Multiple Firings of Kiln Wash

Many people report that they fire multiple times on kiln wash that has not been renewed.  Most add coats over existing kiln wash.  They only remove all the kiln wash when it begins to crack, stick to the glass or gets divots.

We all know that kiln wash fired a second time to full fuse is likely to stick to the glass.  We also know that kiln wash fired to slumping temperatures lasts almost indefinitely.  The kaolin in the kiln wash that allows easy spreading, undergoes a gradual change from platelets to crystals with increasing temperature.  This begins at around 600C/1115F and is complete by 900C/1655F.  The crystalline version of kaolin sticks kiln wash to glass, but as the transition from platelet to crystal is so slow at the lower end of the range, kiln wash on slumping moulds does not exhibit the sticking behaviour even over very many firings.  But, as the temperature rises, the risk of there being enough crystals to stick the kiln wash to the glass also increases.  By full fuse temperatures the proportion of crystalline kaolin is high and becomes complete on the next firing.

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credit: Immerman Glass

It is possible to fire several times to tack fusing temperatures without experiencing the sticking behaviour of kiln wash.  However, the more times and the higher temperature used, the greater risk of kiln wash sticking.

Some people continue firing without adding additional layers of kiln wash until cracks, divots, or sticking occurs.  This leads to creating a fix after the failure of the kiln wash. This requires both finding a means of cleaning the kiln wash residue from the glass, and fixing the firing surface.

Others paint a layer of kiln wash on top of the existing separator before high temperature firings. This continues each firing with a fresh layer of kiln wash.  However, the same cracks, divots, and sticking occurs at some point, requiring a complete re-coating of the shelf, and getting the kiln wash off the glass.

credit: Sue McLeod Ceramics

Re-coating of a shelf takes a couple of minutes and can be done with simple tools.  A broad scraper will remove most of the kiln wash.  This can be followed by rubbing with an open weave sanding sheet as used for plaster board or other dry walling.  If you are worried about the dust – which has less risk than fibre papers – you can dampen the surface before beginning the cleaning process.

If the kiln wash has been on the shelf for many firings, it is more difficult to remove, requiring more effort than a single firing.  High temperature firings as for melts also make the kiln wash more difficult to remove. But the same process is used in these cases.

       

Kiln wash in firings at slump and low temperature tack fuses can be reused as many times as it remains smooth and undamaged since the temperature is not high enough to cause the chemical changes.

The ultimate benefits of renewing kiln wash are that not only less effort is required to clean and re-coat, than to fix pieces, and also the cost of kiln wash is significantly less than fibre papers.

Removing kiln wash

Applying kiln wash

Revisde 18.1.25

Specific Gravity of Unknown Glass

(warning: lots of arithmetic)

Knowing the specific gravity of a glass can be useful in calculating the required amount of glass needed, e.g., for casting, and screen and pot melts, where a specific volume needs to be filled.

Most soda lime glass – the stuff kilnformers normally use – is known to have a specific gravity of approximately 2.5.  That is, one cubic centimetre of glass weighs 2.5 grams. 

If you have glass that is of unknown composition for your casting, you will need to calculate it.

Calculating the specific gravity of unknown glass.

Specific gravity is defined as the ratio of the weight of a substance to the weight of water (in simple terms).  This means first weighing the item in grams.  Then you need to find the volume.

Calculating the specific gravity of regularly shaped items

For regularly shaped item this is a matter of measuring length, width and depth in centimetres and multiplying them together. This gives you the volume in cubic centimetres (cc).

As one cubic centimetre of water weighs one gram, these measurements give you equivalence of measurements creating the opportunity to directly calculate weight from volume. To calculate the specific gravity, divide the weight in grams by the volume in cubic centimetres.

An example:

To find the specific gravity of a piece of glass 30cm square and 6mm thick, multiply 30 x 30 x 0.6 = 540cc.  Next weigh the piece of glass. Say it is 1355 grams, so divide 1355gm by 540cc = s.g. of 2.509, but 2.5 is close enough.

Calculating specific gravity for irregularly shaped objects.

