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What is Redox Theory?
Which chemicals create what colours?
What is refining – How does it work?
How to get rid of or hide the green caused by iron - decolourise?
What is a reduced, neutral or oxidised batch?
What is devitrification?
Do colour changes give you problems - speed, seed & blister?
Flint glass refining, melting & colour
Amber glass & redox
Amber glass
Green glass
Glossary
Glass Redox
Glass Redox can be defined simply as the reduced or oxidised state of a glass. It is affected by the cullet composition, the furnace atmosphere, the batch composition, the thermal history of the glass, etc.
This reduced / oxidised condition in general determines the level of sulphate as SO3 retained in the glass. A highly oxidised glass will contain as much as 0.30% SO3 whereas a highly reduced glass may contain as little as 0.25% SO3.
The lower the glass redox, the lower the solubility of SO3 in the glass and , therefore, the glasses which lose more SO3 and / or retain less SO3 will be better refined. When released out of the glass it is in the form of SO2.
This better refining, which is generally obtained at a lower redox, also allows the furnace to be run at a slightly lower temperature using less fuel.
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Batch Redox Concept
To enable predictions to be made of the glass redox an empirical method of calculating the batch redox was developed by The Calumite Company, USA. By assigning factors to each batch material which may affect the redox state, a total redox number can be calculated for each batch. Reducing materials include Carbon, Anthracite, Iron Pyrites, Iron Chromite, sulphide sulphur from beneficiated slag - with factors ranging from minus 0.10 to minus 9.00. Oxidising material include Sodium Sulphate, Cerium Oxide, Iron Oxide, Sodium and Potassium Nitrate – with factors ranging from +0.19 to +1.20.
Note: a particular composition’s total batch redox number is very useful when altering the batch or glass in that particular furnace. To duplicate the glass in a different furnace the composition gives a good starting point but may need to be altered due to the different furnace conditions.
The total batch redox number is indicative of the resultant oxidation / reduction state of the glass being produced. This is particularly valuable when colour changes are made in the furnace.
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-20 preferably –25 to –30
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The oxidised higher positive redox glasses will retain more SO3 and the reduced more negative redox glasses will retain less SO3 – these are more refined. It should be noted that the rate of refining increases with increased loss of SO3.
For customers of Glassworks Services Limited we regularly calculate out the redox batch number of compositions and advise on subtle changes that can be made to improve stability of colour, improved refining, reduced seed and blister etc.
(Also see Glass Redox)
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Flint Glass – Improving refining, melting and colour
A balance between colour, refining and melting rate has to be achieved, dependent on customer acceptability.
Flint Glass - Refining
Refining can be influenced by a number of factors such as viscosity, the melting time, the glass composition and the rate of loss of SO2. Obviously viscosity can be modified by changes in composition and / or melting temperature. The length of melting time will ultimately modify the final seed count but production levels may not allow that length of time.
It has been shown that refining time is directly related to the quantity of SO3 lost and this in turn can be dependent on the level of sulphate present and the glass composition. These can be shown to relate to the oxygen potential of the glass. In practice it appears that there are two regions of batch redox (see Batch Redox Concept) which result in improved refining. It has been proven in practice, and in theoretical calculations, that the two regions of higher SO2 evolution are generally in the +1 to +5 region and also +15 to +18. Above +18/+20 the refining will deteriorate. These figures will vary from furnace to furnace. (It should be noted, however, that at both these levels there is a progressively better refining achieved with increasing levels of glass grade beneficiated slag in the batch).
Flint Glass - Melting Rate
It has been shown that an excess of sulphate can lead to a film of sulphate surrounding the silica grains of a batch and restrict the action between the various carbonates in a batch and the sand. This is particularly evident in oxidising atmospheres. The breakdown of a sulphate occurs much earlier in the presence of a sulphide and this explains the benefit of the use of beneficiated slag at low redox levels and even at the higher redox levels.
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Flint Glass – Colour / Decolourising (Color)
Batch Redox and colour - As the batch redox is made more positive so the amount of ferrous iron in the glass decreases, also the glass brightness increases and the purity of the colour decreases.
The effect of the total iron in a container glass and its ferrous content can vary considerably. Some glassmakers prefer the less intense yellow-green (ferric iron) glass to the blue-green (ferrous iron) glass. For this reason some companies will produce their flint glass from higher redox batches.
