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.

However, tack fused annealing is much more complicated.  You need to compensate for the fact that the pieces not fully fused tend to react to heat changes in different amounts, 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.

More information is given on these factors and how to deal with them in this post on annealing tack fused glass.

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).

Breaks in Slumping

Diagnosis of breaks during slumping processes is often difficult because the temperature is not high enough to be able to apply the usual rule.

In looking for the reasons for a break in fusing processes, sharp edges imply the break occurred on the way down in temperature, but rounded edges indicate the break happened on the way up to the top heat.

www.warm-glass.co.uk

This not a universally applicable diagnosis.

At low slump temperatures, the edges will be sharp in both the case of a break on heating up, and in the case of breaking on the way down in temperature.

The best test to determine when the break occurs is to observe periodically during the heat up.  You will be able to see if the piece breaks before the top temperature.  If it is whole at top temperature, the break occurred on the way down.

If you have been unable to observe the progress of the firing, you will need to diagnose when the break occurred. The test here is not whether the edges are rounded or sharp, because at normal slumping temperatures, the break will be sharp in both cases. 

If the break occurred before the top temperature, the pieces will shape separately. Therefore, If the pieces no longer fit together, the break was on the rise. If they do, the break was on the way down.  Place the pieces very carefully together to see if they form part of a continuous curve.  If they do, the break was on the cool down.  If they almost  match, or do not match at all, then the break was on the rise in temperature.

In general, when the break is on the cool down, there is an overhang of the glass on the mould which causes the break.  But the most common break of a slumping piece is caused by a too quick rise in temperature.

For a flat 6mm piece, the slump temperature rise should be less than 2/3 as quick as the rise for the fusing.  If you have a tack fused piece to be slumped you should reduce the rate of advance to at least half of that for a smooth, flat piece of 6mm.  Thicker glass with tack fused elements will need to be even slower.

Draping is another story.

Compatibility Tests

These procedures are based on the observation that glasses compatible with the base glass are compatible with each other. This means that you can test opaque colours’ compatibilities with each other by testing each of them on clear strips.

Annealing test

These tests must be combined with an annealing test.  This consists of putting two pieces from the same sheet of glass together - so you know they are compatible - and firing them along with your compatibility test.

Viewing the results of your annealing through the polarised filters shows whether there is stress left in your annealing.  If there is, the compatibility tests are inconlusive as there is no difference in appearance of stress whether from incompatibility or from inadequate annealing.  Once you have the annealing right, you can then interpret the compatibility tests done at the same time.

Strip test

Cut a strip of base glass 75mm/3" wide and as long as convenient for you or your kiln.

Cut clear glass squares of 25mm/1" to separate the colours.

Cut 25mm/1" squares of the colours to be tested.

Start with a clear square at one end of the clear strip and alternate colours and clear along the strip finishing with a clear square.

Place two strips 25mm/1" wide either side of the clear and coloured squares.

Add a stack of two layers of clear to the kiln before firing as a test for adequate annealing. If the annealing is inadequate, then the whole test is invalid.

In this test there is very mild stress showing from the dark brown and dark green.  This is well within the limits for compatibility.

Test the result with polarising filters. Start with the clear annealing test square. If no stress is apparent, go to the test strip. But if stress is apparent in the annealing test, look to your annealing schedule as something needs to change. Usually the requirement is a combination of a longer soak at the annealing temperature and a slower annealing cool.

To test for compatibility, look carefully for little bits of light in the clear glass surrounding the colour. These are indications of stress – the more light or the bigger the halo, the greater the stress. Really extreme stress appears to be similar to a rainbow, although without the full spectrum.

You can use this test to determine if you annealing is satisfactory for larger pieces. In this case you should use at least 100mm squares. Stack them to the height of your planned project and dam them with fibre board or other refractory materials to prevent spread. Fire to full fuse and anneal. When cool check for stresses.


The tile method looks at compressive factors too.

Cut a 100mm/4" square clear tile

Cut two strips of glass 25mm/1" wide and 100mm/4" long for each test

Cut two rectangles of 25mm by 50mm (1" by 2") of the same glass for the two remaining sides

Cut a square of 50mm/2" for the centre. The glass in the middle is normally the test glass. To be very certain of what has happened you can do the reverse lay up at the same time. You put coloured glass around the outside, but in this case the inside needs to be clear or transparent. At least one element needs to be transparent enough to view the stress patterns, if any. So you could have a clear middle and black exterior, and vice versa.

