high value of transmittance, which means that it easily

transmits relatively high share of radiance, fig. 15. Common glass, besides visible radiance,

which is by glass from the reason of expected transmitting of light desired, it lets through

relatively high share of thermal radiance in the infrared area. From this reason spaces with

glass areas are easily heated. Solution of the problem is in the adaptation of glass, which

achieves good trasmittance in area of visible radiance (to circa 1000 nm) and at the same time

the transmittance is lowered in maximal value in the area of IR radiance of wavelength below

3000 nm. Request is fulfilled with glass with content of FeO or glass with content of FeO in

combination with Ni, Se, Cu. Glass with modified chemical composition lets light through

well, and lets through thermal radiation a lot less. If the transmittance is lowered with the

modification of chemical composition of glass, there comes to its warming with the influence

of lowered transmittance of thermal radiance in the area of IR. That is why it is looked for

solution in the modification of surface layer of glass. Then thin layer causes reflexion or

absorption of thermal radiance. Example of solution is in figure 15, where the surface

modifications of glass with trade name LX70 a P70 significantly lower transmittance for IR

radiance up to 2700 nm against common glass.

48

Fig. 15 Relation of transmittance to wavelength of common glass and glass with surface

modification with the trade name LX70 a P70

(http://www.ufrgs.br/casae/systems/acclimatization/selective-coating)

Fig. 16 Transmission of light and IR radiance through layer without modification (left) and

through glass with modification for elimination of transmittance of IR radiance (right)

(http://www.commercialwindows.org/lowe.php)

49

Summarisation of terms

Mechanical characteristics of glass

Resistance of glass against changes of temperature

Optical characteristics of glass

Questions

1. Describe mechanical characteristics of glass.

2. Explain resistance of glass against the changes of temperature.

3. Describe optical characteristics of glass.

50

LITERATURE

HLAVÁČ, J. Základy technologie silikátů. Praha: SNTL, 1985. 516 s. (Basics of technology of silicates)

HANYKÝŘ, V., KUTZENDÖRFER, J. Technologie keramiky. Praha: Silikátový svaz, 2000. 287 s.

ISBN 80-900860-6-3. (Technology of ceramics)

SHELBY, J. E. Introduction on Glass Science and Technology. Cambridge: The Royal Society of Chemistry,

2005. 291 s. ISBN 978-0-85404-639-3

CARTER, C., B., NORTON, M., G. Ceramic Materials Science and Engineering. New York: Springer, 2007.

716 s. ISBN 0-387-4620-8

IMANAKA, Y., et al. Advanced ceramic technologies & products. Tokyo: Springer, 2012. 585 s. ISBN 978-4-

431-53913-1