The unknown glass is not always regular in dimensions, so another method is required to find the volume.  You still need to weigh the object in grams.

Then put enough water in a measuring vessel, that is marked in cubic centimetres, to cover the object.  Record the volume of water before putting the glass in.  Place the object into the water and record the new volume.  The difference between the two measurements is the volume of the submerged object.  Proceed to divide the weight by the volume as for regularly shaped objects.

Credit: study.com

Application of specific gravity to casting and melts.

To find the amount of glass needed to fill a regularly shaped area to a pre-determined depth, you reverse the formula.  Instead of volume/weight=specific gravity, you multiply the calculated volume of the space by the specific gravity.

The formulas are:

v/w = sg to determine the specific gravity of the glass;

v*sg = w to determine the weight required to fill a volume with the glass.

Where v = volume; w = weight; sg = specific gravity.

You determine the volume or regular shapes by deciding how thick you want the glass to be (in cm) and multiply that by the volume (in cc). 

For rectangles

volume = thickness * length * depth (all in cm)

For circles

Volume = radius * radius * 3.14 (ϖ) * thickness (all in cm)

For ovals

Volume = major radius * minor radius * 3.14 (ϖ) * thickness (all in cm)

Once you have the volume you multiply by the specific gravity to get the weight of glass to be added.

Calculating weight for irregularly shaped moulds.

If the volume to be filled is irregular, you need to find another way to determine the volume.  If your mould will hold water without absorbing it, you can fill the mould using the following method.

Wet fill

Fill the measuring vessel marked in cc to a determined level.  Record that measurement.  Then carefully pour water into the mould until it is full.  Record the resulting amount of water. Subtract the new amount from the starting amount and you have the volume in cubic centimetres which can then be plugged into the formula.

Dry fill

If the mould absorbs water or simply won’t contain it, then you need something that is dry.  Using fine glass frit will give an approximation of the volume.  Fill the mould to the height you want it to be.  Carefully pour, or in some other way move the frit, to a finely graduated measuring vessel that gives cc measurements.  Note the volume and multiply by the specific gravity.  Using the weight of the frit will not give you an accurate measurement of the weight required because of all the air between the particles.

An alternative is to use your powdered kiln wash and proceed in the same way as with frit.  Scrape any excess powder off the mould.  Do not compact the powder. And be careful to avoid compacting the powder as you pour it into the measuring vessel.  If you compact it, it will not have the same volume as when it was in the mould.  It will be less, and so you will underestimate the volume and therefore the weight of glass required.

Irregular mould frames

If you have an irregular mould frame such as those used for pot and screen melts that you do not want to completely fill, you need to do an additional calculation.  First measure the height of the frame and record it.  Fill and level the frame with kiln wash or fine frit.  Do not compact it.  Carefully transfer the material to the measuring vessel and record the volume in cc.

Calculate the weight in grams required to fill the mould to the top using the specific gravity.  Determine what thickness you want the glass to be.  Divide that by the total height of the mould frame (all in cm) to give the proportion of the frame you want to fill.  Multiply that fraction times the weight required to fill the whole frame to the top.

E.g. The filled frame would require 2500 gms of glass.  The frame is 2 cm high, but you want the glass to be 0.6 high.  Divide 0.6 by 2 to get 0.3.  Multiply that by 2500 to get 750 grams required.

Regular mould frames

For a regular shaped mould, you can do the whole process by calculations.  Find the volume, multiply by specific gravity to get the weight for a full mould.  Measure the height (in cm) of the mould frame and use that to divide into the desired level of fill (in cm).

E.g. The weight required is volume * specific gravity * final height/ height of the mould.

The maths required is simple once you have the formulae in mind.  All measured in centimetres and cubic centimetres

Essential formulae for calculating the weight of glass required to fill moulds (all measurements in cm.):

Volume of a rectangle = thickness*length*width

Volume of a circle = radius squared (radius*radius) * ϖ (3.14) * thickness

Volume of an oval = long radius * short radius * ϖ (3.14) * thickness

Specific gravity = volume/ weight

Revised 18.1.25

Sticking Fiber Paper

People are reporting different behaviours of their thicker fibre papers such as small fibres sticking to the glass after a fuse, and a different smell from the burning binders.  These are most likely to be from a body soluble refractory fibre paper.

