Tin Bloom

Using float glass sometimes produces partial clouding as though devitrification were present. Although float glass is prone to devitrification, not all the cloudy film on the surface is due to devitrification.

Float glass, which these days, is almost all clear smooth glass, gets its name from the process of floating the glass on molten tin. The tin in compression gives an apparent devitrification effect which is called tin bloom.

it is different from devitrification, to which float glass is particularly subject. Devitrification sprays and solutions will not have an effect on this surface defect called tin bloom. 

When the tin layer is stretched, it does not create a tin bloom on the surface.  Therefore, it is important to have a means to detect which is the tin surface.  Always fire the glass with the tin in the same relative location to each other.  I.e., on several layers of glass have all the tin side down or all up, but not mixed. 

This example of a test by Glass Art by Margot shows the tin bloom on the outer portions of the platter where the tin side was up, causing the tin too be compressed and show.  The flatter portion of the piece did not show this tin bloom as there was not the same extent of compression. You can visit the description of the experiment here.


When forming the glass (slumping, draping, kiln carving) make sure the tin sides will be stretched rather than compressed.  Of course, you can take advantage of the tin bloom by controlling the compression of the tin layers.

Thinfire* and Devitrification

There are reports that Thinfire causes devitrification by rising over the edges of the piece.  There as many saying they have no difficulties with the Thinfire curling.  This indicates there are several factors that may be at work.

If the Thinfire curls over the edge of the glass while firing, it will deposit a fine powder on the edge and perimeter of the piece.  This gives an ideal condition for devitrification to form.

Bullseye recommends placing dams or other kiln furniture on the edges of the paper to resist any tendency for the paper to curl.  Of course, if the paper is put upside down, it is much more likely to rise over the edge.  The smoothest surface should face upwards. Now that Bullseye prints their logo on the bottom, this is unlikely to be a problem.

Cutting the paper to the size of the piece is initially an attractive idea.  However, it does not account for the expansion beyond the initial footprint that glass goes through while heating to the working temperature, and before it contracts to its final size.  The Thinfire must be cut larger than the piece. The amount depends on the thickness of the piece.  6mm larger may be adequate for a 6mm thick piece.

Bullseye does not recommend using Thinfire under multiple small pieces of glass because the paper can shrink and move, disrupting the glass placement on the kiln shelf.  Instead using kiln wash as the separator may be better in these circumstances.

There are other things that can affect the deposit of the separator powder from the Thinfire onto the glass.

Venting – It seems to be good practice to open the peep holes or leave the door/lid slightly ajar during the heat up.  These should be carefully closed once the smell of the binder burning out disappears.  This is usually around 500°C.  The idea here is that the combustion products from the binders are allowed out of the kiln without settling on the glass.  I do not find this necessary, but many do, so it is worthwhile trying it out.  When the smell of the burnout of the binders ceases close the lid slowly and place the bungs gently into the peep holes to avoid disturbing any dust within the kiln.

Opening the kiln or ports - Opening or closing the kiln above ca. 500°C, if done quickly, will create a draft that will distribute the powder around the kiln.  Some of this will land on the surface of the glass. Other parts of the Thinfire will be moved up onto the edges of the piece.  This dust and the pieces of Thinfire will create nucleation points for devitrification.  Always open or close any part of the kiln slowly when there are powders or anything else which can be disturbed by a gentle waft of air.

Over firing - Another element that can bring Thinfire onto your pieces are a too hot a firing.  During high temperature firings, the glass will expand and thin more than usual.  During the cooling phase, the glass will draw back to being 6-7mm thick. This means the glass will have expanded over the Thinfire and drawn some of it back onto the edges as it thickens and retreats.  The solution for this is to reduce the top temperature and possibly lengthen the soak time, but do not do both at the same time.  First see what a lower temperature with a 10-minute soak will do.

Of course, if you are not having problems with Thinfire or Papyros, continue your practice as normal.

*I have used the term “Thinfire” almost exclusively throughout, but remember all these notes apply to Papyros too.

Needling in Bottle Moulds

Sometimes people experience sharp, needle-like points on the bottle after it is slumped.

Causes

As the bottle expands and softens, it conforms to the surface of the mould.  When the cooling begins, some parts of the glass are trapped in the tiny pits of unevenness that always exist in the mould or in the separator.  As the glass retreats, the glass is stretched until it breaks off, leaving the sharp needles.

