Fused Glass in Dishwashers

“Can glass be put into dishwashers?”

image credit: very.co.uk

There are many recommendations to avoid placing fused glass into a dishwasher.

The main reasons given are:

·        Corrosion

·        Devitrification

·        Etching and

·        Breaking.

There are distinct differences between these effects.

Corrosion

Glass corrosion generally comes from constant contact with moisture and has a greasy feel.  As experienced by weather or washing, the wetting of glass is not constant, and it dries between wettings.  No visible corrosion is present on window glass and, although float glass is a little different from fused glass, the same effect applies.

Devitrification

Devitrification occurs at much higher temperatures than those created in a dishwasher, and therefore is not a risk.

Etching

The main risk is etching from the washing process.  This can be mechanical or chemical, and dishwashers combine both. Over time, the glass will be etched just the way lead crystal is in a dishwasher.

Breaks

Glass breaks can occur in the dishwasher because of the shock of hot water.  Most dishwashers rinse while heating the water, so the glass experiences only slow rises in temperature.  Float glass of 4mm can withstand 140˚C differentials according to manufacturers.  Full and tack fused glass is not as homogenous as float glass and will be affected by smaller temperature differentials.  So, there is a small risk of breaks in dishwashers.

Additional risks relate to the layup of the glass. 

• ·   Tack fused glass has a variety of thicknesses that make it more prone to breaks from temperature differentials.
• ·   Contrasting colours can react differently and split at the contact lines.
• ·   Large internal bubbles can cause difficulties, which may arise from the insulating element of the contained air, or simply because of thickness.

 

Slumping Splits

 This is a description of the analysis process to determine the possible causes of a split during a slump.     

Credit: Maureen Nolan

Observe the piece.

It is a tack fused piece, about 20cm (8") square, which has been slumped. 

The base layer is of clear. The piece has three additional layers, but the fourth layer is only of small glass dots and rectangles.  The central, heart, area is made of three layers.

A split has appeared during the slump. It is split irregularly through pieces rather than around them.  It is split through the thickness but only partially across the piece.

In one area the (brown) third of four layers spans the split.  Further to the left a brown second layer seems to have broken, but still spans the split.

Threads and particles of glass are connecting across the split. 

The edges are probably sharp, although only so much can be deduced from a description and one photograph.

History of the Piece

The tack fused piece has been put in a mould to form a platter and has split during the slump.

The schedule in essence was:

139ºC/250ºF to 565ºC/1050ºF for a 30’ soak (some pauses but all at a ramp rate of 139ºC/250ºF)

83ºC/150ºF to 688ºC/1270ºF for 10’

222ºC/400ºF to 516ºC /960ºF for 60’

111ºC/200ºF to 427ºC/800ºF for 10’

167ºC/300ºF to 38ºC/100ºF, off

 

The assumption is that the tack fused piece received a similar annealing soak and cool.

 

Diagnosis

Too fast

Slumping a tack fused piece of three layers plus decorative elements on top needs to be fired as for 19mm (6 layers) minimum (twice the actual).  My work for the Low Temperature Kilnforming* eBook showed best results are achieved by slumping as for one more layer (21 mm/0.825” in this case).  This gives a proposed schedule of:  

120ºC/216ºF to 630ºC/1166ºF (not 688ºC/1270ºF) but for 30 to 45 minutes

AFAP (not 400ºF) to anneal 516ºC/960ºF for 3.5 hours (not 1 hour)

20ºC/36ºF to 427ºC/800ºF, 0

36ºC/65ºF to 371ºC/700ºF,0

120ºC/216ºF to room temperature

 
Commentary on the proposed schedule:

The slump is relatively shallow, so a low temperature with a long soak is the most suitable schedule for this piece.  The drop to anneal is at a sedate rate of 222ºC/400ºF.  This is inappropriate, generally.  Just as there is a rapid rate to top temperature to avoid devitrification, so there needs to be an AFAP drop to anneal, also to avoid devitrification.  The anneal soak was not the cause of the break, but it is worthwhile noting the recommended anneal soak and cool rates are longer and slower than that used.  This is a note for the future.

