Formers

This post is not about the materials that go into the making of glass, but about ways of forming glass once melted or dripped into a space.

Formers are a bit different from moulds.  They are more like the formers used in concrete structures – they are there to resist the movement of the contained materials and give the form or shape desired rather than a natural flow.

These formers can be of anything that can resist the firing temperatures of the process.  Some of the materials are stainless steel, ceramics, fibre board and paper, vermiculite, kiln brick, and I am sure there are others.

Refractory Fibre

Most of these require a separator between themselves and the glass.  The ones which do not are untreated refractory fibre board and fibre paper. 

Most paper is not sufficiently strong to stand on its own. Instead it is used flat and the shape cut out of it.  It can be made in several layers and pinned together to achieve the height desired.  It should be lined in the interior with a thin fibre paper to avoid seeing the layers of the former in the edge of the glass.

For thicker work, fibre board can be used with the shape or form cut from it. Alternatively, it can be used on its side backed up by kiln brick or other material to resist movement. More information on methods and safety are here. 

If hardened, refractory board and paper will need separators between glass and former, just as most other materials will.

Sometimes the fibre board and fibre paper are not heavy enough to resist the flow of the glass.  You can use weights to help resist the movement.  At other times, the glass flows under the fibre and then you need something heavier.  Fortunately, there are a number of refractory materials that can be used.

Other common formers

Vermiculite board is another refractory material that can be cut and shaped much like fibre board.  The vermiculite needs to be covered with kiln wash where it might come into contact with glass or be lined with fibre paper or another separator.

Calcium silicate board can be used in much the same way.  It also needs a separator but does not stand up to such high temperatures as vermiculite.

Ceramics, especially in the form of cut up kiln shelves can be used as straight formers.  They have the advantage, over refractory fibre paper and boards, vermiculite and calcium silicate, of being heavy.  They can resist the movement of thick glass. They need to have a separator and usually a 3mm fibre paper, cut 3mm shorter than the final thickness of the piece, will provide the cushion in the movement that the glass needs.

Kiln brick is an often forgotten former.  The bricks can be cut and formed in many ways, even if not so freely as fibre board and paper.  The bricks do need fibre paper separators to keep the glass from getting into the pores of the brick.

Stainless steel is a common former too.  These are usually formed into an already determined shape and so are not so adaptable as many of the other formers.  Steel contracts much more than glass and needs a cushion of fibre paper, usually 3mm thick to avoid sticking to the glass.

More information on most of these formers can be found here.

Seedy base glass

Sometimes your clear base has bubbles, or as the trade calls it, seeds.  When capped with opalescent glass, in certain circumstances, these tiny bubbles can become larger and rise to the surface, pushing the opalescent aside as it rises.  This leaves a clear spot in the midst of the opalescence.

Clear cap

One way of reducing this problem, is to avoid it altogether.  This can be done by placing the clear on top of the opalescent as a cap.  This way the bubbles, if any, are rising through the clear.

Flip and Fire

If you can’t, for one reason or another, cap the piece with clear, you can fire upside down. Again, the bubbles are rising through the clear.  When the firing is complete, you can flip it over to the right side.  You will need to clean thoroughly and take to a fire polish temperature to get the shiny surface back.

Heat Work

Another way is to fire to a lower top temperature with a longer soak.  This means the glass can take up the profile you want without becoming so soft that the bubbles can rise through the glass.  You will need to observe to determine when the glass has the right profile, and then advance to the cooling and anneal phases.

Low and Slow

This last way of reducing the possibilities of bubbles rising through toward the top is based on the characteristics of glass.  As glass becomes hotter, it becomes less viscous and so allows the air to rise toward the top of the glass surface.  Using a low temperature gives a more viscous glass to resist the bubble movement.  The long soak at the chosen lower temperature allows the surface of the glass to take up the profile you want, as the surface is hotter than the bottom of the glass, therefore reducing the possibilities of bubbles rising.  It does take a longer soak at the top temperature, but it also reduces the marking on the bottom of the piece.