Prevention

Remedies relate to separators and temperatures.  This of course, assumes you already have a good coating of kiln wash or similar separator on the mould.

Separators

These additional separators can be fibre paper or powders.  Thinfire laid on the bottom of a bottle mould can provide additional separation between the bottle and the glass.  This works, because with a slow rate of advance, the Thinfire will have turned to powder before the bottle begins to slump. This powder will not interfere with any designs on the mould.  Papyros will work on smooth moulds, but not so well with textured bottle moulds, because of its more fibrous nature.

This use of powered paper indicates that you could use a cheaper solution.  Just dust a fine film of kiln wash on the mould.  I do this by placing the powdered kiln wash in a sock and shake the sock above the mould.  This will allow an almost invisible layer of fine powder to fall onto the mould.  This is enough to provide an additional layer of separation between the glass and the mould.

Temperature

It is quite common for people to slump bottles at tack fusing temperatures to do both the flattening and the slumping at one firing. This is quite hard on the mould and softens the glass enough to promote the needling. 

It may be better to use two firings – one to flatten using tack fusing temperatures, and one to form the bottle at slumping temperature.  This lower temperature will avoid the needling, as the bottle will not soften enough to form the needles during the slumping. The reason many people avoid this is because the bottles tend to devitrify on second firings.  If you do this two-stage slumping, you will need to apply a devitrification solution to the upper surface of the flat bottle to try to prevent it.

You can take a different solution to the two-stage firing.  As lower temperatures reduce the possibility of needling, you can simply soak for a longer time at the slumping temperature than a normal one stage tack and slump.  You will need to peek in at intervals to determine when the slump is finished, of course.  After a few firings though, you will get a good idea of the amount of time required to complete the slump. An additional advantage is that at the lower temperatures, devitrification is less likely.

Cleaning the Kiln of Dust

Dust is promoter of devitrification. You should do the most you can to keep your kiln free of dust.

Dust can come from the kiln lining materials.  Regular gentle vacuuming of the kiln surfaces will help prevent particles from falling on to you work or other surfaces in the kiln.

It can come from the separators you put in the kiln.  I often see pictures of used fibre paper at the side, or under, the kiln shelf.  This should be cleaned out after each use to provide clean firing conditions.

The main reason for this obsessive cleaning is that dust particles within the kiln will be disturbed by the air movement involved in closing or opening the kiln lid or door. There also is air circulation within the kiln during the heating and cooling phases, although it is not as much as when opening the door/lid.  These disturbed dust particles will settle on the glass and defeat your cleaning of the glass.  

Preventing Devitrification on Cut Edges

“Question-when cutting up a Screen Melt, using a tile saw. How do you NOT get devitrification when laying the slices cut sides up?”

Devitrification occurs where there are differences in the surface.  This means that the surfaces exposed to the heat must be both clean and smooth.  It is not enough for only one of these to be the case, both are required.

First, the sawn edges need to be clean.  A good scrub with a stiff bristle brush is essential.

Second, devitrification sprays of whatever kind do not seem good enough to prevent the devitrification. This is probably due to the thin covering of the differences (scratches, pits, etc.) on the surface.

Beyond that, I know of two ways to prevent or reduce devitrification. That is, providing a smooth surface to resist devitrification.

1 – Grind

This can be done with hand pads, grit slurry or machines such as a Dremel with damp sanding pads or belts, wet belt sanders, or a flat lap.  The grinding should go down to at least 400 grit before cleaning and arranging to fire.

2 – Clear glass

This method relies on putting a layer of clear glass that is less likely to devitrify than the cut edges over the whole surface.  You could use a sheet of glass, although that would promote a multitude of bubbles due to the spaces between the strips and the naturally uneven heights of the strips.

Placing a layer of fine frit on top of the arranged pieces before firing is a way of allowing air out and forming a smooth upper layer by filling the gaps. It is best to avoid powder, as this promotes a multitude of fine bubbles, giving a grey appearance. The layer you apply needs to be an even layer and at least 1mm thick. If you are concerned at getting lots of bubbles, you could use medium frit instead.  In this case, the layer will need to be thicker than 1m to get an even coverage. The whole of the surface of the piece needs to disappear under the layer of frit, and that may be a good guide to the thickness of frit to apply.

