Kilnforming Opalescent Stained Glass

The statement that a sheet of glass can be fused to itself is true in certain circumstances.  It applies to transparent and some streaky glasses best.  These forms of glass are more likely to fuse together successfully although not formulated for fusing.

Transparent and Streaky Glasses

Of course, the best practice is to test for compatibility.  I found in my early days of sticking stained glass together that it was beneficial to test. In doing so, I found Spectrum and Armstrong transparent and streaky glass to be largely consistent across many sheets.  I did not have access to much Kokomo or Wissmach.  I cannot comment on how their glass behaves in terms of compatibility across the production range.  Not all transparent and streaky glass remains stable at fusing temperatures. There are some glasses that opalise, some change colour, some devitrify. This variability makes compatibility testing important - even for the transparent form of stained glass.

Photo credit: Lead and Light

Wispy Glasses

The statement about fusing to itself is less applicable to wispy glass.  Not all the wispy stained glass from the same sheet can be fused.  It seems to be dependent on the amount of opalescence in any one area of the glass.  I found that it is possible - if you are very careful - to fuse certain Spectrum wispies with the clear fusing standard on top, but not on the bottom.  This should be applicable to other manufacturers’ wispy glass too.  There must be a marginal compatibility that is contained by the clear fusing glass on top, but I am not certain.

Photo credit: Lead and Light

Opalescent Glasses

The statement about fusing to itself is almost completely inapplicable to opalescent glass.  Stained glass opalescent glass does not have the compatibility requirements of fusing glasses.  They very often severely devitrify when taken to fusing temperatures.  This devitrification means that opalescent stained glass is often not compatible with itself.  So, no amount of twiddling with schedules will make stained glass opalescent glass fusible, even with itself.

Manufacturers have spent a lot of time and effort to produce fusing compatible opalescent glass.  It is as though there is a minor element of devitrification embodied in the opalising process.  Whether this is so, it becomes very apparent on doing compatibility testing that opalescent stained glass has severe devitrification at fusing temperatures.

Stock photo

Compatibility Testing

It is important to test for compatibility before committing to the main firing.  Some transparent and streaky glass changes colour, devitrifies, and some opalise at fusing temperatures. This applies with even more force to wispies.  They contain a significant proportion of opalescence within them.  Some opalescents are so unstable at fusing temperatures that the devitrification becomes so bad the glass crumbles.

The importance of testing pieces of the sheet for compatibility before committing to a firing is reinforced by these factors.

Slumping

Slumping temperatures are not so high as fusing, and it is often stated that single layers can be slumped.  Again, it is not always true.

Some glasses change colour at slumping temperatures.  A few opalise. It is not always certain what effect moderate temperatures will have on stained glass.  The compatibility testing will show.  Observe the test firing at slumping temperatures.  Also, you will learn if there are changes at moderate temperatures.

One element must be commented upon about slumping.  It is important to have the edges finished to the appearance that you want the final piece to have.  The regularity of the edges without bumps or divots, and the degree of polish need to be showing before the firing starts.  The slumping temperatures are not high enough to alter the shape or appearance of the edges.

Firing of stained glass to itself is normally a low risk activity, but with unpredictable results.  It can teach a lot about behaviour of glass at higher temperatures.  Slumping single layer pieces can give information about the way single layers of glass slump or drape.  But testing is important for fusing.  And can inform about how the glass will react at slumping temperatures too.

CoE as the Determinant of Temperature Characteristics

Many people are under the impression that CoE can tell you a wide number of things about fusing glass. 

What does CoE really mean?

The first thing to note is the meaning of CoE.  Its proper name is the coefficient of linear expansion.  It tells you nothing certain about the expansion in volume, which can be as or more important than the horizontal expansion. 

It is an average determined between 20°C and 300°C.  This is fine for materials that have a crystalline structure. Glass does not.  Glass behaves quite differently at higher temperatures. 

It may have an average expansion of 96 from 20°C-300°C – although there is no information on the variation within that range – but may have an expansion of 500 just above the annealing point. 

The critical temperatures for glass are between the annealing and strain points.  One curious aspect to the expansion of glass is that the rate of expansion decreases around the annealing point.  The amount of this change is variable from one glass composition to another.

The CoE of a manufacturer’s glass is an average of the range which is produced.  Spectrum has stated that their CoE of their fusing compatible glass is a 10 point range.  Bullseye has indicated that their CoE range is up to 5 points. These kind of ranges can be expected in every manufacturer’s compatible glass.