This low temperature process is using the principles of heat work.  The effect on the glass is a combination of temperature and time.  The higher the temperature, the less time is required.  The longer the time, the less heat is required.  The heat work put into the glass to achieve the effect you desire is determined by the combination of temperature and time used in firing the glass.  This principle of heat work is why you can achieve the same effect at very different temperatures, depending on the length of time a piece is soaked.

Batt Wash Sticking to the Glass

The main reasons that kiln wash sticks to glass are:

1. Firing at too high a temperature. The higher the temperature, the more likely the kiln wash will stick to the glass.

2. Firing with opalescent glass against the shelf. Kiln wash sticks to opalescent glasses more easily than to transparent glass.

3. Re-using kiln washed shelves that have been to fusing temperatures already.

4. Using kiln wash with high amounts of china clay makes for more sticking. Thus some brands stick more frequently than others.

Firing at too high a temperature is probably the worse culprit. The second is using opalescent directly on the kiln shelf.


Strategies to avoid this sticking are:

1. Fire at the lowest temperature you can to get the result you want. This often requires slow rates of advance and extended soaks at the working temperature

2. Use Bullseye kiln wash. It is among the best.

3. Have a transparent glass as the bottom layer.

4. Use iridised glass, with the iridised side down to the shelf, as the iridisation acts as a separator. Do not do this with Thinfire, as it can lead to large cavities in the glass.  Fire onto kiln wash.

There are ways to get the kiln wash off but it's easier to avoid it. Using an iridised sheet on the bottom is probably the most effective prevention.  

Multiple firings of kiln wash

Removing Kiln Wash from Shelves

There are at least three ways to remove kiln/batt wash from mullite kiln shelves.

One quick way is to use a broad wallpaper scraper held at a very acute angle to the shelf. This rapidly removes the separator. One down side to this method is that any uneven pressure can put a gouge into the surface of the shelf.

So a more gentle way to remove the wash is to use a drywall/plaster board sanding sheet or other open weave sanding material. This allows the powdered wash to come through the sanding material rather than clog up the material. The disadvantage to this is that it takes longer to remove the wash, although it does leave a very smooth shelf after many sandings.

A third way is to wash off the kiln wash. This is relatively quick, but it gets the shelf wet and requires a longer period before the shelf becomes dry. You can, of course put the next application of kiln wash on as soon as the shelf is clean. They both can dry off at the same time.


Power tools used to clean kiln wash from the shelves can induce low points in the shelf which will promote bubbles during fusing.  It is recommended to avoid power tools in removing kiln wash.

Uprisings at the Bottom of a Slumped Bowl

“I just finished slumping a dish and I got a big lump in the center of the bottom. This is not an air bubble, just a lump. What should I do to avoid this again?”

Several suggestions are possible.

Ensure there are holes at the bottom of the mould that allow air to get out into the kiln. Prop the mould up on stilts if the hole does not go directly out of the mould. Alternatively, drill a hole in the side to allow the air to escape from under the mould.

Firing for too long or at too high a temperature will cause the glass to continue sliding down. Having nowhere else to go, the weight causes the bottom to begin rising. This is a consistent experience across several kilns and with multiple users.

So keep the temperature down to the minimum required. To find that out, watch the slumping in stages (do not stare!). Look at the piece for a second or two every five minutes after you reach your desired temp.

If it already has slumped adequately, you are firing too high. Reduce your temperature in subsequent firings and watch to find what the required temp and time is. There is absolutely no substitute in slumping but to watch and learn what your mould and glass require.

If you are slumping at such a temperature to seal the glass to the mould, you are firing too hot anyway. Or put more positively, use a low temperature slump, that is, a slump at the lowest temperature to achieve the desired result over an extended period of your choice.

A low temperature slump will allow the glass to conform to the shape of the mould without softening so much that it takes up all the markings of the mould. Therefore, there are spaces for the air to escape from under the glass all the way to the top as well as through the air holes at the bottom. It also gives the most mark-free slump possible for your shape.

Changing Grits on a Belt Sander

When to Go to a Finer Grit on Belt Sander?

If you change the direction the belt moves along the glass with each step it will be easier to see if you have removed each of the previous step's scratches. If you see marks at another angle than your present direction, you need to continue the current step a little while longer.