Slumping Glass that is not Tested Compatible

Is it Possible?

It is possible to slump unknown glass. This glass might be art glass, window glass, bottles, or any other glass whose characteristics are unknown by you.  There are some suggestions about the characteristics of some glasses in this post that can be used as a starting point.

Preparation of the Glass

Prepare the edges to their final finish before slumping.  This because the slumping temperature will not be enough to alter the finish of the edge significantly.  This preparation can be done with diamond hand pads, or wet and dry sandpapers.  Start with a relatively coarse grit. You may wish to do the initial shaping on your grinder. This will be between 80 and 100 grit.  Continuing with a 200 grit and working your way through 400 and then 600 grit will give you an edge that will become shiny during the slumping.

Cleaning

Clean thoroughly.  This is especially important when using glass that is not formulated for fusing.  Devitrification is more likely on these glasses.  Water with a drop of dishwashing liquid can be enough unless your water has high mineral content.  Then distilled water or a purpose made glass cleaner such as Bohle or Spartan should be substituted.  Finish with a polish to dry with clean paper towels. More here. 

Firing the Slump

Fire up slowly.  You should advance at about 100°C to 150°C per hour.  Set your top temperature around 630°C for a simple slump, for soda lime stained glass.  For bottle or window glass you will need a temperature closer to 720°C although the also are soda lime glasses.

It is best to start with simple curves, as there are fewer difficulties in determining what the glass is doing.  It will help you to learn the characteristics of the glass before you tackle the difficult stuff, such as compound curves or texture moulds.

Observation

It is necessary to observe the progress of the slump as you do not yet know the slumping temperature.  You want to know when the glass begins to deform so that you do not over fire.  Start watching the glass at about 10 minute intervals from about 580°C for stained glass and 680°C for window and bottle glass.  There is not much light in the kiln at these temperatures, so an external light is useful.  You can also observe the reflections of the elements on the glass.  When the image of the elements begins to curve, you know the glass is beginning to bend.

Altering the Schedule

Soak for at least 30 mins at the temperature when the glass begins to visibly drop. This may or may not be long enough.  Continue checking at 5-10 minute intervals to know when the slump is complete.  If the glass is completely slumped before the soak time is finished, advance to the next segment.  If not fully slumped, you need to extend the soak time. This means that you need to know how to alter your schedule in your controller while firing.  Consult your controller manual to learn how to do these things.

Stop the soak when complete and advance to the anneal. Continue the slumping soak if not complete after the 30 mins.  In some cases, you may need to also increase the temperature by 5-10°C.

Annealing

The annealing point will be about 40°C below the point that the glass visibly starts the slump. If you want a more accurate determination of the annealing point, this post gives information on how to conduct a test to give you both the slump temperature and the annealing point.  It also helps to determine the lower part of the tack fusing range (the lamination state), since it is not far above the slumping point that you will observe.

The annealing soak for a single layer, 3mm glass need not be long – 15 to 30 minutes.  The annealing cool can be as fast as 120°C down to 370°C.  For thicker glass and slumped bottle glass you will need a longer soak – 30 to 60 minutes – and a slower cool.  The annealing cool in this case could be about 60°C/hour to 370°C.  You can turn the kiln off at 370°C, if you wish, or keep the temperature controlled to about 50°C.  The rate for the final cooling can be approximately double the first cooling rate.  For a single layer of stained glass this could be 240°C, and for thicker glass about 120°C

Borax solutions

A borax solution can act as a devitrification spray. That is its usual application in kiln forming.  But it can be used in other ways too.

Borax is a flux helping to reduce the firing temperature of glass. So, it can be used as a medium for powdered mica which can be painted or sprayed onto the glass. It also helps reduce the oxidisation of included metals.

Obtain borax that has no additives. Put a couple of teaspoons into water and bring to a simmer. Remove from the heat and cool. Decant the almost clear liquid off the sediment and you have a saturated solution of borax ready to use. 

If you are really parsimonious, you can add water to the crystals remaining in the pot and heat to get another saturated solution. You could do this until there was no residue, but that would get tedious.

Add a couple of drops of washing up liquid to the solution. This is enough to break the solution's surface tension. It helps to give an even distribution of the solution across the clean glass by reducing the beading of the liquid that otherwise occurs.