For factories using selenium (adding a pink to physically decolourise, or mask, the iron from lower iron sands) it will be better for them to work at a lower redox where the selenium will not oxidise to its colourless state.
Decolourisation of colour (decolorising) - It is extremely difficult to attenuate the green tint created in glass by chromium oxide, usually originating from sand and other minerals and so we will concentrate on the colour produced by iron.
The physical method of decolourising has been to introduce a complimentary colour of pink from selenium and blue from cobalt oxide to counter the various shades of green produced by iron. Obviously, as more masking decolourisers are added to the glass the more it will become grey and lose its brightness.
Iron in the ferric state is a yellow-green and is less intense the blue-green ferrous iron. In the past the tendency has been to oxidise the glass to the ferric state so that the less intense colour could be more easily masked by selenium. However, selenium if highly oxidised becomes colourless.
Decolourising reduced flints with lower levels of beneficiated slag has not presented greater problems. In a reduced glass there is an increased retention of selenium in the glass and also it has remained in the pink state. Often the presence of the slag enables a lower temperature to be used enabling the selenium to be more effective. Therefore, despite a relatively more intense iron colour, no more selenium is usually required.
There has been a trend towards using cerium oxide in cosmetic ware and table ware glass as it changes the valent state of the iron to a colourless form. In most of these situations 1 to 2kg of cerium oxide for every 2000kg of sand is usually sufficient. (At the 2kg level it also reduces UV transmission to less than 20%) The use of cerium with low levels of selenium is not advantageous as it can oxidise the selenium to its colourless state.
When attempts are made to decolourise higher levels of iron with high levels of selenium the melting conditions need to be carefully controlled or grey-green selenides can be formed.
Reducing the level of iron in the melt through careful magnetic separation and washing of the cullet, assessing alternative sands, calcium sources and alumina sources (i.e. calcined alumina) may be effective in producing a premium priced product.
Glassworks Services Limited will give customers help in refining, melting, colour, and colour changes based on the information provided by the customer regarding available materials local to them and their original batch composition.
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Amber Glass and Redox
The amber chromophore has been shown to be a tetrahedrally co-ordinated ferric ion, with one sulphide and three oxygen ligands. Therefore there are limits of redox state, which must be observed, for the amber glass formation:-
- if the melt is too oxidising there will be an excess of ferric iron and insufficient sulphide;
- if the melt is too reducing there will be insufficient ferric iron and excess sulphide.
The amber formation is affected by:-
- the atmosphere above and within the melt;
- glass composition; the ferrous/ferric ratio;
- the sulphide/sulphate ratio;
- the solubility of sulphate.
The total iron in the glass and the total sulphur in the batch are also important.
It has been found in practice that most container glass compositions that the above requirements are met if the total iron in the glass is 0.25% Fe2O3 and sulphide sulphur is added to provide a theoretical sulphide in the glass of 0.065% to 0.075%. Furthermore, the correct oxidation conditions to obtain the correct ferrous/ferric ratio, etc. to provide the normal amber colour required for containers is developed with an approximate –25 / -30 batch redox.
Methods of calculating a batch to suit the above condition are available, in particular to ensure no excess sulphide remains in the glass which would produce a susceptibility to reboil and blister formation and also to allow the use of an iron silicate to give the correct iron content without affecting the redox state.
(Also see Amber Glass)
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Green Glass
Emerald Green – Iron chromite is the essential ingredient for making standard emerald green glass. About 6 to 10 kgs of chromite is used per tonne of sand. Ideally the base glass will be a reduced flint glass with a redox of about +5; the added chromite would take the redox to about minus 7. A small amount of anthracite or carbon can be used as a reducing agent to trim and control the colour. To satisfy the customer various colour shades can be produced by adding small amounts of copper oxide, cobalt oxide or nickel oxide.
Iron chromite is often referred to as chrome flour due to the need for it to be 400 mesh or finer (38 micron or lower). If the chromite is coarser it will not melt in the molten glass resulting in black specks (referred to as black stones) in the finished product. If the chrome flour is damp it agglomerates resulting in similar black specs; by mixing the chrome flour with an equal quantity of soda ash (sodium carbonate), which absorbs any moisture, this problem can be avoided.