It seems more suppliers are selling the body soluble versions of fibre paper. It sticks to glass and it gives off a smell of volatile chemicals. I don't like it, but I may have to use it due to the unavailability of the more health risky refractory fibre that worked very well without so much sticking.

There are several ways to minimise the fibres sticking to the glass.  They all relate to adding a separate coating of separator to the fibre paper before firing.  Among the coatings that can be used are 

• shelf paper on top, 
• a kiln wash solution brushed on, 
• kiln wash powder dusted over, 
• sprinkled alumina hydrate, and 
• boron nitride (Zyp is one brand name).  

Others have found that simply soaking the fired glass in water overnight allows the fibres to be brushed off with stiff brushes.

It seems body soluble refractory fibre papers tend to stick to the glass at anything over low temperature tack fuses.  This requires an additional layer of separator to be applied over the paper.  It is each person’s choice, of course, but I will continue to attempt to get the older version of fibre paper.

Leading Procedure

Cut the leads exactly as the cartoon indicates. In other words, where one line runs into another, that is generally a stopping/starting point for the came.

Always lead to the cartoon line, not the glass. This ensures accurate completion of the panel. If the glass is slightly too small, the cement will take up the gap (assuming the flange of the came covers the glass – if not, you need to cut another piece of glass that fits). If the glass overlaps the cut line, it needs to be reduced.  A description of the process is given here.

This shows the use of a gauge to determine where to cut the horizontal lead came.

Cut the ends of the came shorter than the glass. The best way to determine this is to place a piece of came of the dimensions being used for the next edge on the cut line. Use it to determine the length and angle for the cut. The object is to have each piece of came butt squarely against the passing came, to make a strong panel and to make soldering easier.

Revised 6.1.25

Relative stress in Tack and Full Fused Glass

There is a view that there will be less stress in the glass after a full fuse than a tack fuse firing.

This view may have its origin in the difficulties in getting an adequate anneal of tack fused pieces and the uncritical use of already programmed schedules. There are more difficulties in annealing a tack fused piece than one that has all its elements fully incorporated by a flat fuse. This does not mean that by nature the tack fused piece will include more stress. Only that more care is required.

Simply put, a full fuse has all its components fully incorporated and is almost fully flat, meaning that only one thickness exists.  The annealing can be set for that thickness without difficulty or concern about the adequacy of the anneal due to unevenness, although there are some other factors that affect the annealing such as widely different viscosities, exemplified by black and white.

Tack fused annealing is much more complicated than contour or full fusing.  You need to compensate for the fact that the pieces which are not fully fused tend to react to heat changes in differently, rather than as a single unit.  Square, angled and pointed pieces can accumulate a lot of stress at the points and corners. This needs to be relieved through the lengthening of the annealing process.

The uneven levels need to be taken into consideration too.  Glass is an inefficient conductor of heat and uneven layers need longer for the temperature to be equal throughout the piece.  The overlying layers shade the heat from the lower layers, making for an uneven temperature distribution across the lower layer.

The degree of tack has a significant effect on annealing too.  The less incorporated the tacked glass is, the greater care is needed in the anneal soak and cool.  This is because the less strong the tack, the more the individual pieces react separately, although they are joined at the edges.

If you have taken all these factors into account, there will be no difference in the amount of stress in a flat fused piece and a tack fused one.  The only time you will get more stress in tack fused pieces is when the annealing is inadequate (assuming compatible glass is being used).

More information is given on these factors and how to deal with them in this post on annealing tack fused glass and in the eBook Low Temperature Kilnforming available from Bullseye and Etsy.

Revised 5.1.25

Came: Straighten vs stretch

In dealing with lead came there is often reference to “stretching the lead”.  This frequently leads to emphasis on making the lead came longer. However, this is a misinterpretation of the phrase.

The object in pulling on the lead is to straighten it.  No more effort needs to be put into the lead once it is straight.  In fact, further stretching can lead to weakness.