A second method that I use more often is to use a paint marker to show up the areas not ground. You need to dry the piece before applying the paint marker and let the paint dry before continuing with the next grit. The paint should be applied thinly to avoid transferring the paint to the belt, which reduces the efficiency of the grits. Any low spots or scratches will show up as white areas or lines on the surface.

The process of drying and painting also helps to provide a break in the sometimes tedious process of grinding and polishing.


These two methods can also be used for cold working by hand.

Striking glass

Yes, much glass is striking in its effect.  But the term is used in a technical sense to indicate the glass has not reached its intended colour without further firing.

A striking glass is one that changes to its true colour. Not one which takes up a different colour.  There seem to be differing ideas on how striking works, but it is an intentional process.

Several glasses coloured with copper or silver strike to Their final colour when heated.  It seems that copper when used to make red (rather than blue or green) can undergo a chemical change during the heating.  The copper oxide used is normally Cu₂O.  When heated the copper and oxygen molecules can separate and form bonds with other molecules.  The rapid cooling that is used in glass prevents the copper and oxygen from combining in the Cu₂O formation.  The extent of this dissociation determines the degree of colour change.  Thus, the colour is affected by the heat work given to the glass – assuming the starting proportions of materials are the same.  This can occur with some other colouring metals too.

Another form of striking is caused by the growth of crystals within the glass. In these cases, usually in silver bearing glass, the metals separate from the silica and form small crystalline structures which are also fixed by the rapid cooling required for glass.

There is another theory that the colour change is due to the orientation of the colouring molecules within the glass matrix.  The idea is that the molecules will change from the clearer state to the struck colour due to the orientation caused by reheating and cooling.

The actual process seems to be unknown in a definitive sense.  What is known is that temperature, a reducing or oxidising atmosphere, and heat work will vary the intensity of the strike in colour.  This means that where the project is especially sensitive, you must undertake experiments to help predict the colour that will be achieved with the conditions you choose to use.

Break Diagnosis in Slumping

The usual advice in looking at the reasons for breaks in your pieces must be considered in relation to the process being used.  Breaks during slumping need to be considered differently to those occurring during fusing.  

The advice normally is that if the edges are sharp, the break occurred on the way down in temperature. Therefore, the glass must have an annealing fracture or a compatibility break.  It continues on to say if the edges are rounded it occurred on the heat up, as it broke while brittle and then rounded with the additional heat.

This is true, but only on rounded tack and fused pieces.

When the process is a slump, there is not enough heat to round the edges.  So, the edges will be sharp whether the break was on the heat up or the cool down.

How can you tell in a slump process when the break occurred?

If you can put the pieces of the slump back together and they fit perfectly, the break was on the cool down, as the piece was already fully formed.

If the pieces do not fit together perfectly, the break was on the heat up.  This is because the break occurred, and then the two (or more) pieces slumped independently, thus leaving slightly different shapes at the break line.

There is a special case here, of course.  Sometimes the break is only a split in the bottom, that does not come all the way to the top of the piece. This split (or splits) occur when the heat up is too fast.  The top becomes plastic while the bottom is still brittle/stiff.  The weight of the hotter, more pliable glass overcomes the strength of the cooler and heat stressed bottom, causing it to split.  More information is given here: Diagnosis of Breaks

There is also more extensive information on diagnosis of breaks in this blog entry on slumping cracks.  

Slip cast moulds

Hard spots in some moulds are the result of the method of creating the moulds. Most of the ceramic moulds we use in kilnforming are slip cast.

This diagram shows the main stages of slip casting

Slip casting is a way of quickly producing multiples from a mould.  The original shape is surrounded by a one - or multiple - part plaster mould.  This mould is used to contain the clay slip which is poured in.  

The plaster absorbs water from the slip, stiffening the clay in contact with the plaster. After a defined time, the remaining slip is poured out of the mould.  The clay remains in the mould a short time until it begins to contract from the plaster mould and is described as leather hard.  

It is then de-moulded, trimmed and cleaned before it is further dried.  When appropriately dry, it is fired.