UVA Emerald Green – This is a highly oxidised glass with the required chrome content normally provided by sodium or potassium dichromate. However, the dichromate is a hazardous material to handle and it would be worthy of consideration to use chrome flour instead. The glass would then have to be a very highly oxidised state to accommodate the reducing nature of the chrome flour. In some cases the iron content of the chrome flour may be a limiting factor in its use in this type of glass. The chrome content of UVA emerald green is about half that of a standard emerald green. Sometimes a little cobalt oxide or copper oxide is use to adjust the green shade.
Georgia Green – This is a pale green required by some drinks manufacturers to give added distinction to their product. A level of chrome flour of one-tenth that used for a standard emerald green is typical.
Dead Leaf Green – This colour can be produced using a combination of chome flour (about 1 kgs per tonne of sand) and iron pyrites (about 2 kgs per tonne of sand). These quantities and the redox state of the glass can be modified to obtain the required depth and shade of colour. The redox state has to be maintained at a constant level and using anthracite or carbon as the reducing agent achieves this. Similar colours have been achieved using low levels of chromium and manganese oxides. The level of each ingredient will depend on the depth of colour and tint required by the ultimate user.
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Amber Glass
Amber glass produced using carbon and sulphur is a most unstable glass susceptible to seed and blister (reboil).
Using iron pyrites is one of the more reliable ways of producing an amber glass. Adding the appropriate amounts of iron and sulphide sulphur in combination at the correct redox forms the amber colour. The amount of iron pyrites used will be approximately 2 ½ kgs per tonne of sand. Normal beer bottle amber will contain 0.25 to 0.30% of Fe2O3 and if sufficient iron is not available from the sand and pyrites more can be added using iron silicate or using red iron oxide. Using additional iron pyrites to increase the iron is not recommended, as this will add too much sulphide and an unstable glass will result. A golden amber will contain about 0.15% Fe2O3 and dark ambers as much as 0.40%. In some circumstances the dark ambers can be made even darker by using colours at various levels of light transmission i.e. by adding small amounts of cobalt oxide and / or copper oxide.
Where it is economically available beneficiated slag is used. It is composed mainly of calcium, alumina and silica but also contains sulphide sulphur. This is a dilute source of sulphur which can be used as an alternative to iron pyrites in all green and amber glasses – whilst also bringing melting advantages. In emerald green it can contribute about 10% of the batch, compared to the sand weight, and in amber as much as 15%. Using these quantities it can replace the alumina source. Even in flint glasses it contributes about 5%, compared to the sand weight, and in all cases improves both the refining, melting and, consequently, the fuel economy of a furnace.
(Also see Amber Glass and redox)
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Colour changes without foaming
Changing from an oxidised glass, such as flint or green, to a reduced glass, such as amber, has historically required the furnace to be drained. Changing from flint to green did not cause any major problems but when changing from glasses of substantially different oxidation states severe foaming was often encountered.
No-one wants a furnace full of foaming molten glass trying to force its way out.
The batch redox number concept allows the state of oxidation to be predicted from the batch composition, thus enabling controlled and predetermined changes to be made to the redox state of the glass. Using this approach, when changing from amber to flint, the amber is allowed to become less reduced until it is at its limit of colour stability and quality acceptance (enabling packing of glass until the last possible moment) the glass is then held at this state for a time and then moved through the critical sulphur solubility zone in a series of small changes until it becomes oxidised. Once through this zone it can be changed more quickly to the desired oxidation state of a good flint glass.
The redox number concept enables calculated changes to be made to the batch. At zero redox the sulphur is at its lowest solubility and therefore susceptible to foaming but by controlling the passage through this zone there is less chance of any foam being produced. A similar procedure is used to move from an oxidised glass to a reduced amber. It can enable longer packing before the final change and it usually ensures a quicker and safer changeover time.
(Also see Batch Redox Concept)
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Refining – all types of glasses
Soda-Lime - In the past soda-lime glass makers used arsenic trioxide with sodium nitrate as their main refining agents. Later carbon with a “sulphate” was used in addition to, or as an alternative to, the arsenic/nitrate method. The sulphate can be sodium, calcium (anhydrite or andricite) or barium. Currently sodium sulphate (also know as saltcake) is the more economic source
In both cases there was a stirring action in the melt and an improvement in the seed count due to the large bubbles produced collecting, and absorbing, the smaller ones as they moved to the surface. At about 1040oC it collects at the interfaces of the melt between the particles of un-dissolved batch, the gas bubbles and on the surface. It therefore acts as a surfactant and increases the melt fluidity. At about 1288oC the thermal decomposition of the sulphate in the glass becomes significant – ultimately producing a stirring action at the interfaces which accelerates the dissolution of unmelted particles and allows bubbles to rise more rapidly through the melt. When sulphate is decomposed by a sulphide this begins at about 900oC and the reactions mentioned above occur earlier. The most economic source of the sulphide is from beneficiated slag such as the Calumite product. In addition the sulphate-sulphide reaction causes most of the sulphur in the batch to be ejected as SO2 thus reducing the possibility of foaming or reboil later in the melting process.