You can paint the solution onto the material - glass or metal - with a soft brush such as a hake brush, or you can spray it on with a pump spray container.  Be careful to clean the spray container immediately, as borax crystals form quickly.

Permanance 

Description of devitrification
Temperature range

Characteristics

Single Layer Slumping

Almost all glass can be slumped as a single layer, whether produced for kiln working or not.  A few are extra sensitive at even slumping temperature and change character at around 630C-650°C, but all others can be slumped.  This posts concentrates on slumping of single layers of non-fusing compatible glass, but most of these elements can be applied to fusing compatible glass too.

The things you need to take care about are:

• Temperature
• Soak Times
• Edges
• Devitrification
• Annealing
• Testing

It certainly is possible to slump single layers. The resulting glass will be slightly less robust than two or more layers of glass, but simply because it is thinner.

Temperature

The temperature that you use needs to be high enough to allow the glass to take the shape of the mould, but low enough that the glass does not distort or stretch and thin.  This is a balance that you can achieve through observation of the firing. 

It most often is best to use the lowest practical forming temperature that you can.  Practicality here is about how long you want to wait for the glass to conform to the mould.  It is possible to take the glass to about 580°C and soak for multiple hours, but not very practical.  It does depend on the glass as to the temperature to be used for the slump.  There are two sources here that can help: the slump point test  and this table of glass characteristics. 

Soak times

A practical soak time will be 30 – 90 minutes, which will avoid marking the underside of the glass.  This means that the temperature will need to be lower than the softening (or slump) point of the glass. Your slump point test will tell you the temperature at which the glass begins to deform.  That is the best temperature to use.  If it is taking too long, advance the temperature by about 10°C.  If you used the table of glass characteristics to find a softening point, reduce that temperature by about 30°C as a starting point.

Edges

The temperature that you will choose to use is not high enough to allow the edges to change as they would in a fuse.   This means that you need to have the edges exactly as you want them in the finished project.  This will require cold working by hand or machine.  Neither will take a long time, but require the correct tools. This post gives you the comparison of fused and cold working methods.

Devitrification

While most glass can be slumped you need to be careful with opalescent glass, as it can devitrify easily.  Most wispy glasses are fine, but the more opalescent wisps they have, the more difficult there may be.  Streaky and single colour glasses are usually fine. 

Annealing

Another element in slumping glass not formulated for kiln working is the annealing of the glass after the slumping.  The annealing temperature can be estimated as 40C below a low temperature slump of a 280mm span of glass. The slump point test mentioned earlier will help determine the annealing point. You need to soak for a time - maybe 30 minutes - at the estimated annealing temperature and then cool slowly in case you have miscalculated on the annealing temperature.  In any case, a long slow anneal cool will pay dividends in a more robust glass.

Testing

You will find some manufacturers’ glasses are less adaptable to kiln forming than others.  So, it is best to run tests on the glass before committing to larger projects.

Remember TADSET - temperature, annealing, devitrification, soak, edges, test.

Devitrification on Ground Edges

The first element in preventing devitrification is cleaning.  Making sure all the edges of the glass are clean will help.  OK, you have cleaned the edges well after grinding. You still get detrification, so you want to know

Why do ground edges get devitrification? 

To answer this question, you need to think about how glass behaves in the kiln. As it heats up the glass expands, pushing the cut edges into the separator on the shelf. The pits caused by the grinding have not yet become fire polished.

When the glass retreats on cooling the pits in the edges of the glass, although very small, pick up some of the separator. These small particles act as the nucleation points for the crystallisation of the glass which is generally called devitrification.

The glass of a single 3mm layer retreats further on a single piece than on a 6mm piece. This rolls the devitrified glass upward onto the upper edge of the piece.

Prevention of devitrification of the ground edge is to have the pits in the glass edge finer than the particles of the separator. This is more than just washing the glass immediately after grinding to remove the glass powder from the grinding scratches.  Yes, this will reduce the chance for devitrification, but not totally prevent it.  As noted above, the pits in the glass will pick up particles of separator on expansion, giving nucleation points for the devitrification.

Further coldworking beyond the initial grinding is required to reduce the devitrification possibilities.  This involves using finer grinding bits or smoothing by hand with finer grits.  This does not have to take long, as the shape has been achieved by the grinder.