Some moulds we receive show a spot where the kiln wash does not cover the surface in the same way as the rest.  This is a result of the method of pouring the slip into the mould.  Slip that is hand poured does not fall in the same place for long.  But industrially poured slip often falls in the same place for the whole of the pour.  This creates a hard spot - an area where the slip is more compacted than the rest of the object.

This hard spot does not affect the appearance or performance of the object.  However, it does not absorb the water from the kiln wash as well as the other areas. And this is when the hard spot becomes apparent. It will still have enough separator to keep the glass from sticking, although visually it appears bare. If concerned, you can coat that area more than the rest after the kiln wash has dried a little.  You need to be careful that you do not introduce an unevenness into the kiln washed surface, as that might appear on the slumped surface of the glass.

Float Annealing Temperatures

Float glass annealing temperatures vary quite a bit from one manufacturer to another; and even within one manufacturer’s product line.

Comparisons of various float glasses

Some companies are more informative that others.  Pilkington are one of the more open of European glass manufacturers on various bits of information.

Pilkington Float

CoLE 83 *^10-5

Softening point:  715°C

annealing point:  548°C

strain point: 511C

Pilkington Optiwhite ™

Softening point:  ca. 732°C

annealing point:  ca. 559°C

strain point:  ca. 526°C

There is a difference of 11C between two of the Pilkington product lines for the annealing points.  The softening and strain points are slightly wider.

Glaverbel, a Belgian company, restricts their information to CoLE and the softening point.

CoLE 91 * 10^-5

Softening point: 600°C

Saint-Gobain, a French company, shows some more of the variation in the product lines, although they do not give specific annealing points for the different products.

CoLE 90 * 10^-5

annealing range:  520 - 550°C

Low E glass

softening – 840°C

strain - 617°C

R glass (sound reducing)

softening – 986°C

strain - 736°C

D glass (decorative)

softening point – 769°C

Compatibility

Even this small sample of float glasses shows there is a significant difference between manufacturers for the softening, annealing and strain points.  This means that, unless you are sure of the glass merchant’s source of glass, you will need to test each batch of glass for compatibility with previous batches, if you are combining from different suppliers.

I included the CoLE numbers (which all the manufacturers specified as an average change in length for each degree C increase in temperature from 0 to 300°C) to show the variation and to challenge anyone to find Bullseye and Saint-Gobain or Glaverbel compatible with each other.  My experience has shown that the Optul coloured frit and confetti is more likely to be compatible with Pilkington than the other two.

Annealing

I have been beginning my annealing of float glass at 525°C.  This little bit of literature research shows that my annealing soak should be starting higher, possibly at 540°C, certainly no lower than 530°C.  Other areas of the world may find their float glass has significantly different annealing ranges.

Broken base glass

Firing a piece with a partially covered base layer requires more care than two even layers to avoid the fracture of the glass during the heat up stage of a firing.  Slower rates of advance need to be used.

Glass is a poor conductor of heat and electricity. This can be good in certain circumstances but is usually one of the limitations in kilnforming.  The poor conductivity of glass means the top layer of glass will need to be heated before it begins to transmit heat to the glass below. 

A while back an example was shown that is a special case, but also illustrates the general principle (apologies to the poster, as I didn’t take down the name at the time and can’t find the original post now).

This sheet of clear glass was covered by an arrangement of stringers, with a border of clear exposed.  I don’t know positively, but I presume this was done in the knowledge that the single sheet of clear glass would become smaller, and the border would be cut down to the appropriate size.

Be that as it may, the exposure of the clear allowed the edges of the clear to heat up faster than the covered part of the sheet.  The stress of the temperature differential between the centre and the edges led to the fracture of the glass during the heat up.  This can be confirmed by the rounding of the broken edges.  It is further confirmed, by observing the relative straightness of the stringers, that the break occurred before the stringers became sticky enough to even laminate to the base glass - the clear glass broke underneath, leaving the stringers relatively undisturbed. It is also an indication that the glass broke earlier than the slumping temperature, as the stringers would have been sticky enough to break with the clear otherwise.