The upper strectched came has an orange peel texture and the lower straightened does not

You will see an “orange peel” texture on the surface of the came when it has been stretched beyond its tensile strength.  This indicates considerable weakness in the metal.

The upper piece illustrates the visual effect of over stretching lead, weakening the came

A test to show relative strengths in stretched and straightened came uses two short pieces of came from the original pair.

After three 90° bends from the straight to a right angle, the stretched came has begun to break.  The straightened came is deformed at the inside bend, but not broken. 

This test shows stretching the came to the extent that there is an "orange peel" appearance to the surface, dramatically weakens the lead came.  Only draw the lead came to make it straight, not to lengthen it.

When you are trying to get kinks and twists out, there is a point between straight and stretched where you begin to weaken the came instead of simply making it straight. There is a point in straightening linked or twisted lead that goes so far in trying to get it straight that the whole is weakened. When the orange peel appearance shows on the came, you have stretched to the weakening point. 

It is often better with kinked and twisted came to cut out the damaged portions and straighten the rest.

Revised 5.1.25

Soldering Iron Maintenance

“How do I maintain my soldering iron?  I see so many different methods online that I find it confusing.”

Regular cleaning

There at least two reasons for regular cleaning of the solder bit.

The first is to avoid the build-up of carbon and other contaminants which impedes the transfer of heat from the soldering bit to the solder and surfaces to be joined.

Many soldering stations come with a sponge which, when wet, is used to quickly swipe the iron's tip clean. A small amount of fresh solder is usually then applied to the clean tip in a process called tinning.

The second is to maintain the soldering bit in good condition.

The copper that forms the heat-conducting bulk of the soldering iron's tip will dissolve into the molten solder, slowly eroding the tip if it is not properly cleaned. As a result of this, most soldering iron tips are plated to resist wearing down under use. To avoid damaging the plating, abrasives such as sand paper or wire brushes should not be used to clean them. Tips without this plating or where the plating has been broken-through may need to be periodically sanded or filed to keep them smooth.

To avoid using abrasives, cleaning with sal ammoniac is recommended. This comes in a block. You rub the hot soldering iron bit on the surface. As the surface becomes hot, it begins the cleaning process, noted by the smoke rising from the block. When the block under the bit becomes clear, the bit will be clean and can be tinned as above. If this is done at the end of each session of soldering, the bit will last longer and will be ready for soldering immediately when you next need to use it.

Turn off the Iron

The most important element in the deterioration of soldering iron bits is long idle times. This is where you leave the iron on, and not in use, for a long time.

Have everything ready when you start soldering, so the iron will be used continuously, and will not sit there building up heat, while you get ready to use it again. An idle iron without internal temperature control will keep heating to its maximum capacity and, without anything to transfer the heat to, it will start burning off the tinning after a short while. If you will not be using the iron for a while turn it off until you are ready again.

Tinning

If a bit has not been properly tinned, solder will not wet to it. Without solder on the bit heat transfer from the bit to the work surface may become extremely difficult and time consuming, or even impossible.

You will understand that proper wiping and continuous wetting is important and a lot easier than continually having to clean and re-tin the bit, especially at the risk of damage to the plated surface because of accidentally scratching, or over abrading it.

When you notice that an iron is not performing as well as it did when it was new you will find that poor thermal transfer from the soldering bit to the work is usually the cause. Improper care and maintenance and the lack of a periodic cleaning of the bit can cause a layer of oxides to form, which will inhibit the transfer of heat through the bit.

These factors are reasons why keeping a film of solder on the bit (tinning) is important in maintaining the long life of the soldering bit.

Courtesy of American Beauty Tools

Cleaning the whole Bit.

Each soldering bit has a shank which fits into a heating collar of one kind or another.  The bit should be removed at periodic intervals and the build-up of oxides should be cleaned from the shank.  The oxides inhibit the transfer of heat from the elements to the soldering bit.  This cleaning work, of course should be done when the iron is cool.  You can use fine abrasives on the shank to remove the oxides.  You can also make a tube of fine sand paper to clean the inside of the heating collar.  This should not be done on ceramic heated soldering irons such as the Hakko.