This test is a more time consuming process and you may wish to use it only for larger projects.

Also look at the use of polarising filters

Large Bowed Pieces

Occasionally, large pieces in the kiln develop a bow at the end of firing.  The most obvious is when the bow is upwards, but it also occurs that the piece is domed.  This is much more likely to be observed when there are complete sheets, rather than ones interrupted with other design elements which break up the whole sheet.

This is a result of a slight mismatch of compatibility.  One glass is expanding and contracting slightly more than the other.  The bow is always toward the glass which expands the most.  When it contracts, it also contracts more than the other glass, drawing the sheet with lower expansion toward it to form a bow.

This is a form of mild stress.  It can sometimes be seen in large sheets of streaky or flashed glass which are not completely flat.

It is not a fatal flaw.  A piece of this nature can survive many years in that state.  I once had a large window to repaint, because of a football impact.  When re-assembled, it showed that it had been bowed from the outset, almost 90 years before.  It is not in a suitable state for wall pieces or other things that need to be flat, of course.

Remedies

The remedies most often relate to reducing the stress in the piece.

This of course, relates to the firing schedule.  Increasing the length of the soak at the annealing point is one method.  This combined with reducing the rate of cooling can be effective.

Another method can be employed also.  This is to soak the glass just above the upper strain point of the glass.  This soak should be equal to the one planned for the anneal.  The upper strain point temperature – that point above which no annealing can occur -  is about 40C above the annealing point.  Thus, this soak should occur about 55C above the annealing point of the glass concerned.  Then proceed at a moderate pace to the annealing point.  This rate may be the same as the second stage of the anneal cool (as a starting point). Then anneal as usual for the thickness of the piece.  This method can, of course, be combined with the extended soak and reduced cooling rate as first suggested.

A third method can be employed, if the first two do not work.  This assumes one of the sheets of glass is clear.  Place a sheet of clear on the opposite side of the piece to form a glass sandwich with the two pieces of clear.  Then fire as for a three-layer piece of glass.  The assumption behind this is the same as for toughened glass.  The outer layers will hold the inner layer in compression.  But more importantly, will equalise the slight stress, allowing the piece to remain flat when the firing is completed. This can be used with any transparent glass, but the colour change may not be acceptable.

A fourth method is possible.  Turn the fired piece over and fire, to allow the weight of the glass to overcome the tension of the contraction of the more expansive glass.  This can be successful, but it does retain the stress within the resulting piece.  As such it is not a remedy for the stress, but is a way of flattening.

Placement

The place of the glass in the kiln can have an effect too.  If the sheet is near the side of the kiln, there can be a stress inducing effect.  All kilns are a bit cooler at the perimeter than at the interior.  This applies to circular, oval and rectangular kilns.  Rectangular kilns have additional cool spots at the corners.  If the glass is near the capacity of the kiln, the cooler corners can induce this bowing stress to otherwise compatible glass.  The thing to do is to stay about 50mm away from the edges of the kiln when firing large sheets into one piece.

Testing

The ideal is to know before firing the large piece whether there will be a problem to overcome. This requires a simple test of the glass to be used.

Assuming the final piece is to be two layers thick of different glass colours, cut a strip of each colour about 50mm wide and as long as the final piece.  Assemble them in the same order as you plan for the final piece.

Add an annealing test square of the two glasses stacked on top of one another.  If one is opalescent and the other is transparent. Make the transparent larger than the other.  If both are opalescent, you will need to run a compatibility test at the same time as this test.  In simple terms, it is to put each of the opalescents on a strip of clear or transparent with the gaps between the opals filled with the transparent.  This test will tell you whether you have fired so fast as to induce stress and so invalidate the test.

Fire as though for a 50mm piece of jewellery – about 200C to bubble squeeze - but without a soak - and then at 400C to top temperature.  Cool to annealing temperature for 15 minutes and cool at 120C per hour to 370C and turn off.