The logic of prevention is to have the glass edge smoother than the particle size of the separator, so the finer and smoother the separator, the smoother the surface of the glass edge must be.  

But my devitrified edge was on top of other glass

The follow-on question is about why devitrification occurs on ground edges that are not near the kiln shelf.  There are two elements to consider.

It is claimed that the fumes of the binder burning off can settle in the pits of the ground glass, providing those nucleation points for the glass crystalisation. The suggested solution is to vent the kiln to about 400C to allow the combustion fumes out of the kiln rather than keeping them inside the kiln.

The second and more certain element is that the grinding creates microscopic pits and fractures in the glass where the powder from grinding settles.  Almost no amount of cleaning will completely remove this residue from the tiny pits and fractures resulting from grinding. 

There are at least two solutions to this cleaning problem. Don't grind unless absolutely necessary - groze instead.  The second is to lightly cover any ground edges with clear powder frit.  You could of course consider ultrasonic cleaning or power washing, either with a dishwasher, or outdoor power washer.  Both these seem to be so completely out of proportion to the problem, that I have never used them.

Kiln Washing Kiln Lids

It is frequently recommended that the bottom of the kiln should be kiln washed to prevent any spilled glass from sticking to the kiln brick.  You should remember that this is applicable to brick lined kilns.

This in itself is a little clue.  You do not need to kiln wash any insulation fibre in the kiln. If any glass were to stick to the fibre, it would come away easily.  In any case, most insulation fibre blanket will not stick to the glass.

The recommendation often goes on to advocate kiln washing the sides.  There is a caution that the side elements (if any) should not be kiln washed. The caution comes from the knowledge that water and electricity should not be mixed.  The kiln should not be on when applying kiln wash anyway.  If kiln wash is splashed onto the elements, it is simply a matter of letting the whole kiln dry naturally with the lid open before firing.

The extension to this series of recommendations is that the whole of the kiln should be kiln washed, including the lid.  This is not a good idea.  The wash on the lid will soon fail and drop dust and debris onto and into your work.  The glass should never touch the top of the kiln anyway.  If the elements are in contact with the glass, the glass will either stick to them or break.  You have to ensure you do not put glass nearer than about 20mm to the elements or lid. In any case, the glass will fall to the bottom of the kiln, not the top or sides – unless the kiln is not level.

BUT

The whole idea of kiln washing the interior of the kiln is suspect in some ways.  Anyone who has had glass drip off the shelf and onto the brick during an over-firing will know the glass eats into the brick through the kiln wash.  Kiln wash will only protect the brick at full fuse or less temperatures. But it is a good precaution to keep the pieces of frit that fall off the shelf from sticking to the brick. It does not do much more than that.

The application of kiln wash to the kiln creates another source of dust within the kiln.  Dust and general uncleanliness in the kiln is a main potential source of devitrification. Thus, the application of kiln wash should be the minimum necessary and does not need to go up the side beyond the elements or the lowest shelf height, whichever is less.

There is a strong argument to be made that laying a sheet of 0.5 mm fibre blanket on the floor of the kiln will provide better protection of the kiln than any amount of kiln wash.  It is less likely to fail, it is not a source of additional dust, it provides a better protection during any kiln runaway, and it is easily replaceable.

Devitrification on Repeated Firing

 Devitrification is defined as the crystallisation of the glass, making it a non-vitreous substance.

Molecular level difference between vitreous and devitrified silica
from Digitalfire.com

You can see that there is not much difference between the the two states of the glass in structure, but mainly the arrangement of molecules.

The appearance of devitrification has a range of appearances from a mild smeary look through a dull surface to a crazed, crumbly aspect in severe cases. 

Mild devitrification

Medium level devitrification requiring abrasive cleaning

Causes of devitrification are related to slow changes of temperature (up or down) and most importantly nucleation points such as dust, oils, or cleaning residues. So, thorough cleaning is most important. 

Causes in repeated firings of the same piece relate to:

        Cleaning. 

It is important to thoroughly clean the piece before each subsequent firing.  Many times abrasive cleaning such as sandblasting is important to clean out impurities from the previous firing.  The resulting surface from any abrasive cleaning requires further cleaning with lots of clean water and a thorough drying with clean cloths or paper.

        Slow cooling or heating. 