Wattage

Another element in the maintenance of soldering irons is to have an iron of high enough wattage to readily melt the solder and be able to reheat fast enough to maintain the necessary melting temperature. An iron with enough power will reduce the strain on the heating element of the iron and the strain on the user while waiting for the iron to catch up.

For example, an 80-watt iron is sufficient to solder with, but it will continue to get hotter, as it has no temperature control, becoming too hot for stained glass soldering, and often causing breaks in the glass. An iron of this type is often used with a rheostat in order to prevent overheating while it is idling. However, this  reduces the power to the iron and so increases the time needed to recover sufficient heat to continue soldering.  Also, a rheostat only slows the heat up, it does not limit it, so eventually the iron will still become too hot if left to idle.

Most temperature-controlled irons seem to be produced in 100 watts or higher. These irons attempt to maintain a constant temperature. Their ability to do so depends on the wattage and the amount of heat drained from the bit during soldering. The temperature-controlled irons are normally supplied with a 700°F bit (identified by the number 7 stamped on the internal end of the bit) and is sufficient to melt solder without long recovery times. You can obtain bits of different temperature ratings, commonly 800°F and 600°F. The 800°F bit is particularly useful when doing a lot of copper foil soldering, because it recovers to a higher temperature, allowing much more continuous soldering action.

An increasingly popular soldering iron has a ceramic heating element, requiring less time to recover heat, and with a lower wattage.  Most of these have a temperature dial for setting the soldering temperature, and most find 410C suitable for copper foil work, although 380C may be enough for leaded glass soldering.

You can also get several sizes of tips for different detail of work.  Upon first sight a fine tip would be useful for fine copper foil work.

But fine tips loose heat quickly, requiring the user to wait while the tip regains the required heat.  A 6mm to 8mm wide bit is useful to maintain the heat during the running of a long bead.  Of course, the bit is wider than the bead being run, but the solder has enough surface tension, while molten, to draw up into a bead the copper foil without spreading – unless too much solder is being applied. Really big bits of 12mm or larger are not practical – long initial heat up times, and too much area is covered, even though there is enough heat stored for really long solder beads.

Revised3.1.25

White solder beads

It is relatively common for questions about white deposits on the solder beads of copper foiled pieces to be raised. In reflecting on the cause of the white deposit on solder beads, I recalled that some people use baking soda to neutralise the flux.  

I put this together with some work on lead corrosion.  I have been doing a bit of research on lead came corrosion in another context.  One of the forms of lead corrosion is white lead corrosion, or lead carbonate.  It has the chemical compound PbCO₃.  It is a curious compound, as it is soluble in both acid and alkaline solutions.  

Excess whiting (or chalk) has a carbonate chemistry, which left on lead cames to give rise to this form of white corrosion. Baking soda has a chemical formula of NaHCO₃.  Solder contains a significant amount of lead – usually 37-40%.  The chemical reaction of lead and baking soda gives lead carbonate - PbCO₃ and NaH -sodium hydride.  The sodium hydride is soluble in water, leaving the white deposit of lead carbonate as a corrosion product on the surface.

Putting these things together leads me to recommend that baking soda and other carbonates should not be used in cleaning solder beads.  Some other non-carbonate neutralising or rinsing agent should be used instead.

Revised 2.1.25

Heat Work

“Heat work” is a term applied to help understand how the glass reacts to various ways of applying of heat to the glass. In its simple form, it is the amount of heat the glass has absorbed during the kiln forming heat-up process.

There is an relationship between how heat is applied and the temperature required to achieve the wanted result.  Heat can be put into the glass quickly, but to achieve the desired result, it will need a higher temperature. If you put the heat into the glass more slowly, the reverse applies.

For example, you may be able to achieve your desired result at 816C/1500F with a 400C/hr (720F/hr) rise and 10min soak. But you can also achieve the same result by using 790C/1454F with a 250C/hr (450F/hr) rise and 10min soak. The same amount of heat has gone into the glass, as evidenced by the same result, but with different schedules. This can be important with thick glass, or with slumps where you want the minimum of mould marks. Of course, you can achieve the same results with the a rise and a longer soak at the lower temperature, e.g. a 400C/hr (720F/hr) to 790C with a 30 min soak, but you will have more marking and difficulty with sticking separators.