If the long strip is bowed, and the anneal test piece shows no stress, there is enough compatibility mismatch to require the use of one of the remedy methods outlined above for the main piece. It may of course, cause a reconsideration of the glasses to be used or the size of the piece.

Reducing Stress Points in Tack Fusing

Stress is greater in tack fused pieces than in full fused. Tack fused pieces to some greater or lesser extent behave independently from the base and surrounding pieces.  This means that more care must be taken in the anneal cooling of the glass.

Stress is dissipated more evenly in rounded tack fused pieces so the stress is not concentrated as individual points around the edge of the glass.

Stress is however, concentrated in corners of rectangles and in points of triangular and the ends of thin pieces.

By nipping the corners off these sharp angled pieces, the amount of stress concentrated there can be reduced.  Very little needs to be removed to have the effect. So the appearance of the angles is hardly affected.

Using your grozing pliers, you can take a small piece of the corner off.  It needs not be much more than a large grain of sand. This should be done at all corners and points.  It will not reduce the amount of annealing or the rate of cooling, but will assist in reducing the possible stress built up in the tack fused piece.

Relieving Stress at Corners

The most frequent locations of high stress in a piece is at corners or points.  The stress seems to be concentrated there and thus they become the most vulnerable parts of the piece.

Although the above image is of a plastic drawing triangle, it illustrates the point. The stresses are concentrated at the points and right angles whether inside or at the edge. The rainbow effect of some of the stress points show that those are the location of extreme stress.  If you see any of that in your glass, you need to check for compatibility and certainly anneal it again more slowly if it is compatible.  Remember though: slow annealing of incompatible glass will not enable incompatible glasses to fit together and become compatible.

Of course, the main thing that we do is to ensure the anneal is adequate to reduce the stress at these points.  It is important in a piece that has points, right angles and other abrupt changes in angle that you are more conservative in your annealing soak and cool. 

Further, if you are tack fusing, the stresses will be greater than on a full fuse. This is because the pieces of glass are not fully incorporated and tend to expand and contract independently of each other and of the main piece.  Also, the lower glass is shaded from the heat by the upper pieces on heat up. On cool down, the lower glass looses heat more slowly.  These two main effects, although there are others, require that the annealing is done much more slowly - two to four times more slowly than a piece of the same thickness.

One simple means of reducing stress before the start of the fusing process is to nip the corners off.  And slightly round the internal angles.  This requires only a very small piece to be taken from the corner or point to reduce the stress in the final piece. This is particularly important in tack fusing projects.

This nipping of the corners also removes the frequentl sharp points that some right and more acute angles develop during the cool down.  Glass, even of 6mm and more expands with the heat of the fusing.  As it cools toward the annealing temperature, it contracts.  The glass at the corners has to contract further than the edges, and so leaves a sharp point where it was unable to fully round. Removing only a small piece of glass from the corner removes enough mass to counteract this effect of contraction.

Slump Point Test

Revised 7.7.21

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.  This is called the slump point test.

This test can be used to determine both the slumping point and the annealing soak temperature. This used to be required when the manufacturers did not publish the information. 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 strip should be 305 mm x 25 mm.  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.  

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.

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 will need to be modified during the firing.
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.

Fire at a moderate rate initially – 200C/hr to 500C - and then at 50C/hr 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 onward, unless it is float glass you are testing. Then you can start observing from about 580C. Record the temperature in Celsius 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 from the recorded touch down temperature to obtain an approximate upper annealing point.  The annealing point will be 33C below the upper point.  This is approximate as the touch down temperature is by the nature of the observation also 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 is a problem. 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 to discover the difficulties 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 above to 10C 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. 

You Can Re-fire 3 Times Only - Kiln Forming Myths 25

Bullseye claims that you should only fire a piece 3 times

       

No. They only say the glasses are tested three times and that you are on you own after that.

There is not a general answer that can be given for the number of times you can fire a piece.  In general, Bullseye glass (and probably others, although they do not state what their limits of confidence are) can be fired three times with confidence.  Beyond that you need to do your own testing.