Lead - In refining lead glass melts, in particular with pot melting, the glassmaker traditionally used arsenic trioxide and sodium and/or potassium nitrate for 30% PbO compositions and antimony trioxide and nitrate for lower lead levels. In recent years for environmental and legislative reasons, most glassmakers have changed to the use of antimony trioxide. In practice 15% to 30% of the arsenic introduced into the batch can evaporate from the melting batch in open pots while for antimony the figure is closer to 10%; however, the antimony has a somewhat weaker oxidising effect on Fe than the arsenic.
In the early stages of melting the trioxide changes to the pentoxide taking in oxygen from the nitrate. Small bubbles produced during the melting are difficult to remove however at the later stages of founding when temperatures are being lowered as the pentoxide releases oxygen and reverts to the trioxide. The large bubbles produced collect up the small bubbles present in the melt and take them to the melt surface. The final part of the refining process consists of reducing the temperature of the furnace to about 100oC below the normal working temperature for about an hour before raising again in order to begin work. This final action helps to take any remaining seed into solution.
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Devitrification
Devitrification of a glass means that some of the molten glass has crystallized. Corresponding to every glass composition there is a limiting temperature below which, given sufficient time, crystalline material will appear in the glass. This temperature will be constant for any given glass composition. The above limiting temperature is normally referred to as the liquidus temperature. The normal working temperature ranges of commercial tank furnace glasses are comfortably above their liquidus temperatures and devitrification problems are rare.
A glass should be designed so that the production process is complete sufficiently quickly so that the glass passes through the liquidus temperature without allowing the glass time to form crystals. Where it is not possible it is advisable to make sure that the liquidus temperature is quite low through attention to the glass composition. In the production of float glass and tubing, where cooling is relatively slow, some of the CaO is replaced by MgO, or additional MgO is added, thus lowering the liquidus temperature substantially.
The rate of devitrification of glasses tends to increase as the temperature falls below the liquidus temperature however devitrification is opposed by the increasing viscosity of the glass. In a glassmaking furnace the bottom glass will generally be colder than that on the surface. If there are corners with poor glass flow, where the glass is likely to be colder, there is a possibility of “devit” being formed. This can also occur when the tank is nearing the end of its campaign and the glass contact refractories are wearing thin resulting in the adjacent glass becoming cooler than the main glass flow.
Running pots and furnaces at lower temperatures to save on fuel can lead to “devit”. The bigger the difference between the liquidus temperature and the main tank temperature the lower the possibility of devitrification. Increasing the tank temperature or introducing bubblers or boost into colder areas of the tank may be uneconomic and increase refractory wear . A change to the glass composition to lower the liquidus temperature within the constraints of the required working properties and glass/batch cost is the usually the best way.
If increasing calcium to create a quickly hardening glass the likelihood of “devit” increases – magnesia can be substituted as discussed.
It is possible that inadequate mixing or incorrect sizing of batch materials ( or segregation due to mixed batch standing far too long) will lead to localised differences in a glass composition with higher than expected liquidus temperature and a possibility of small outbreaks of devitrification. If this occurs in a glass the materials and the standard of mixing should be investigated.
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Coloured Glasses
Colour is the light that transmits through or reflects off an object. The colour in glass comes from one, or a combination of factors or materials. Colour is rarely a constant produced by a particular oxide in isolation. The arrangement of electrons surrounding the atoms of a particular colouing material are affected by the light energy and by the magnetic energy of adjacent atoms. The remainder of the batch can have significant effect i.e. NiO in a Soda-Lime batch (or lead or barium) will create brown/grey colours – in a Potash Batch (potassium Carbonate) the nickel will create a violet glass.