One speculation given for the break was that it was affected by the size.  You can see the size is relatively large for the kiln.  This may have had some influence on the fracture as well.  But it is not so much the size as the shielding of the heat from above for a large part of the base sheet. We don’t know if this was a side fired kiln, but if it was, there would be an increased exposure of the edges of the glass to the heat and so increase the likelihood of temperature differentials leading to too much stress for the base glass.

The rate of advance for partially covered sheets needs to be reduced to be slower than for evenly covered base sheets.  Even on evenly covered base sheets, there is a risk of breakage of the bottom sheet, if the rate of advance is too quick.  Slower heating reduces the temperature differentials, as the gradual rise in heat allows the glass to be closer in temperature from top to bottom.

Specific Gravity, CoLE, and Colourants of Glass

I’ve been asked the question “is there is differential in specific gravity as related to COE or colorant used in the glass (white opal v clear)”? 

Using the typical compositions of soda lime glass (the stuff we use in fusing), both transparent and opalescent and combining the specific gravity of the elements that go to make up the glass, I have attempted to answer question - the last part of the question first.

Difference in specific gravity between transparent and opalescent glass

Transparent glass

Typical transparent soda glass composition % by weight (with specific gravity) Material                         Weight        S.G. Silicon dioxide (SiO2)           73%         2.648 Sodium oxide (Na2O)            14%         2.27 Calcium oxide (CaO)               9%         3.34 Magnesium oxide (MgO)          4%         2.32 Aluminium oxide (Al2O3)          0.15%    3.987 Ferrous oxide (Fe2O3)               0.1        5.43 Potassium oxide (K2O)             0.03       2.32 Titanium dioxide (TiO2)            0.02        4.23

There are, of course minor amounts of flux and metals for colour in addition to these basic materials.

The specific gravity of typical soda lime glass is 2.45.

Opalescent glass

Initially opalescent glass was made using bone ash, but these tended to develop a rough surface due to crystal formation on the surface.  The incorporation of calcium phosphate (bone ash) and Flouride compounds and/or arsenic became the major method of producing opalescent glass for a time.

The current typical composition by weight (with specific gravities) is:

Silicon Dioxide (SiO2) –             66.2%,     2.648 SG

Sodium Oxide (Na2O) –            12%,        2.270

Boric Oxide (B2O3) –                10%,        2.550

Phosphorus pentoxide (P2O5) –  5%,         2.390

Aluminum Oxide (Al2O3) –         4.5%,      3.987

Calcium oxide (CaO) –              1.5%,      3.340

Magnesium oxide (MgO) -         0.8%,      2.320

The combined specific gravities are within 0.03% of each other -  a negligible amount.  So, the specific gravity of both opalescent and transparent glass can be considered to the same. For practical purposes, we take this to be 2.5 rather than the more accurate 2.45.

Other glasses exhibit different specific gravities due to the materials used, for example:

Lead Crystal Glass

Lead Crystal glass contains similar proportions of the above materials with the addition of between 2% and 38% lead by weight.  Due to this variation the specific gravity of lead crystal is generally between 2.9 and 3.1, but can be as high as 5.9.

Borosilicate glass

Non-alkaline-earth borosilicate glass (borosilicate glass 3.3)

The boric oxide (B₂O₃) content for borosilicate glass is typically 12–13% and the Silicon dioxide (SiO₂) content over 80%. CoLE 33

 

Alkaline-earth-containing borosilicate glasses

In addition to about 75% SiO₂ and 8–12% B₂O₃, these glasses contain up to 5% alkaline earths and alumina (Al₂O₃).  CoLE 40 – 50

 

High-borate borosilicate glasses

Glasses containing 15–25% B₂O₃, 65–70% SiO₂, and smaller amounts of alkalis and Al₂O₃

All these borosilicate glasses have a specific gravity of ca. 2.23

Correlation between CoLE and and specific gravity?

This comparison of different glasses shows that the materials used in making the glass have a strong influence on the specific gravity.  However, there does not appear to be a correlation between CoLE and specific gravity in the case of borosilicate glass.  If this can be applied to other glasses, there is no correlation between specific gravity and CoLE.

Correlation between specific gravity and colourisation minerals and CoLE?