Bullseye states: 

At Bullseye, glasses known to be fairly stable are tested by firing to a top temperature of 1500°F (815°C) and soaking for 15 minutes before annealing. Once cooled, these tests are viewed for stress through polarized light and graded accordingly. We fire glasses known to be less stable three times to make sure they'll perform well under multiple firing conditions, such as those used to fuse and slump a plate.

If you have plans for multiple re-firings, tests are needed. The tests should replicate the temperatures, colours and thickness of the proposed project.  You probably do not need to reproduce the size of the project in these tests though.

Results from each firing should be tested for stress and these tests should include a test for annealing each time. 

You may wish to note that I have fired up to 7 times on several two layer with powder pieces.  Many people fire more times successfully.  It is my belief, but I have no proof, that multiple firings of a piece to slightly lower than full fuse will be more successful than each of them being to the full fuse.  My practice is to go to a rounded tack each of the firings subsequent to the first full fuse, but the final firing will be to a full fuse if I wish a gloss finish.  If I do not, my final firing will be about 10C - 15C below full fuse.

Short Holds in Schedules - Kiln Forming Myths 5

Frequent short soaks on the way up will make a schedule safer

Safer in this context usually means less subject to thermal shock.  To determine the validity of this requires a bit of understanding on how the glass takes up heat, and as effected the lay-up.

Glass is a good insulator, both of heat and electricity, although we are only concerned about heat here.  This means that glass transmits heat poorly or, as it may be thought of, slowly.  A steady input of heat at an appropriate rate is less likely to shock the glass than quick rises with (catch up) soaks. 

In general, there is not much change in the rate required when you go over to a single rate without soaks.  For example, a ramp rate of 200°C from 20°C to 400°C with a 20 min soak, then 300°C to 540°C with another 20 minute soak could also be written as 193°C/hr to 540°C - both take 2.8 hours to achieve the same temperature. So the rate is not very different, but the way the heat is put into the glass is.

The glass is subject to heat shock below its softening point, and so rapid increases in temperature at the start of the schedule increase the risk of thermal shock below the 540C region.

When you have uneven coverage of the base glass, as most of us do, more care is required than when we have evenly thick glass.  This relates to the poor heat conductivity of glass.  The need is to have all the glass heat up at the same rate.  This is relatively simple when there are no partial layers on top as when doing a decorative tack fusing.  The pieces on top insulate the heat from the glass immediately below.  This gives a cool spot under the top glass, in relation the uncovered glass. To avoid this difference in temperature, which causes stress, becoming too great you need to slow the rate of advance as well as keeping it a steady increase.  This indicates you should be scheduling the rate of increase as though there were two more layers over the base glass.

The steady input of heat also becomes more important with thicker glass or more than two layers of glass.  The rate of heat input needs to decrease rapidly with increasing thickness – there is not a linear relationship.  For example, doubling the thickness from 6 to 12mm requires a reduction of 2.3 times the rate of advance.  Increasing the thickness by 4 times to 25mm requires a reduction of 10 times the 6mm rate of advance.

Other factors that require slower and steady increases in temperature are where you have dark and light glasses next to one another.  The same applies where you have a viscous and a less viscous glass together.  The classic is black, the least viscous of the glasses, and white, the most viscous.

However there is at least one circumstance where soaks are useful.  When draping over steel or ceramic, the free hanging glass heats up more than the centre where it is resting on the mould.  In this case, the mould forms a heat sink, drawing the heat away from the glass into itself.  You need to go very slowly or insert a few soaks to allow the supporting mould to heat up. More information here.

All myths have an element of truth in them otherwise they would not persist.

They also persist because people listen to the “rules” rather than thinking about the principles and applying them.  It is when you understand the principles that you can successfully break the “rules”.

Thermal Shock

Thermal shock is a term for a break caused by a too rapid change of temperature within a piece of glass.

"Glass tends to be 

1) very brittle, 

2) expand and contract quickly when subjected to temperature changes, and 

3) is an insulator (when solid) and therefore does not readily conduct heat. 
That is why glass is highly susceptible to thermal shock"

http://www.glassfacts.info/indexf286.html?fid=210

This can occur on both an increase or decrease in temperature. Glass conducts heat poorly.  The ideal is to keep the temperature differentials within the glass to 5C or less.  This is the purpose of the anneal cool.  The risk of thermal shock can be increased by different thicknesses across the piece. Greater care is required in cooling these pieces than those of uniform thickness.