CoE does not tell us anything about viscosity, which has a bigger influence on compatibility than CoE alone. 

Comparison of CoE and Temperature

Among the things people assume CoE determines is the critical temperatures of the strain, annealing and softening points of various glasses.

Unfortunately, CoE does not necessarily tell you fusing or annealing temperatures. 

“CoE 83”

Most float glass is assumed to be around CoE 83.  The characteristics depend on which company is making the glass and where it is being made.

Pilkington float made in the UK has an annealing point of 540°C and a softening point (normally the slump point) of 720°C.

Typical USA float anneals at 548°C and has a softening point of 615°C.

Typical Australian float has a CoE of 84 and anneals in the range 505°C -525°C.

“CoE 90”

Uroboros FX90 has an annealing point of 525°C compared to Bullseye at 482°C, and Wissmach 90 anneal of 510°C. 

Wissmach 90 has a full fuse temperature of 777°C compared to Bullseye's 804 - 816°C.   

There is a float glass with a CoE of 90 that anneals at 540°C and fuses at 835°C.

Bullseye has a slump temperature of 630°C-677°C and Wissmach’s 90 slumps between 649°C and 677°C, slightly higher.

“CoE 93”

Kokomo with an average CoE of 93 has an annealing range of 507°C to 477°C. Kokomo slumps around 565°C

“CoE 94”

Artista with a CoE of 94 has an annealing point of 535°C and a full

fuse of 835°C, almost the same as float with a Coe of 83. 

“CoE96”

Wissmach 96 anneals at 482°C with a full fuse of 777°C and a slump temperature of 688°C.

Spectrum96 and its successor Oceanside Compatible anneals at 510°C and full fuses at 796°C.

Conclusion

In short, CoE does not tell you the temperature characteristics of the glass. These are determined by several factors of which viscosity is the most important. More information can be gained from this post or from your own testing and observation as noted in this post.

Re-firing

A frequently asked question is “how many times can I re-fire my piece?”

This is difficult to answer as it relates to the kind of glass and the firing conditions.

Kind of glass

Float glass is prone to devitrification. This often begins to appear on the second firing. Some times it may be possible to get a second firing without it showing. Sandblasting the surface after getting devitrification will enable another firing at least.

Art glass is so variable that each piece needs to be tested.

Fusing glasses are formulated for at least two firings, and experience shows may be fired many of times. The number will depend on the colours and whether they are opalescent. Transparent colours on the cool side of the spectrum seem to accept more firings than the hot colours. Both of these accept more firings than opalescent glasses do.

Firing conditions

Temperature

The higher the temperature pieces are fired at, the fewer re-firings are possible. So if multiple firings are planned, you should do each firing at the lowest possible temperature to get your result. This may mean that you have relatively long soaks for each firing. The final firing can be the one where the temperature is taken to the highest point.

Annealing

You do have to be careful about the annealing of pieces which have been fired multiple times. A number of people recommend longer annealing soaks. However, I find that the standard anneal soak for the thickness is enough. What is required is cooling rates directly related to the anneal soak.  This is a three-stage cooling as described in the Bullseye chart Annealing Thick Slabs.  The slump firing can be annealed at  the standard. 

Slumping

In general, slumping is at a low enough temperature to avoid any creation of additional stress through glass changes at its plastic temperatures.  But any time you heat the glass to a temperature above the annealing point, you must anneal again at least as slowly as in the previous firing. Any thing faster puts the piece at risk of inadequate annealing.  Of course, having put all this work and kiln time into the piece, the safest is to use the cooling rate as for a piece one layer thicker.  My research has shown that this gives the least evidence of stress.

Testing

Testing for stress after each firing will be necessary to determine if there is an increase in the stress within the piece. In the early stages of multiple firings, you can slow the annealing and if that shows reduced stress, it will determine your previous annealing schedule was inadequate. When reducing the rate of annealing does not reduce the stress, it is time to stop firing this piece at fusing temperatures.

Revised 6 May 2023

Digest of Principles for kiln forming

Some time ago people on a Facebook group were asked to give their top tips for kiln forming.  Looking through them showed a lot of detailed suggestions, but nothing which indicated that understanding the principles of fusing would be of high importance.  This digest is an attempt to remind people of the principles of kiln forming.

Understanding the principles and concepts of kilnforming assists with thinking about how to achieve your vision of the piece.  It helps with thinking about why failures have occurred.

Physical properties affecting kiln work

 Heat

Heat is not just temperature. It includes time and speed.

 Time

       The time it takes to get to working temperatures is important.  The length of soaks is significant in producing the desired results.