 

Suspect Tack Fuse

If the tack fuse schedule was like the slump schedule, the slump was started with inadequate annealing in the previous firing.  More importantly, the evidence for an inadequate tack fuse is that the split under the brown rectangle was through the clear and red on top, but the split left the brown intact.  This means it was not securely fixed to the red below it. 

 

If the condition of the tack fuse is not sound, it is probable that difficulties will be experienced in the slump.  The poster commented “… why do [these splits] happen only when slumping – it came through tack just fine.”    It is probable the tack fuse was not “just fine”.  The way to be sure the previous firing was just fine, is to test for stress.

 

There is enough clear in this piece that an inspection for stress could be conducted by use of polarising filters before the slump.  Testing for stress is a simple viewing of the piece between two sheets of polarised light filters.  Doing this test will give information on the amount of stress, if any, in the flat tack fused blank.

 

Slump Split

During slumping the glass is subjected to more movement and therefore stress than while being fired flat.  The glass is often only barely out of the brittle zone when being slumped and that makes the stress more evident during the early part of the slump. This requires careful inspection of the failed piece.

 

Look at the glass surrounding the split.  My opinion is that the edges are sharp.  If rounded, the threads of glass from the edges of white would have melted to the edges of the split rather than spanning it. 

 

It appears the top layers were hot enough for less viscous glass on top to form stringers that span the break as the underlying layers split.  It is probable that the split was during the plastic phase of the slump for the upper glass, but  the lower layers were not as hot and suffered thermal shock. 

 

This split of lower layers, while the overlying ones are whole, is often seen in tack fuses, although the top ones do slump into the gap as the firing proceeds.  In a slump there is not enough heat, time or space, for the brown piece to slump into the gap.  Both splits appear to be a result of too rapid firing.  In the flat fusing work, the split results from too fast a ramp rate during the brittle phase of the glass.  But the slumping splits appear to occur after the brittle phase, almost as a slow tear in the glass. This may result from the differential heating of the layers if not fully combined.  It may also indicate the split developed slowly. 

 

One other observation is that these splits seem to be more frequent during the slumping of tack fused pieces.  As speculated above, it may be the inadequate tacking together of the pieces of glass during the first firing, which can form a discontinuity in transmitting heat.  And it may be that the different thicknesses across the tack fused piece allow stress to build from differential heating of the glass.

 

Rates

 

Whichever of these speculative effects may be true, it appears the ramp rates are suspect.  As mentioned elsewhere* (and in Kilnforming Principles and Practice to be published soon), the reasons for these splits are not fully known.  Even microscopic examination by Ted Sawyer has not produced a satisfactory explanation.  The only practical approach that has been successful is to slow the ramp rates.  However, the appearance of these splits is essentially random (with our current understanding), so prevention is difficult.

 

Conclusion

The probable cause of the split in the slump has been that the ramp rates were too fast.  This may have been made worse by the too short anneal soak, and the too fast cool of the tack fused blank.

 

Remedy

There is no practical rescue for this piece.  Prevention in the future is to use ramp rates that are for at least one layer thicker, if it is full fused.  If it is tack fused, firing as for twice the thickest part plus one additional layer is advisable to slow the ramp rates, allowing all the glass to heat and form at the same rate.

 

 

*Low Temperature Kilnforming; an Evidence-Based Approach to Scheduling.  Available from:

Bullseye

and

Etsy

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

CoE Varies with Temperature

Information from Bullseye shows that the Coefficient (average) of Linear Expansion changes rapidly around the annealing range.

The following is from results of a laboratory test of Bullseye clear (1101F)
Temperature range.......................COE
20C-300C (68F -­ 572F).................90.6
300C-400C (572F - ­752F).............102.9
400C-450C (752F - 842F).............107.5
570C-580C (1058F-1076F)............502.0

Bullseye glass is probably typical of soda lime glasses designed for fusing.