A piece showing large differences in thickness and so at greater risk of shock

Identification

The break normally is straight through the glass without following the edges of the various pieces of glass.

This shows the break crossing multiple colours of glass

The line of the break will be rounded if it parted on the heat up. In some cases, the glass will have stuck back together if it was dammed or the break was gentle enough to avoid pushing the glass apart.

If the shock occurred on the cool down, the edges will be sharp.  

The edges will also be sharp in a slump whether the break occurred on the advance or the reduction in temperature.  If the pieces fit together perfectly the break is likely to be in the down phase.  If the pieces are slightly different shapes the break likely occurred in the rise in temperature phase.

Other kinds of breaks are possible and are described elsewhere.

Compatibility Shift at Higher Temperatures

People experience breakages of their pot and screen melts that do not seem to have anything to do with annealing or glass sticking to the shelf. The common suggestion is that there has been a compatibility shift of the glass. This view is re-enforced by the opalisation of the transparent hot colours experienced by most.

Bullseye indicates in their glass notes that some colours are not suitable for high temperature work. This probably applies to other fusing glasses too. My experience leads me to believe that this compatibility shift occurs with all the opalescent glass colours as well as the hot ones. Further work will appear soon. is required to determine if there are any general indicators of the kinds of glass that are likely to develop incompatibility at high temperatures.

If you are concerned about the lack of durability of your piece due to possible incompatibility, you need to include tests with the firing. To make this test, place a piece of each colour used in the melt on a double layer of clear. If you are using a single base piece, ensure you leave space between the colours. It is best to place each colour on its own stack of clear. Also place a stack of clear glass as thick as your blank along side the other test pieces. Put all those pieces somewhere within the kiln out of the way of the area the melt will occupy and fire the lot together.

When cool, take all the pieces from the kiln and check the test pieces for compatibility. Do this check with a polarising filter to determine whether there is any incompatibility by looking for the halo showing the degrees of incompatibility.

If any or all, of the the pieces show stress, check the clear stack for stress. If the clear also shows stress, the annealing has been inadequate, rather than just the compatibility shift. Ideally, this process should be conducted in every firing.

Performing these tests will give you confidence in the durability of your piece, as it will show the levels of stress in the finished piece.

Annealing Unknown Glass

Sometimes you may want to use a glass in kiln forming when its characteristics are not known, such as for bottle slumping. It is possible to determine the approximate annealing point of this glass in your own studio. This tip on slump point testing gives you the information to do the test and calculations.

If you do not want to go to that detailed effort for a one-off process, you can adopt the shotgun annealing approach. This does require some observation of the glass, of course.

You need to observe when the glass has reached the temperature for the process you are performing. This will enable you to compare the behaviour of this unknown glass with what you normally use. This will give some idea of the relative annealing temperature to use. If a higher temperature is required for this glass than your normal glass, a higher annealing point can be assumed. The difference in top temperature can be added to the annealing point of your known glass.  If the top temperature is lower, you subtract the difference from the known glass' annealing point.

Set the annealing temperature to be 10C to 20C above the predicted annealing temperature and soak there for 30 to 60 minutes. This will help ensure the glass is all at the same temperature throughout. Set the annealing cool to be at about 30C per hour for pieces up to 6mm for the first 55C. The next segment should be about twice that to 110C below your chosen annealing temperature. The final segment can be around 150C per hour to 100C.  For thicker glass, the annealing cool should be proportionately slower.

This may seem an excessive, overly cautious process, but as you get to know the characteristics of the glass, you will be able to alter the schedule. This is a conservative and safe process to ensure your glass is well annealed.  And to be certain, you should check the cooled glass with polarised light filters.

amended 22.12.18

Tack Fusing Considerations

1 – Initial Rate of Advance

Tack fuses look easier than full fusing, but they are really one of the most difficult types of kiln forming. Tack fusing requires much more care than full fusing.

On heat up, the pieces on top shade the heat from the base glass leading to uneven heating. So you need a slower heat up. You can get some assistance in determining this by looking at what the annealing cool rate for the piece is. A very conservative approach is needed when you have a number of pieces stacked over the base layer.  One way of thinking about this is to set your initial rate of advance at approximately twice the anneal cool rate. More information on this is given in this entry. 