In short, this means that heat work is a combination of time and temperature.  The same effect can be achieved with: 
- fast rates of advance and high temperatures.
- slow rates of advance and low temperatures.

- long soaks at low temperatures.

You obtain greater control over the processes when firing at slower rates with lower temperatures.  There is less marking of the back of the piece.  There is less sticking of the separators to the back and so less cleanup.  There is less needling with the lower temperature.  More information on heat work is here.

The adage “slow and low” comes from this concept of heat work. The best results come from lower temperature processing, rather than fast processing of the kiln forming.

More information is available in the eBook Low Temperature Kilnforming available from Bullseye and Etsy.

Revised 1.1.25

Breaks after the Piece is Cool

People sometimes fire a piece only to have it break after it is cool.  They decide to re-fire with additional decoration to conceal the break.  But it breaks again a day after it has cooled.  Their questions centre around thermal shock and annealing. They used the same CoE from different suppliers, so it must be one of these elements that caused the breakage.

Thermal Shock

This is an effect of a too rapid heat changes.  Its can occur on the way up in temperature or on the way down.  If it occurred on the way up to a fuse, the edges will be rounded.  If it occurred on the way up to a slump the edges may be sharp still, but the pieces will not fit together because the slump occurred before the slump.  It the break occurs on the way down the pieces will be sharp.  The break will be visible when you open the kiln.  More information is here.

If the break occurs after the piece is cool, it is not thermal shock.

Annealing

Another possible cause of delayed breakage is inadequate annealing.  Most guidelines on annealing assume a flat uniform thickness.  The popularity of tack fused elements, means these are inadequate guides on the annealing soak and annealing cool.  Tack fused items generally need double the temperature equalisation soak and half the annealing cool rate. This post gives information on how the annealing needs modification on tack fused items. 

The annealing break usually crosses through the applied pieces and typically has a hook at each end of the break.  If the piece has significant differences in thicknesses, the break may follow the edge of the thicker pieces for some distance before it crosses it toward an edge. This kind of break makes it difficult to tell from an incompatibility break.

Compatibility

The user indicated all the glass was of the same CoE.  

This is not necessarily helpful. 

Coefficient of Linear Expansion (CoE) is usually measured between 20°C and 300°C. The amount of expansion over this temperature range is measured and averaged. The result is expressed as a fraction of a metre per degree Celsius. CoE90 means that the glass will expand 9 one-thousandths of a millimetre for each degree Celsius.  If this were to hold true for higher temperatures, the movement at 800C would be 7.2mm in length over the starting size.  However, the CoE rises with temperature in glass and is variable in different glasses, so this does not tell us how much the expansion at the annealing point will be.  It is the annealing point expansion rate that is more important.  More information is here.

• Compatibility is much more than the rate of expansion of glass at any given temperature.  
• It involves the balance of the forces caused by viscosity and expansion rates around the annealing point.

Viscosity is probably the most important force in creating compatible glasses. There is information on viscosity here.  To make a range of compatible glass the forces of expansion and viscosity need to be balanced.  Each manufacturer will do this in subtly different ways.  Therefore, not all glass that is claimed by one manufacturer to compatible with another’s will be so. 

All is not lost.  It does not need to be left to chance.

Testing glass from different sources is required, as you can see from the above comments.  It is possible to test the compatibility of glass from different sources in your own kiln.  The test is based on the principle that glass compatible with a base sheet will be compatible with other glasses that are also compatible with that same base sheet.  There are several methods to do this testing, but this is the one I use, based on Shar Moorman’s methods.  

If you are buying by CoE you must test what you buy against what you have.

If you are investing considerable effort and expense in a piece which will use glass from different sources or manufacturers, and which is simply labelled CoE90, or CoE96, you need to use these tests before you start putting the glass together.  The more you deviate from one manufacturer’s glass in a piece, the more testing is vital. 