The change of CoE by temperature is further illustrated by Kugler (a blowing glass) who state their CoE by temperature range. Remember CoE is an average expansion over a stated range of temperatures)
CoE 93 for the range 0C-300C
CoE 96 for the range 20C - 300C
CoE 100 for the range 20C - 400C

The extension of the range by 100C beyond the brittle phase of glass has a distinct effect on the average expansion over the (larger) range. 

This shows why it is not helpful to refer to CoE without also mentioning the range of temperature.

In addition, here is an illustration of the effect. 

Image credit: Kerwin and Fenton, Pate de Verre and Kiln Casting of Glass,2000, p.32

It is understandable and common sense that as the temperature increases, so the rate of expansion increases and this applies to most solids.  Glass behaves differently as the graph above shows.  The change in expansion of Bullseye glass shows a relatively consistent average expansion until the strain point is reached.  Once out of the brittle phase, glass expansion rates change very much more rapidly.  It is not be coincidence that viscosity of glass changes at almost the same rates.  It is the viscosity that is controlling the CoE, not the other way around.   

Revised  5.1.25

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

Slump Point Test

At a time when we are all going to be trying a variety of glass of unknown compositions to reduce costs of kiln working, the knowledge of how to determine the slump point temperature (normally called the softening point in the glass manufacturing circles) and the approximate annealing temperature becomes more important.  The slump point test can be used to determine both the slumping point and the annealing soak temperature.  This was required when the manufacturers did not publish the information, and it continues to be useful for untested glasses.

The method requires the suspension at a defined height of a strip of glass, the inclusion of an annealing test, and the interruption of the schedule to enter the calculated annealing soak temperature.

A strip of 3 mm transparent glass is required.  This does not mean that it has to be clear, but remember that dark glass absorbs heat differently from clear or lightly tinted glass. The CoE characteristics given are normally those of the clear glass for the fusing line concerned.  The strip should be 305 mm x 25 mm.  

Suspend the strip 25 mm above the shelf, leaving a span of 275 mm. This can be done with kiln brick cut to size, kiln furniture, or a stack of fibre paper.   Make sure you coat any kiln furniture with kiln wash to keep the glass from sticking.

The 305mm strip suspended 25mm above the shelf with kiln furniture.

Place some kiln furniture on top of the glass where it is suspended to keep the strip from sliding off the support at each end. Place a piece of wire under the centre of this span to make observation of the point that the glass touches down to the shelf easier.

The strip held down by placing kiln furniture on top of the glass, anchoring it in place while the glass slumps.

If you are testing bottles, you may find it more difficult to get such a long strip.  My suggestion is that you cut a bottle on a tile saw to give you a 25 mm strip through the length of the bottle.  Do not worry about the curves, extra thickness, etc.  Put the strip in the kiln and take it to about 740C to flatten it. Reduce the temperature to about 520C to soak there for 20 minutes.  Then turn the kiln off.  

Also add a two layer stack of the transparent glass near the suspended strip of glass to act as a check on whether the annealing soak temperature is correct. This stack should be of two pieces about 100 mm square. If you are testing bottles, a flattened side will provide about the same thickness.  This process provides a check on the annealing temperature you choose to use.  If the calculated temperature is correct there should be little if any stress showing in the fired piece.

The completed test set up with an annealing test and wire set at the midpoint of the suspended glass to help with determining when the glass touches down.

The schedule will need to be a bit of guess work.  The reasons for the suggested temperatures are given after this sample initial schedule which needs to be modified during the firing.

In Celsius
Ramp 1: 200C per hour to 500C, no soak
Ramp 2: 50C per hour to 720C, no soak
Ramp 3: 300C per hour to 815C or 835C, 10 minute soak
Ramp 4: 9999 to 520C, 30 minute soak
Ramp 5: 80C per hour to 370C, no soak
Ramp 6: off.