Devitrification normally occurs in the range of 670⁰C to 750⁰C. This is the reason for the rapid rates of advance in this temperature range rather than other factors.  It can form both on the rise and on the fall in temperature. Slower rates in the devitrification range allow enough time for the crystallisation to begin.

        High temperatures.

Both high temperatures and long soaks can promote devitrification.  It is not just the slow rise or fall in temperature, but long periods at high temperature can lead to devitrification even though other precautions have been taken.

Changes in the composition

High temperatures and many repeated firings of the piece can lead to changes in the glass.  Some metals and fluxes are more likely than others to change composition or oxidise at extended soaks at high temperatures.  This can reduce the ability of the glass to resist devitrification.

Prevention/Correction

Prevention relates to thorough a) cleaning and b) firing rates.

All correction of devitrification relates to the modification of the surface.  If the problem is only at the surface, you can use either abrasive cleaning or the addition of fluxes to the surface, or a combination of the two. 

Where you have a mild dulling of the surface due to devitrification you can apply a flux.  This softens the surface by reducing the melting temperature of the glass and so reverses the crystallisation at the surface. The devitrification solution can be a proprietary spray such as Super Spray. Be aware that some sprays use lead particles as the flux, so are inappropriate for pieces intended to be food bearing. You can make your own devitrification solution by dissolving borax in distilled water.  When the devitrification is wide spread or deep, abrasive cleaning is required.

Abrasive cleaning can be by hand with sandpapers or diamond pads.  Be sure to keep them damp.  This keeps dust from rising, and the sanding surfaces clean for better working.  Sandblasting can be quicker, especially on uneven surfaces or where there are deep imperfections.  The surfaces resulting from abrasive cleaning need to be scrubbed clean with sufficient water, and then polished dry as for a finished piece.

It is possible to combine both these methods to be more certain of a shiny finish.  When combining, you need to do the abrasive cleaning first, then the wet cleaning and finally add the devitrification solution.

A fourth possibility is to sprinkle a fine but consistently thick layer of clear fine frit or powder over the piece.  This, when fused, provides the new surface concealing the devitrification below.  Again, this must be done at a full fuse, so it is not applicable to items you wish to remain tack fused.

However, if the devitrification has progressed to a crazed appearance, it is so deep as to be almost impossible to reverse.  The piece will also probably have developed incompatibilities. So the only real option in crazed pieces is to dispose of them.  They will not be useable in combination with any other glass. They will make any glass with which they are combined subject to devitrification and possible breakage.  These are pieces which truly cannot be cut up and re-used.

Use of Untested Glass - Kiln Forming Myths 22

You must use art glass rather than recycled glass.

This seems to refer to the use of untested glass in kiln forming.  If you are going to use untested glass for kiln forming, it does not much matter which you use.  Because, in every case you will need to test for forming and annealing temperatures to be able to make use of the glass with unknown properties. 

Of course, people use glass that is not tested fusing compatible in many circumstances.  Float glass is frequently used in many kiln forming applications.  And bottle glass is of very little different in composition.  So-called art glass can be used in a variety of ways also.  There are many other variations of glass including handmade, casting, lamp working, and borosilicate, among others.  Each has their own set of characteristics, which overlap with each other.  The forming and annealing temperatures must be determined to enable you to use them. Some of this information is often available from the manufacturer’s web site or other sources.  Many times you have to do the testing for yourself.  One guide to help determine the critical temperatures is here. 

One characteristic that all untested glasses share is a tendency to devitrify by the second or third firing, so attempting to get the most work done in the fewest firings is a good idea.  This tendency to devitrify is frequently shown when manipulating bottle glass.

Crash Cooling - Kiln Forming Myths 20

Crash cooling will harm your kiln or break your glass.

Crash or flash cooling was often a requirement in the early days of fusing to avoid devitrification. The kilns used were ceramic ones that did not lose heat very quickly.  The glass also was more subject to devitrification than the glass being made now.  Since those early days, kiln design has advanced so the kilns lose heat more quickly, although still well insulated; and the glass is more resistant to devitrification.  Thus, crash cooling is no longer advised.

If you have a brick lined kiln, crash cooling is hard on the bricks.  The cold air causes rapid shrinking of the brick.  The more rapidly the brick heats and cools, the more fractures will develop in the brick.  This effect will take place over many firings before there is any noticeable damage to the structure of the brick.  However, if you have brick tops or lids, there is the increasing likely development of crumbs of brick falling onto your work.  Brick lids and tops should be vacuumed frequently to remove the crumbs as they form.