In the past, people found ways of combining glass that was not necessarily compatible, by different layering, various volume relationships, etc.  But the advent of manufacturers’ developing compatible lines of glass eliminated the need to do all that testing and experimenting.  While the fused glass market was small, there were only a few companies producing fusing glass.  When the market increased, the commercial environment led to others developing glass said to be compatible with one or other of the main producers of fusing compatible glass.

An incompatibility break may occur in the kiln, or it may occur days, months or years later.  Typically, the break or crack will be around the incompatible glass.  The break or crack may follow one edge of the incompatible glass before it jumps to an edge.  The greater the incompatibility, the more likely it is to break apart.  Smaller levels of incompatibility lead to fractures around the incompatible glass pieces, but not complete breaks.

If the break occurs some length of time after the piece is cool, it can be an annealing or a compatibility problem.  They are difficult to distinguish apart sometimes.  There is more information about the diagnosis of the causes of cracks and breaks here.

The discussion above shows that even with the best intentions, different manufacturers will have differences that may be small, but can be large enough to destroy your project.  This means that unless you are willing to do the testing, you should stick with one manufacturer of fusing compatible glass. 

Do not get sucked into the belief that CoE tells you much of importance about compatibility.

Revised 30.12.24

Effects of Annealing at the Top End of the Range

It is possible to begin your annealing at any point in the annealing range.

The annealing point is the temperature at which the glass most quickly relieves the stress within.  This occurs at the glass transition point. 

The  annealing range is between the softening point and the strain point of the glass.  No annealing can be achieved above the softening point, nor below the strain point.  This range, for practical purposes can be taken to be 55°C above and below the published annealing point.  For thick slabs, Bullseye has chosen to start the anneal 34°C below the published annealing point of 516°C.

High Annealing Point

A high annealing temperature, even up to 571°C, the approximate strain point of the glass could have been chosen, but this is impractical.  The effect of this is a greater slow cool range and so an extended anneal cool.  The reasons are as follows:  

• The anneal cool range is greater as the first rate of cool needs to be maintained to the strain point.
• The anneal cool has to extend to at least just below the strain point.
• The highest practical annealing temperature is determined by the viscosity of the glass.  Any soaks above that temperature are ineffective in production of soundly annealed glass.
• The purpose is to get all the glass at the same temperature in preparation for  cooling.  It is more difficult to maintain the small differentials in temperature achieved by the annealing soak over a large range of temperature.

Low Annealing Point

Starting the anneal cool closer to the strain point requires a slightly longer soak to ensure the glass is all at the same temperature (+/- 2.5°C, often called the Delta T=5C) before the anneal cool begins.  Typically, this initial soak would be for an hour before the initial cool begins (for a 6mm/0.25" thick piece).

Effect of the Differences in Approach

The advantages and disadvantages centre around the need to:

• soak long enough to get all the glass to the same temperature, and to 
• cool slowly enough to maintain the delta T throughout the glass.

Example

If you think of an example of a piece of Bullseye glass 12mm/0.5" thick, it will show the differences in approach.

High temperature soak

A soak of 120 minutes at 571°C/1060°F (the highest possible start for an annealing soak) is still required to even the temperature.  To ensure the temperature differentials in the glass do not deviate from the Delta T, the cool needs to be at 18°C/32°F per hour down to 427°C/800°F.  It is possible then to increase the speed to 36°C/65°F per hour down to 370°C/700°F.  This gives you a total annealing cool of just over 11.5 hours.

Low temperature soak

Starting the anneal at 482°C still requires a two hour soak followed by a decrease in temperature of 18°C/32°F per hour to 427°C, and an increased rate of 36°C/65°F to 370°C/700°F.  This gives an anneal cool time of just over 6.6 hours.

The example shows how, although the annealing result may be the same, there is considerable time saved (and especially for thicker pieces) in using the lower part of the annealing range to begin the annealing.  It also will save some electricity.

However, an anneal of two hours at 516°C with a cool of 18°C/32°F per hour to 427°C/800°F and 36°C/65°F to 370°C/700°F will still give a perfectly adequate anneal for 12mm thick pieces even though it will take about 2 hours longer.

Revised 30.12.24