In Fahrenheit

Ramp 1: 360F per hour to 932F, no soak
Ramp 2: 90F per hour to 1328F, no soak
Ramp 3: 540F per hour to 1500F or 1535FC, 10 minute soak
Ramp 4: 9999 to 968F, 30 minute soak
Ramp 5: 144F per hour to 700F, no soak
Ramp 6: off.

Fire at the moderate rate initially, and then at 50C/90Fper hour until the strip touches down. This is to be able to accurately record the touch down temperature.  If you fire quickly, the glass temperature will be much less than the air temperature that the pyrometer measures.  Firing slowly allows the glass to be nearly the same temperature as the air.  

Observe the progress of the firing frequently from 500C/932F onward.  If it is float or bottle glass you are testing you can start observing from about 580C. Record the temperature when the middle of the glass strip touches the shelf. The wire at the centre of the span will help you determine when the glass touches down.  This touch down temperature is the slump point of your glass.  You now know the temperature to use for gentle slumps with a half hour soak.  More angular slumps will require a higher temperature or much more time.

Once you have recorded the slump point temperature, you can skip to the next ramp (the fast ramp 3).  This is to proceed to a full fuse for soda lime glasses. Going beyond tack fusing temperatures is advisable, as tack fuses are much more difficult to anneal and so may give an inaccurate assessment of the annealing. Most glasses, except float, bottles and borosillicate will be fully fused by 815C. If it is float, bottles or borosilicate that you are testing, try 835C. If it is a lead bearing glass, lower temperatures than the soda lime glass should be used. In all these cases observation at the top temperature will tell you if you have reached the full fuse temperature. If not add more time or more heat to get the degree of fuse desired.

While the kiln is heating toward the top temperature you can do the arithmetic to determine the annealing point.  To do this, subtract 40C/72F from the recorded touch down temperature to obtain an approximate upper annealing point.  The annealing point will be 33C/60F below the upper point.  This is approximate as the touch down temperature is, by the nature of the observation. approximate.  

The next operation is to set this as the annealing soak temperature in the controller. This will be the point at which it usually possible to interrupt the schedule and change the temperature for the annealing soak that you guessed at previously. Sometimes though, you need to turn the controller off and reset the new program.  Most times the numbers from the last firing are retained, so that all you need to do is to change the annealing soak temperature.

The annealing soak should be for 60 minutes to ensure an adequate anneal. This may be excessive for 3 mm glass, but as the anneal test is for 6 mm, the longer soak is advisable. The annealing cool should be 83C/hr down to 370C. This is a moderate rate which will help to ensure the annealing is done properly. The kiln can be turned off at that temperature, as the cooling of the kiln will be slow enough to avoid any thermal shock to the annealing test piece.

When cooled, check the stack for stress. This is done by using two polarised light filters. See here for the method. 

Squares of glass showing different levels of stress from virtually none to severe
 (no light emanating for no stress to strong light from the corners indicating a high degree of stress.)

If the anneal test piece is stressed, there could be a number of reasons for the inadequate annealing. It could be that the glass has devitrified so much that it is not possible to fuse this glass at all. If you also test the suspended strip for stresses and there is very little or none, it is evidence that you can kiln form single layers of this glass. You now know the slumping temperature and a suitable annealing temperature and soak for it, even though fusing this glass is not going to be successful.

Other reasons for stress due to inadequate annealing could be that the observations or calculations were incorrect.  

• Of course, before doing any other work, you should check your arithmetic to ensure the calculations have been done correctly. I'm sure you did, but it is necessary to check.  If they are not accurate, all the following work will be fruitless.

• The observation of the touch down of the suspended strip can vary by quite a bit - maybe up to 15C.  To check this, you can put other annealing test pieces in the kiln.  This will require multiple firings using temperatures in a range from 10C/18F above to 10C/18F below your calculated annealing soak temperature to find an appropriate annealing soak temperature.

• If stress is still showing in the test pieces after all these tests, you can conduct a slump point test on a strip of glass for which there are known properties. This will show you the look of the glass that has just reached touch down point as you know it will happen at 73C above the published annealing point.  You can then apply this experience to a new observation of the test glass. 

Revised 30.12.24