 Gravity

       Gravity affects all kiln work.  The glass will move toward the lowest points, requiring level surfaces, and works to form glass to moulds.

 Viscosity

       Viscosity works toward an equilibrium thickness of glass. It varies according to temperature.

 Expansion

       As with all materials, glass changes dimensions with the input of heat.  Different compositions of glass expand at different rates from one another, and with increases in temperature.

 Devitrification

       Glass is constantly tending toward crystallisation. Kiln working attempts to maintain the amorphous nature of the molecules.

 Glass Properties

·        Glass is mechanically strong,

·        it is hard, but partially elastic,

·        resistant to chemicals and corrosion,

·        it is resistant to thermal shock except within defined limits,

·        it absorbs and retains heat,

·        has well recognised optical properties, and

·        it is an electrical insulator. 

These properties can be used to our favour when kiln working, although they are often seen as limitations.



Concepts of Kiln Forming

 Heat work

       Heat work is a combination of temperature and the time taken to reach the temperature.

 Volume control

       The viscosity of glass at fusing temperatures tends to equalise the glass thickness at 6-7mm. 

 Compatibility

       Balancing the major forces of expansion and viscosity creates glass which will combine with colours in its range without significant stress in the cooled piece.

 Annealing

       Annealing is the process of relieving the stresses within the glass to maintain an amorphous solid which has the characteristics we associate with glass.

 Degree of forming

       The degree of forming is determined by viscosity, heat work and gravity.  These determine the common levels of sintering, tack, contour, and full fusing, as well as casting and melting.

 Separators

       Once glass reaches its softening point, it sticks to almost everything.  Separators between glass and supporting surfaces are required.

 Supporting materials

       These are of a wide variety and often called kiln furniture.  They include posts, dams, moulds, and other materials to shape the glass during kilnforming.

 Inclusions

       Inclusions are non-glass materials that can be encased within the glass without causing excessive stress.  They can be organic, metallic or mineral. They are most often successful when thin, soft or flexible.

A full description of these principles can be found in the publication Principles for Kilnforming

Characteristics of Some Glasses

This information has been taken from various sources. Some manufacturers may change the composition of their glasses or the published information about them from time to time. Therefore, this information can only be used as a guide. If the information about strain, annealing, and softening points is important, contact the manufacturer for the most accurate information.

The temperature information is given in Celsius.

Strain point – the temperature below which no annealing can be done.

Annealing point – the temperature at which the equalisation soak should be done before the annealing cool.

Softening point – the temperature at which slumping can most quickly occur.

Armstrong – Now made by Kokomo

Typical Borosilicate – nominal CoE 32

Strain point – 510 - 535C / 951 - 996F

Annealing point – ca. 560C/1041F
Softening point - ca. 820C/1509F

Blackwood OZ Lead – nominal CoE 92

Annealing point - 440C/825F

Blenko – nominal CoE 110

Annealing point – 495C/924F

Bullseye – nominal CoE 90

Transparents

Strain point - 493C/920F

Annealing point - (532C)  Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 677C/1252F

Opalescents

Strain point - 463C/866F

Annealing point – (501C)  Note that Bullseye has changed this to 482C900F for thick items

Softening point - 688C/1272F

Gold Bearing

Strain point - 438C/821F

Annealing point - (472)   Note that Bullseye has changed this to 482C/900F for thick items

Softening point - 638C/1182F

Chicago – nominal CoE 92

Desag  Note that this glass is no longer produced

Artista – nominal CoE 94

Strain point – 480 - 510C / 897 - 951F

Annealing point – 515 - 535C / 960 - 996F

Softening point – 705 – 735C / 1302 - 1356F

Fusing range – 805 – 835C / 1482 - 1537

Float Glass (Pilkington UK)