The minerals that colour glass are a very small proportion of the glass composition (except copper where up to 3% may be used for turquoise).  The metals are held in suspension by the silica and glass formers.  That means the glass is moving largely independently of the colourants which are held in suspension rather than bring part of the glass structure. There is unlikely to be any significant effect of the metals on the Coefficient of Linear Expansion.  The small amounts of minerals are unlikely to have an effect on the specific gravity.  So, the conclusion is that there is no correlation between CoLE, specific gravity, and colouring minerals.

The short answer

This has been the long answer to the question.  The short answers are:

·         The specific gravity of soda lime transparent glass and opalescent glass is the same – no significant difference is in evidence.

·         There appears to be no correlation between specific gravity and CoLE.

·         There is unlikely to be any correlation between colourant minerals and CoLE or specific gravity.

Lubrication for cutters

You can cut glass without oil.  It has been done for a long time.  But it has been found that there are advantages to using oil on a score line.

The purpose of oil:

·        A minor element is to oil the cutter wheel.

·        A major element is to oil the score line.  An oiled score line stays open longer than a dry one.  

·        An oiled score line reduces the amount of visible chipping from the score line.

The kind of oil

·        Mineral oil does not oxidise to gum up the scoring wheel.

·        Any light mineral oil from sewing machine oil to WD40 is acceptable. Some use very light oils such as turpentine or white spirits.

·        There are cutting oils that are synthetic and easier to clean than the standard oils and spirits, in that less residue is left when the oil is wiped off.

·        Vegetable oils might appear to be a good substitute.  But they oxidise and become sticky, attracting dust and other particles which soon block the turning of the scoring wheel.  This requires frequent checking and cleaning.  Avoid vegtable oils.

Methods of applying

·        The oil can be put into the cutters that have a reservoir.

·        The cutter can be dipped into a container of oil with or without an oil-soaked material.

·        The oil can be painted onto the glass before scoring.

  Any combination of the above will work.

Frit by thermal shock

Frit can be created by thermal shock.  You will still need to do some manual breaking up. The principle is that you heat the glass and then cool it rapidly, causing the glass to break into pieces.

Place the glass in a stainless steel bowl and heat as fast as possible to 300C – 400C. Turn the kiln off and pull out the bowl, using heat resistant gloves and dump the hot glass into a large bucket of water. Once the glass is cool, pour off the water and dry the glass.  When dry, you can break the crazed glass into smaller bits just as you would with other glass.  Note that pouring water over the glass has two disadvantages – one, it does not completely thermal shock the glass, and two, the large amount of steam released is very dangerous.

The advantages of this quenching method of obtaining frit are that you can create frit with less effort.  You also get less fines and powder with this method. And less effort is required to smash up the glass.

Some indicate that ice cold water to quench the glass is a good idea.  This is because warm water will not provide enough of a shock to the glass to craze it throughout.  But if you have a large bucket of water, there is no necessity, as the volume of water will cool the glass quickly enough.  Of course, if you are planning another quenching, you need to renew the water, as it will not be cold enough to thoroughly craze the glass.

You can, in part, control the size of the resulting frit.  Firing at 300C results in larger frit than firing at 400C.  However, firing at 500C does not provide even smaller frit.  The best results are between 300-400C, although frit can be made at 200C as well.  Experiment with temperatures to get the frit you want.

Once you have dried the frit, you can begin to break it up. Some can be done by hand, but the pieces are often sharp, so gloves are essential.  The other standard methods of breaking up glass to make frit are applicable. But it does not take as much effort as breaking from cullett.

Annealing vs toughening

The statement “annealing stained glass makes it stronger” appeared on the internet some time ago.  Of course, without annealing there is no glass, it would simply crumble.  Annealing is the process of allowing the glaseous state to be achieved.

I think the statement is more about the difference between annealed and toughened/tempered glass.  In summary, it relates to the amount of stress within the glass.  Well annealed glass has less stress than inadequately annealed glass and so is more stable.  Toughening is a process that balances stress and tension in the glass.

The processes are for different purposes and follow different processes. 