The oxidising or reducing atmosphere (furnace conditions) can have a significant effect as some of the colours can exist in the glass in more than one valent state. Iron oxide can reduce to ferrous oxide and produce blue-green or it can oxidise to ferric oxide to produce yellow-greens and browns of bottle glass. Copper oxide is blue-green in oxidised alkaline glass but can be reduced to reds or even to produce a thin metallic layer on the glass surface. Glasses coloured by copper, gold, selenium and silver often need to be reheated to enable the colour to strike. The correct temperature and time required for them to strike varies considerably and will only be right for that furnace, or glory hole, at exactly those conditions, at that humidity, etc. It should be repeatable but over confidence beware. Selenium is a major problem when instead of red-orange a black, brown, green or yellow may appear due to its different valent states depending if oxidised or reduced.
Sometimes several “colouring oxides” are added to create an overall colour i.e. “black” is made up of oxides each affecting light transmission at different wavelengths:- blue, purple, grey, green, red etc. Sometimes two different materials are added which combine together i.e. Cadmium Sulphide and Selenium create Cadmium Selenide which creates a ruby red.
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Fe2O3 + CoO, CuO + Cr2O3, FeO
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NiO + above in Soda-Lime glasses
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Yellow / green fluorescence
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Se + CdS not in PbO glasses
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Copper Ruby or Antique Ruby
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MnO2 in Soda Lime Glasses
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NiO in PbO + in Potash Glasses
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Glossary
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These glasses have high deformation temperatures and are suitable for the envelopes of high pressure discharge lamps. Typical composition 55.3 SiO2, 7.2 B2O3, 0.6 Na2O, 0.4 K2O, 22.9 Al2O3, 8.5 MgO, 4.7 CaO
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The removal of undesirable stress by suitable heat treatment. The glass is held at a constant temperature in the annealing range until the stresses have almost completely disappeared, then cooled very slowly until considerably below the lowest possible annealing temperature and then finally cooled rather more rapidly to room temperature. Annealing allows finishing operations such as cutting, grinding and polishing to be carried out safely. The temperature, and speed of cooling depends upon the type of glass, thickness, thermal properties of the glass i.e. dark coloured glass has different heat transmission properties to clear glass.
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The temperature at which a fibre of glass of length 230mm and diameter 0.65mm extends at a rate of 0.14mm/min under a load of 1kg. It is the temperature at which internal stress is relieved.
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The mixture of raw materials used to make glasss.
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A machine that charges batch into a tank furnace, at a rate adjusted to balance the rate at which glass is removed from the other end of the furnace and keep the glass level constant. It is usually linked to the glass level controller.
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The buildings in which the raw materials for making glass are stored, weighed out, and mixed together before being melted are collectively known as the batch house.
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The time needed to complete the melting reactions in a glass melt is known as batch-free time. It includes the times required 1) to heat the batch until it begins to react; 2) to complete the vigorous initial melting reactions; and 3) to dissolve the residual sand grains.
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Due to their resistance properties it is used in sodium vapour lamp glass. Also in optical applications due to their high indices with modest dispersion
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Ovenware and sealing glasses in the electrical valve and lamp industries. Typical composition of ovenware 80.6 SiO2, 11.9 B2O3, 4.5 Na2O, 0.5 MgO, 0.5 CaO, 2.0 Al2O3
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A transverse wall built across a tank furnace to divide the tank into two separate chambers, the melting end and working end.
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Bottles, jars, or any container which may be subsequently capped or closed. Clear glass is known as “Flint” and typical composition 73 SiO2, 2.0 Al2O3, 13.0 Na2O+K2O, 11.0 MgO+CaO, 0.5 BaO
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Refers to regenerative furnaces in which the flames pass across the width of the furnace. The width must be such as to allow proper development of the flames. The number of ports along each side of a cross-fired furnace is relatively small.
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The roof of a glass tank furnace is referred to as the crown. It is usually a free-standing arch (part of a cylinder or sphere) built of high quality silica bricks. Since it is supported only along the edges (or around the circumstances) it must be heated to operating temperatures very carefully.
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Optical glass of relatively low refractive index and high Abbe number v.
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Previously melted glass, often waste from the manufacturing process, added to the furnace with the batch. Until recently cullet formed perhaps 20-30% of total batch but this proportion can now be very significantly higher. Problems can arise if the cullet is not close to the composition of the glass being melted.