2 – Annealing 

The tacked glass can be considered to be laminated rather than fully formed together. This means the glass sheets are still able, partially, to act  as separate entities. So excellent annealing is required.

Glass contracts when it's cooling, and so tends to pull into itself. In a flat, symmetrical fuse this isn't much of a problem. In tack fuses where the glass components are still distinct from their neighbours, they will try to shrink into themselves and away from each other. If there is not enough time for the glass to settle into balance, a lot of stress will be locked into the piece that either cause it to crack on cool down or to be remarkably fragile after firing. In addition, in tack fusing there are very uneven thicknesses meaning it is hard to maintain equal temperatures across the glass. The tack fused pieces shield the heat from the base, leading to localised hot spots on cool down.

On very difficult tack fuses it's not unusual to anneal for a thickness of four to six times greater than the actual maximum thickness of the glass. That extended cool helps ensure that the glass has time to shift and relax as it's becoming stiffer, and also helps keep the temperature more even throughout.

So in general, tack fused pieces should be annealed as though they are thicker pieces. Recommendations range from the rate for glass that is one thickness greater to at least twice the maximum thickness – including the tacked elements – of the whole item. Where there are right angles - squares, rectangles - or more acutely angled shapes, even more time in the annealing cool is required, possibly up to 5 times the total thickness of the piece.

It must be remembered especially in tack fusing, that annealing is much more than the annealing soak. The soak is to ensure all the glass is at the same temperature. The anneal cool over the next 110ºC is to ensure this piece of different thicknesses will all react together. That means tack fusing takes a lot longer than regular fussing.

3 – Effects of thicknesses, shapes, degree of tack

The more rectangular or pointed the pieces there are in the piece, the greater the care in annealing is required. How you decide on the schedule to use varies. Some go up two or even four times the total thickness of the piece to choose a firing schedule.

A simplistic estimation of the schedule required is to subtract the difference between the thickest and the thinnest part of the piece and add that number to the thickest part. If you have a 3mm section and a 12mm section, the difference is 9mm. So add 9 to 12 and get 17mm that needs to be annealed for. This thickness applies to the heat up section as well.

Another way to estimate the schedule required is to increase the length the annealing schedule for any and each of the following factors:

·         Tack fusing of a single additional layer on a six millimetre base

·         Rectangular pieces to be tack fused

·         Sharp, pointed pieces to be tack fused

·         Multiple layers to be tack fused

·         Degree of tack – the closer to lamination, the more time required

The annealing schedule to be considered is the one for at least the next step up in thickness for each of the factors. If you have all five factors the annealing schedule that should be used is one for at least 21mm thick pieces according to this way of thinking about the firing.

4 – Testing/Experimentation

The only way you will have certainty about which to schedule to choose is to make up a piece of the configuration you intend, but in clear. You can then check for the stresses. If you have chosen twice the thickness, and stress is showing, you need to try 3 times the thickness, etc. So your annealing soak needs to be longer, if stress shows. You can speed things by having your annealing soak at the lower end of the annealing range (for Bullseye this is 482C, rather than 516C).

You will need to do some experimentation on what works best for you. You also need to have a pair of polarisation filters to help you with determining whether you have any stress in your piece or not. If your piece is to be in opaque glasses, you need to do a mock up in clear.

Using Space on Shelves

Often there is unused space on the kiln shelves when you are firing a project. With a bit of planning, you can make use of the spaces for a variety of things.

Frits fired on fibre paper

Bowl made from frit balls

You can place piece of frit in the clear areas to make frit balls.

You can make colour tests on plaques of glass to see the results of strikers, powder combinations or results of various depths of colour.

Compatibility tests can be done with pieces of glass of which you are not certain.

simple stress testing set-up

Strip of fired glass samples for testing

Results - those with halo are stressed

In the same way, annealing tests can be conducted.

You can fire small pieces of jewellery at the same time as your larger pieces.

You can also prepare elements for incorporation into other fusing projects and lay them out in the open spaces on the shelf.  Your use of the spare space is related both to your imagination and to your future projects.