Crash or flash cooling from top temperature toward annealing temperature is unlikely to break any glass other than thick glass pieces.  However, when using glass formulated for kiln forming, you do not need to crash cool. The crash cooling may be more useful when using glass that is not formulated for kiln forming.  The purpose in this case would be the same as that for the early fusing – avoiding devitrification by moving as quickly as possible through the devitrification range.

Sometimes flash/crash cooling is required to fix a free drop in place.  If allowed to cool on its own, the glass will continue to move for a while.  If the extent of the drop is critical, crash cooling is required.  This should be to a point below the slumping but above the annealing temperature.  The flash cooling will cool the outer portions of the glass, stopping any further movement. Meanwhile the inner portions are still hot.  This sets up significant stresses.  By stopping the cooling just below the slumping temperature, you allow the internal and external temperatures of the glass to approach one another before going into the anneal soak where the temperature equalises throughout if the differentials are not too great from the flash cooling.

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”.

Rapid Rates of Advance to Avoid Devitrification - Kiln Forming Myths 18

Firing as fast as possible, or at least, very fast above annealing point will avoid devitrification.

Of course, this is true in one sense. Moving quickly through the devitrification range will reduce the time the glass has to crystallise – the action we call devitrification.

It will not on its own prevent devitrification.  Nowadays fusing compatible glass is formulated to resist devitrification during the firing.  However, devitrification still occurs during prolonged soaks at high temperatures, and slow rises or falls in the temperature range of 720°C to 760°C.  So you should always be trying to fire quickly through this range, whether up or down.

The contaminants that can form nucleation points for crystal growth can be oils from fingers, or cutters, residue from glass cleaner or refractory fibre papers, or even dust. 

This means the first line of defence against devitrification is cleaning.  Cleanliness is next to perfect results in kiln forming.  Use glass cleaners without additives.  In the UK, Bhole produce excellent glass cleaners.  In the USA, Spartan glass cleaner is recommended by Bullseye.  These may be better than clean water if your water supply contains a lot of minerals or additives for health purposes.

If you feel the need to make your own cleaning fluid do not use denatured alcohols such as rubbing alcohol.  They contain additives which may leave residues.  Use something like isopropyl alcohol and distilled water.

The drying of the glass should be accomplished with a thorough buffing to squeaky clean with plain paper towels or lint free cloths that have been washed without softeners in the washing.

The burn off of organic binders in fibre papers can produce enough residue to affect your glass, so it is best to keep your kiln vented until the burnout has completed – around 400°C.

To prevent dust settling on your pieces, clean and place into the kiln immediately.  If this is not possible, make sure the surface is well covered until placing in the kiln.

Borax Characteristics

Borax is a glass making flux used to reduce the melting temperature of glass. 

It is almost colourless - grey, white, or yellowish; seldom bluish or greenish; and colourless in transmitted light.

The chemical composition of Borax is:  Na₂(B₄O₅)(OH)₄ · 8H₂O

It has a hardness rating of 2 – 2.5, about half that of glass at approximately 5.5.

The melting point is 878°C. At this temperature borax dissolves numerous metal oxides. In spite of this high melting temperature, it acts as a flux reducing the softening point at the surface of the glass at kiln forming temperatures.

The specific gravity of borax is approximately 1.7, considerably ligther than glass at ca.2.5.

Borax is sparingly soluble in cold water, although readily soluble in boiling water. It is insoluble in ethanol.

Description of devitrification
Temperature range

Devitrification spray

Float Glass

A question about sharp raised points on the corners of a square bubble plate made of window glass is the occasion to discuss some characteristics of float glass. 

It is necessary with float to find out which is the tin side and which is the air side. The tin layer of the glass produces a bloom that resembles devitrification when compressed. Put the tin side down for a slump.  If you slump with the tin side up, you will create a tin bloom by compressing the tin. If the tin is on the bottom, you will be stretching the tin and so avoid the tin bloom.