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A mechanized method of making rod and tubing. A steady stream of glass falls on to the upper end of a rotating refractory mandrel inclined at about 30 degrees to the horizontal. As it spreads towards the tip it forms a layer of glass which is drawn off and turned horizontally by a drawing machine. To make tubing air is blown down the hollow central core of the mandrel.
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A small tank furnace which is used like a pot furnace: batch is charged in the late afternoon, melted overnight, and worked the next morning / day. Most often used when the glass is corrosive or the melting temperature is so high that pots would have a very short life.
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The growth of crystals which can occur when the melt is held too long at a temperature just below the liquidus temperature. This can happen unintentionally in glass manufacture – see tips.
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A furnace in which the flames travel lengthwise through ports in the end wall. Not as common as cross-fired types.
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A mechanism fitted to the end (nose) of a forehearth for measuring out gobs of glass of accurately controlled weight and delivering them to the forming machines at the proper rate.
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Optical glass of relatively high refractive index and low v value.
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The modern method of making high quality flat glass which has now largely replaced both sheet and plate glass in many applications. A horizontal stream of glass “floats” on to a bath of molten tin in such a way that both surfaces become flat under the influence of surface tension and gravity. The atmosphere above the tin must be kept strongly reducing to prevent oxidation of the tin which would then react with the glass. When almost rigid the fire-finished ribbon is lifted off and passes into the lehr. The thickness of the ribbon can be changed from it’s natural value of approx. 6mm by special techniques. There is no need for expensive grinding and polishing stages in the process.
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A channel along which molten glass flows from the furnace to the forming machines. There is now full automotive control of the temperature distribution, both transverse and longitudinal, so that the glass is brought to the correct temperature for feeding to the machine. There may be cooling vent and burners in various postions to control the temperature. Forehearths vary in size but typically might be 6m in length and 0.5m in width. The depth of the glass would be perhaps 0.2m.
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A glass, or a substance in the glassy or vitreous state, is a material formed by cooling from the normal liquid state, which has shown no discontinuous change (such as crystallization or separation into more than one phase) at any temperature, but has become more or less rigid through a progressive increase in its viscosity. As a material, glass behaves like a typical solid in certain respects such as mechanical properties but has a non-crystalline structure
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Unlike earlier machines for mass production of container glass these machines have independent sections which are placed side by side in sections. The moulds on each section can be adjusted independently of the other sections and the parison and blow moulds remain stationary except for the lateral movements involved in opening and closing. Typically each section will have 1, 2, 3 or 4 moulds working simultaneously and the machine would have 8, 10, 12, 16 or more sections. Typical is a 10 section double gob machine (2 moulds in each section) but for small ware such as baby food jars or stubby beer bottles 12 or 16 section triples or quads are not uncommon.
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Flint Optical Glasses contain significant proportions of lead oxide but the term usually refers to “Lead Crystal” glasses of 24% approx. or “Full Lead Crystal” of 30/32% approx. content.
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A furnace in which a suitable temperature profile has been established in order to provide adequate annealing for the ware in question. The ware moves on a continuous belt and experiences a period of stress release, slow cooling to minimize the re-introduction of stress, and a faster rate of cooling to allow the ware to be handled at the exit to the lehr.
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The highest temperature at which a liquid can exist in equilibrium with crystals of its primary crystalline phase is its liquidus temperature. Above this all crystal dissolve, below crystallization may not occur and the glassy “solid” can be created without crystals.
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The main chamber of a tank furnace into which the batch is charged and melted. Much of the refining and homogenising also occurs in this area.
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The circular opening in the bottom of the forehearth through which the feeder forms gobs and delivers them to the forming machine. The orifice can be changed so that the gob size is suitable for the ware being made.
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The shaped and partly blown object formed as the first stage in making a container (or other article) by a two stage process. The paraison is also called the blank.
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An iron mould having a lining of solid porous material. It is dipped into water before use so that a cushion of steam forms between mould and glass. The porous layer may be carbon made by carefully heating a coating of cork granules mixed with a suitable adhesive already applied to the mould.
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Cast between water cooled rollers which smooth the glass surface as the glass passes through. Both faces are then ground and polished subsequent to annealing.
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A container used for melting glass in amounts from only a few grams to several hundred kilograms. The largest pots are made from special clays which are carefully dried and fired before use. Small pots may be either refractory or metal and will be chosen according to the nature of the melt.