Annealing

Annealing of glass is a process of slowly cooling hot glass to relieve residual internal stresses introduced during manufacture. Annealing of glass is critical to its durability. Glass that has not been properly annealed retains thermal stresses caused by rapid cooling, which decreases the strength and reliability of the product. Inadequately annealed glass is likely to crack or shatter when subjected to relatively small temperature changes or to minor mechanical shock. It even may fail spontaneously from its internal stresses.

To anneal glass, it is necessary to soak it at its annealing temperature. This is determined mathematically as a viscosity of 10¹³ Poise (Poise is a measure of viscosity). For most soda lime glass, this annealing temperature is in the range of 450–540°C, and is the so-called annealing point or temperature equalisation point of the glass. At such a viscosity, the glass is too stiff for significant change of shape without breaking, but it is soft enough to relax internal strains by microscopic flow. The piece then heat-soaks until its temperature is even throughout and the stress relaxation is adequate. The time necessary for annealing depends on its maximum thickness. The glass then is cooled at a predetermined rate until its temperature passes the strain point (viscosity = 10^14.5 Poise), below which even microscopic internal flow effectively stops and annealing stops with it. It then is safe to cool the product to room temperature at a rate limited by the thickness of the glass.

At the annealing point (viscosity = 10¹³ Poise), stresses relax within minutes, while at the strain point (viscosity = 10^14.5 Poise) stresses relax within hours.  Stresses acquired at temperatures above the strain point, and not relaxed by annealing, remain in the glass indefinitely and may cause either immediate or delayed failure. Stresses resulting from cooling too rapidly below the strain point are considered temporary, although they may be adequate to promote immediate failure.

But annealed glass, with almost no internal stress, is subject to microscopic surface cracks, and any tension gets magnified at the surface, reducing the applied tension needed to propagate the crack. Once it starts propagating, tension gets magnified even more easily, causing it at breaking point, to propagate at the speed of sound in the material.

In short, the aim of annealing is to relieve the stress to create a stable piece of glass. The above describes when and how that occurs.

Toughened/Tempered Glass

Toughening or tempering glass starts with annealed glass to form one type of safety glass.  This done through a process of controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tension. Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards as annealed glass does. The granular chunks are less likely to cause injury – thus safety glass.

Toughened glass is stronger than normal glass.  The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses internally. For glass to be considered toughened, the compressive stress on the surface of the glass should be a minimum of 69 megapascals (10,000 psi). For it to be considered safety glass, the surface compressive stress should exceed 100 megapascals (15,000 psi).

It is the compressive stress that gives the toughened glass increased strength. Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.

Toughened glass is normally made from annealed sheet glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace that heats it well above its transition temperature of ca. 540°C (depending on the glass concerned) to around 620°C. The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time.

An alternative chemical toughening process involves forcing a surface layer of glass at least 0.1 mm thick into compression by ion exchange of the sodium ions in the glass surface with potassium ions (which are 30% larger), by immersion of the glass into a bath of molten potassium nitrate. Chemical toughening results in increased toughness compared with thermal toughening and can be applied to glass objects of complex shapes. 

This blog entry is largely based on Wikipedia

https://en.wikipedia.org/wiki/Toughened_glass

and other sources.

Slumping Different Glasses in the Same Firing

The question has arisen as to whether it is possible to slump Bullseye and Spectrum in same slump firing.

Yes, it is possible.

But precautions are necessary.

Different temperatures are generally recommended for Spectrum and Bullseye.  Spectrum is generally expected to do the same slump as Bullseye at 25C less.

This implies that Bullseye should be put in larger or easier slump moulds than Spectrum and fired to the lower temperature required by Spectrum. The thinking behind this is that smaller spans require longer or more heat to slump.  Steeper moulds require more time and heat than less steep ones.

In general, shallow slumps will work better for both glasses together than more steep or textured ones.

To be certain of a good result, you should fire as low as practical for an extended soak.  Follow this with an extended annealing and a slower cooling rate than normal for Spectrum.

This applies to almost all the glass that is being produced with the aim of being compatible with these two glasses.  It is not possible to get a good result for float glass if it is put into the same firing as for Bullseye or Spectrum.