Sharp, pointed and raised corners are the result of devitrification.  Devitrification is the crystallisation of glass. Mild devitrification appears to be dirty streaks across the surface. Extreme devitrification produces a crumbling glass surface. Raised, sharp corners are the result of intermediate devitrification. The tin side does not protect against devitrification.  It does provide a separating action when against the shelf, although kiln wash is still needed.  Float glass devitrifies easily. I have only ever been able to get two firings without devitrification.

Cleaning is of great importance in avoiding devitrification. Clean well with only a little detergent, rinse and then polish dry with paper towels. Any residues left on the surface will promote devitrification.

A general way of reducing sharp corners is to nip or round the corners with diamond pads. I nip the corners - it is quicker and does not leave any microscopic pits for devitrification formation.

Paint, stains and enamels will interact with the tin to produce variants of the colours.  Stains most often become darker than when put on the air side. Powder, frit and mica will not usually react to the tin.

Remember, float glass is not manufactured to be a kiln forming glass.  You will always be at risk of devitrification.

Tack Fuse Temperature

The tack fuse range is around 730C – 780C. This will give a graduation in profile from the very sharp, almost barely laminated, to one very rounded almost flat. Choosing the right heat for the right profile is one of balancing several elements: temperature, time, speed.

Low temperature, high tack fuse

If there were no other considerations, you could go slowly up in temperature and peek in at infrequent intervals until the right profile had been achieved. However this tack fusing is happening in the devitrification range, so slow rises in temperature are not advisable.

Medium temperature, mid tack fuse

So an alternative strategy would be to go quickly through the devitrification range (700C to 760C) and soak for a bit longer above that range. However, often the desired profile may has disappeared by the time you get to 770C.

High temperature, rounded tack fuse

It would seem that you can attempt to balance the temperature, time and speed equation by firing quickly (such as 330C/hr) to your desired temperature and soak there for 10 minutes only.

To ensure you get the profile that you want you should begin to observe from at least 10C below your chosen temperature. If you do not get the profile you want, you can extend the soak until the desired effect is achieved. On a subsequent firing, you can set the top temperature a bit higher, but with the 10 minute soak and again observe. This can be repeated until the desired combination is achieved.

Each of these attempts needs to be completely recorded so that the results can be used in later firings if slightly different profiles are needed.

Also look at this entry for annealing of tack fusings.

Recognising Devitrification

The appearance of devitrification varies from mild streaks as a dirty appearance on the surface, to at worst a granular surface that breaks away in small pieces. The glass will often have raised sharp corners in cases of severe devitrification.

Avoiding devitrification relates to cleaning, firing rapidly through the devitrification range, avoiding devitrification-prone glasses, and grinding edges as little as possible.

Repairing devitrification requires the removal of the devitrified surface. This can be done by sandblasting, sanding the surface by hand, using acid pastes to remove the surface. Then the piece needs to be fired again to a fire polish.

To ensure a polished surface a devitrification solution may need to be applied. It can be a commercial product or a borax solution.  Any devitrification solution should be applied evenly.

Organic Burnout Marks

Occasionally there is a haze at the centre of the back of large pieces of fired glass. This seems to happen when a large piece of glass is placed over fibre paper (of whatever thickness) that has not been pre-fired. 

 This is based on my experience of doing large pieces on thinfire or other fibre paper with a relatively fast rate of advance. What seems to happen is that the edges of the glass soften enough and early enough that not all the binder in the fibre papers can burn out and the combustion gasses escape from under the glass. The resulting haze is the remnants of the combustion product fired to the surface of the glass.

I have found that flipping the piece over and taking the glass to a low temperature fire polish is enough to return the glass to its usual appearance. You can, for extra insurance, apply a devitrification spray, although I have not found it necessary.

You could, of course, work the back of the glass with pumice and cerium oxide to bring back the original shine without firing. But my impression is that the areas with haze are fractionally depressed into the back surface. This means that a lot of glass has to be removed to reach and polish the hazy areas.

Pink Confetti

Because confetti needs to be so heavily saturated with colour, some of the opalescent colours tend to devitrify. The pink is particularly prone to devitrification. There are several ways to prevent this: 

• cap (which can lead to bubbles), 
• add a devitrification spray, or 
• cover with clear powder or frit.

Covering completely with a fine layer of powder gives the most even result. Using frit can provide a speckled appearance that is useful in some circumstances.

This tendency of pink opal to devitrification applies to all formulations – Bullseye, Uroboros, S96 and float.