Optiwhite

Strain point – 525 - 530C / 978 - 987F

Annealing point – 559C/1039F

Softening point – 725C/1338F

Optifloat

Strain point – 525 - 530C / 978 - 987F

Annealing point – 548C/1019F

Softening point – 725C/1338F

Float Glass (typical for USA) nominal CoE 83

Strain point - 511C/953F

Annealing point - 548C/1019F

Softening point – 715C/1320F

Float Glass (typical for Australia) nominal CoE 84

Strain point - 505-525C / 942 - 978F 

Annealing point – 540 -560C / 1005 - 1041F

HiGlass “GIN” range – nominal CoE 90

Annealing point - 535C/996F

Gaffer colour rod – nominal CoE 88

Gaffer NZ Lead – nominal CoE 92

Annealing point - 440C/825F

HiGlass

Annealing point - 495C/924F

Kokomo – nominal CoE 92 - 94

Cathedrals

Strain point - 467C/873F

Annealing point - 507C/946F

Softening point - ca. 565C/ca.1050F

Opal Dense

Strain point - 445C/834F

Annealing point - 477C/891F

Softening point – ca. 565C/1050F

Opal Medium

Strain point - 455C/834F

Annealing point - 490C/915F

Softening point – ca.565C/1050F

Opal Medium Light

Strain point - 461C/863F

Annealing point - 499C/931F

Softening point – ca.565C/1050F

Opal Light

Strain point - 464C868F

Annealing point - 502C/937F

Softening point – ca.565C/1050F

Kugler – nominal CoE

Annealing point - 470C/879F

Typical lead glass – nominal CoE 91

Lenox Lead – nominal CoE 94

Annealing point – 440C/825F

Merry Go Round – nominal CoE 92

Moretti/Effetre – nominal CoE 104

Strain Point: 448C/839F

Annealing Range: 493 – 498C / 920 - 929F

Softening Point: 565C/1050F

Pemco Pb83 – nominal CoE 108

Annealing point – 415C/780F

Schott Borosilicate (8330) nominal CoE 32

Annealing point - 530C/987F

Schott “F2” Lead – nominal CoE 92

Annealing point - 440C/825F

Schott “H” & “R6” rods - nominal CoE 90

Annealing point – 530C/987F

Schott “W” colour rod – nominal CoE 98

St Just

MNA

Strain point - ca.450C/843F

Annealing point – ca. 532C/ca. 991F

Spectrum

System 96 – nominal CoE 96

Transparents

Strain point – 476C  +/- 6C  /  890F +/- 11F

Annealing point – 513 +/- 6C  /  956C +/- 11F

Softening point – 680 +/- 6C  /  1257F +/- 11F

Opalescents

Annealing point – 505 -515C  /  942 - 960F

Spruce Pine 87 – nominal CoE 96

Annealing point – 480C/897F

Uroboros system 96 – nominal CoE 96

Transparents

Strain point - 481C/899F

Annealing point - 517C/964F

Opalescents
Strain point - 457C/855F
Annealing point - 501C/935F

Uroboros - nominal CoE 90

Transparents

Strain point - 488C/911F

Annealing point - 525C/978F

Opalescents

Strain point - 468C/875F

Annealing point - 512C/955C

Wasser - nominal CoE 89

Annealing point – 490C/915F

Wissmach
Wissmach 90
Annealing point - 483C/900F
Softening point - 688C/1272F
Full Fuse - 777+

Wissmach 96
Annealing point - 483C/900F

Softening point - 688C/1272F

Full Fuse - 777+ / 1432+

• 
  

Placing of glues

The placing of glues to hold the glass pieces temporarily is important.  Often unsightly black marks appear due to inappropriately placed glues.  Bubbles can form between layers  and even appear to come from underneath the glass for the same reason.  Placing is often more important than the amount of glue used.  Still, the amount used should be the minimum to hold the glass from moving from bench to kiln.

Place glues at edges of the pieces to be secured during movement.  This allows the burn-off of the glue to evaporate without being trapped under the glass.  If you use very runny or diluted glues, the capillary action will draw the required amount of glue under the glass piece to form a secure adhesion.

Glues burn off and leave the glass pieces unsecured long before the glass becomes tacky enough to stick together.  This means that if your stack of glass will not stay in place without glue as you build it, the glass will collapse or move in the kiln.  Glues are only suitable to stabilise the glass pieces while moving to the kiln.

Two recommended glues that burn off cleanly are the Bullseye Glasstac (more fluid) and the Glasstac gel (more viscous)

Uneven Slumps

A common problem in kilnforming is that the glass slumps into the mould unevenly. Several of reasons are given in this post about high temperature or fast slumps for uneven results.

There are two other things that can be done to alleviate uneven slumps.

Place the mould in the centre of the kiln to reduce any uneven heating of the glass.  Uneven heating is a common cause of off-centre slumps.  Where you have persistent uneven slumping with a mould it may be better to fire it on its own so the conditions can be best for it.  Sometimes it is more economical to fire a single item rather than a crowded kiln shelf where the firing conditions must be for an average rather than the optimal firing schedule and conditions for one mould.  Less of the resulting slumped glass is disappointing.