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A furnace designed and built to hold one or more pots for melting glass. Many pot furnaces are roughly circular in plan and hold several pots of glass of the same type.
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A structure for transferring heat from furnace waste
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The internal stress present in glass after annealing. Complete removal of residual stress would take extremely long times and annealing schedules are therefore designed to reduce the stresses below the acceptable maximum value. The permissible residual stress is much lower for optical glass than for containers – their annealing control is a major part of their quality process.
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A chamber for transferring heat from furnace waste gases to incoming air and which has one set of passages constructed from bricks. The latter are heated by waste gasses and then used to heat the air as it passes in the opposite direction around the same bricks. There must always be two regenerators so that one is heated while the other is being cooled, and the direction of firing of the furnace must be reversed at regular intervals.
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A machine with two continuous tracks, one above the other; glass is fed between the tracks, the upper one carries blow moulds. The gobs of glass hang down from the continuous ribbon are blown into thin-walled articles such as electric lamp bulbs. The speed of operation will depend on the size of the article.
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An open lattice structure built with refractory bricks on top of the bridge wall of a tank furnace. By reducing the radiation received from the flames in the melting end the shadow wall keeps the temperature in the working end considerably lower than it would otherwise be.
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A furnace in which the bottom part (the tank) is filled with molten glass. The tank is constructed from very resistant and close fitting refractory blocks. Capacity vary from a few hundred tonnes to several thousand tonnes, and the hottest parts operate at temperatures up to 1600oC
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The single passage at the bottom of the bridge wall through which the melt flows from the melting end to the working end in a tank furnace
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A type of narrow tank furnace with burners along both sides and no heat recovery equipment or, sometimes, a recuperator.
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A continuous method of making rod or tubing in which the stream of glass flows vertically downwards out of an orifice in the bottom of a forehearth and is then drawn off horizontally.
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The smaller chamber of a tank furnace in which the melt cools down from the melting temperature (say 1500oC) to near working temperature (perhaps 1200oC). In a container glass tank the working end is separated from the melting end by the bridge wall and the shadow wall. In a flat glass tank the two ends of the furnace are not divided by a bridge wall. Some refining and homogenising occurs in the working end.
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Bubbles which are present in the finished glassware. They are generally quite large.
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Threadlike tails of glass produced by the complex chemical reactions involved in melting. They are obvious because they have a different refractive index to the surrounding glass. Poorly mixed batch, cullet can be the source of the problem.
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A high concentration of gas bubbles near the surface of a glass melt. Such a layer is a good insulating material and can lead to serious problems in the melting process.
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Similar to cord but occurs in layers rather than threads. More common in flat glass where the layers are be drawn out.
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The formation of gas bubbles at refining temperatures which can lead to blister.
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Material floating on the surface of a glass melt. Usually solid and often siliceous in nature
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Relatively small bubbles either in the body of the melt or present in the finished article are known as seed.
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Any crystalline inclusion in the glass such as incompletely dissolved batch, devitrified material, or refractory inclusion. The presence of stones is a serious cause of defects since they can often initiate fracture.
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A form of very fine cord which would be troublesome in optical glass.
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Essentially any optical media in which the cross-section is very small relative to the length of the medium. If the cross-section is also small relative to the wavelength of the radiation being carried then the fibre can act as a waveguide. In nearly all cases, whether the fibre acts as a waveguide or simply transmits the beam, the fibre is immersed in another medium of slightly lower refractive index. Fibres can be used in bundle form either to transmit images or give high illumination, or as single fibres to carry information as in optical communication systems.
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Materials in which there is no resistance to a permanent change in shape and in which deformation continues as long as the force is applied are said to be viscous. The level of resistance is its viscosity. Water is viscous : treacle is far less viscous.
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Thermal Expansion Coefficient
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All materials either expand or contract upon heating at constant pressure. This coefficient is not constant with temperature and at absolute zero of temperature it vanishes. The coefficient is usually quoted along with the temperature range that it relates to. There is a marked dependence on the constitution of the material. For isotropic materials, like glass, the volume coefficient is three times the linear coefficient. Thermal expansion is extremely important in glass science since it is involved in annealing, thermal toughening, sealing etc. It may be noted that very large mechanical stresses can be generated by modest temperature changes.
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Once formed, thermally toughened glass cannot be cut or processed because it would break into small, relatively harmless fragments. Heated to a temperature near the top of the annealing range and cooled extremely quickly.
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