There is an alternative. Cut the glass so the fused piece will be slightly smaller than the mould top. This will allow the glass to sit inside the mould rather than on top. Frequently there is evidence of the glass hanging up on the side of a mould.  Sometimes there are spikes where the glass stuck and stretched. (Another reason for Low and Slow)

A third method has been suggested, but I have not tried it.  This is to lightly bevel the underside of the piece to be slumped.  The basis for this suggestion is that a bevelled edge will fit the mould better by having a slope rather than a relatively sharp edge resting on the mould surface.  I do know the other two suggestions work, but not this one, although it sounds logical.

More detailed information is available in the e-book: Low Temperature Kilnforming.

Slumping sizes

A question about slumping into a bowl mould.

·        Will the end result always be that the glass drops down into the mould and so the bowl size is smaller than the mould?

·        Does the firing schedule affect the outcome?

You need to think about what is happening to the glass in the slumping process to understand what the outcome of a slump will be.

During slumping, the glass is being bent rather than stretched (very much) into the mould.  In other, higher temperature processes the glass is being both stretched and thinned.

The reason for using low temperatures is to reduce the thinning effect while obtaining the shape of the mould.  If you use higher slumping temperatures, the glass will slide down the mould more than at low temperatures, and will begin to thicken, or create an uprising, near the lower portion of the mould.

Take the simple case of a ball mould. One which has no flat bottom but a simple curve from edge to edge.

If the glass was 300mm diameter when flat, it will still be (approximately) 300mm from edge to edge on the bottom.    If you put a flexible tape measure around the bottom from one side to the other, it will be very near the 300mm diameter around the curve.

This means the new diameter of the rim will be smaller than the flat diameter.

Steep moulds have less change in diameter, but greater change in the height of the piece in relation to the mould.  Again, the glass measured from side to side on the bottom will be about the same as the flat piece.  To achieve this, it will slide down into the mould more than a shallow one.

More detailed information is available in the e-book: Low Temperature Kilnforming.

Texture Mould Firings

Texture moulds are essentially permanent kiln carving moulds.  These are moulds that use different levels within the mould to develop the imagery by giving different thicknesses to the glass. Temporary or single use moulds can be made from fibre paper, although not with the same subtlety as the ceramic ones.

Single layers

Many people wish to use a single layer in these texture moulds. For a single layer, a tack fuse is a high as you can take the temperature. This will not give you the definition that you could get with higher temperatures unless you use very long soak times. 

Good definition

To use higher temperatures, you need at least two layers (6mm) to avoid distortion, dog boning, possible bubbles and needling.  With two layers you can go to full fuse temperatures to get the best conformation to the mould.

Low temperature firings

You can get better definition at lower temperatures by going slowly to your target temperature. This slow rise in temperature – ca. 100C - or less - per hour – all the way to the lower end of the tack fuse range – ca. 730C to 750C -  needs to be combined with a long soak, possibly two or more hours.  This long soak allows the glass to sink into the depressions of the mould without shrinking, dog boning or needling.  This shows that the speed you use has a major effect on the target temperature.

Separators

Another element of difficulty in the use of these moulds is the separator used. Kiln wash is adequate, especially if you are using the lower temperatures.  Boron nitride is a popular choice for those going to higher temperatures.  Using iridescent glass with the coated side down to the mould provides an additional separator, allowing higher temperatures to be used.  It can enhance the appearance of the piece too. 

The way you schedule for texture moulds is an interaction between the thickness of the glass, the rate of advance, the target temperature and the kind of separators used. With these four factors interacting, the choices are not simple.

More detailed information is available in the e-book: Low Temperature Kilnforming.

Gravity

One of the fundamental elements in kiln forming is gravity. When glass is hot it moves according to the effects of gravity and you have to remember that gravity has a big effect on all your firings.

The effects mainly cause:

• Uneven thickness on shelves that are not level.
• Uneven slumps into moulds which are not level or the glass is not levelled.
• Uneven forming due to varying viscosities. Gravity acts on the softest parts of the glass first.
• Faster or slower forming due to span width. With greater span, gravity pulls the glass into the mould more quickly than with a small span.
• Gravity acts on things of greater thickness more quickly than those of lighter weight. So a thick piece will form more quickly than the same sized thin piece.
• Surface tension (affected by viscosity and heat) is affected by gravity also. The glass will attempt to draw up or spread out to about 7 mm if there is enough heat, time, and low viscosity.
• The effect of gravity causes upper pieces to thin lower ones, as it presses down while the glass is plastic. This has the effect of making the colour of the lower piece less strong.

More information on each of these effects can be found throughout this blog.