Visible Devitrification

"Why does devitrification appear on slumped pieces?"

A brief explanation 

Scientific research in developing a glass matrix to support bone grafts gives some information.  This kind of glass matrix requires to be strong.  Development showed that devitrification weakens the matrix.  The crystals in a matrix are not as strong as the amorphous glassy state.  So, devitrification needs to be avoided.

The research to avoid devitrification showed that it begins at about 600˚C/1110˚F.  It only begins to become visible above 700˚C/1300˚F.  The process developed was to introduce a “foaming” agent.  The process fired slowly to 600˚C/1110 ˚F and then quickly to 830˚C/ 1530˚F.  It left a strong open matrix around which bone can grow. Although the research used float glass, it is also a soda lime glass, just as fusing glasses are.  The formation of devitrification begins at the same temperature for fusing glasses as for float.

The result of this medical research shows that devitrification begins on glass before it is visible. Devitrification is cumulative. A little becomes greater with another firing.  This is so even with good cleaning between firings. The new devitrification builds on the previous.  It does this from 600˚C/1110 ˚F.

A subsequent firing can continue this devitrification to the point where it is visible. This can happen, although the temperature at which we can see it after one firing has not been reached.  This continued devitrification at low temperatures can become great enough to be visible at the end of one or multiple slumps.

Credit: Bullseye Glass Co.

What can we do?

Clean all the glass before every firing very well.

·         Avoid mineralised water.

·         Final clean with isopropyl alcohol.

·         Polish dry at each stage with white absorbent paper.

 

Soak longer at lower temperatures.

·         Use longer soaks to achieve the slump.

·         Keep the temperature low.

·         Observe the progress of the firing with quick peeks.

 

Use slower ramp rates.

·         Slower rates enable the heat to permeate the glass.

·         Enables a lower slump temperature.

 

If there is any hint of devitrification after the first firing,

·      use a devitrification spray, or

·      provide a new surface.

• o   remove the surface by abrasion on sandblasting,
• o   cap with clear, or
• o   cover whole surface with a thin layer of clear powder.

·      Fire to contour fuse to give a new smooth surface.

·      Clean very well and proceed to slump.

Kilnforming with 3mm Glass

 A power point presentation I made a few months ago to the group Lunch with a Glass Artist.

It is 33 slides long.

Kilnforming with 3mm Glass.pptx

Flows

 

Credit: Marcy Berman

I have not had much success [with] the Patty Gray mould despite using the recommended firing schedule. I always have holes or bubbles and the edges are not smooth.

The schedule for Oceanside was:

• 111°C/200°F per hour to 537°C1000°F for 15 minutes
• 167°C/300°F per hour to 662°C/1225°F for 30 minutes
• 195°C/350°F per hour to 798°C/1470°F for 20 minutes
• 9999 to 510°C/950°F for 120 minutes
• 55°C/100°F per hour to 371°C/700°F off

 Your picture shows a bottom view of the piece - made of cullet pieces - as fired. Two large bubbles show to have been created from the bottom rising through the glass to the top.

 Although a long bubble squeeze will not prevent this, it will help to reduce the number of bubbles, and especially large ones. Because of the number of pieces and the thickness of the glass put into the mould, a longer bubble squeeze would benefit this piece.

 The bubble squeeze can be as you have done this – at a single temperature – with a soak. In this case, I would have used 60 to 90 minutes as the soak.

 The other bubble squeeze method is to start the squeeze about 55C/100F below the top of the bubble squeeze. Most people use a soak of about 30 minutes there. They then proceed at a rate of between 30C/55F and 55C/100F to the top of the bubble squeeze and soak there for another 30 minutes. The rates and soak times will vary according to the thickness or complexity of the piece.

 I dispense with the soak at the beginning of the bubble squeeze on the grounds that at 610/1130F so little movement will be created that it is a waste of time. I would prefer to have a slower ramp rate to the top temperature and a longer soak there. I know the glass will be moving at those temperatures. Many people find the soak at the beginning of the bubble squeeze successful.

 The schedule to the top of the fuse is faster than the rest of the schedule. When I want a piece to flow, and especially, to fill gaps, I slow the rate. In this case a rate of between 100C/180F and 167C/300F would be slow enough to allow the glass to flow to fill gaps.

 I want to ensure the glass has enough time when it is flowing most freely at the top temperature to level out. This requires scheduling a longer soak at the top and observing how well the glass is levelling out. If more time is required you can add it on the “run,” and advance to the next segment when the surface is as wanted. Read up in your kiln manual how to do both these things.

 Yes, the rate is one which will enable devitrification to form on flat glass. The soak at top temperature is even more likely to promote it. However, as the glass is flowing, less devitrification has an opportunity to form. The crystallisation – which is what devitrification is - of the glass takes time to form. The movement of the glass surface is sufficient to reduce the formation of those crystals. It is of course likely there will be some devitrification, but not as much as the slow rates and long soaks would lead you to think. 

 But for these flows there always is the possibility of devitrification. You have to plan a method of removing it. Unless the surface is very flat, grinding the top is not a fast way to remove it. Sandblasting is a quick way to remove devitrification. Another way is to sift a thin layer of clear glass powder over the surface. This is an increasingly popular way to deal with devitrification for those without access to sandblasting facilities. When fired again, the powder melts and forms a new shining surface. The piece will need to be fired fire again whether sandblasted or covered in glass powder.

The summary for flows:

• Slow down to top temperature.
• Give sufficient time there to get the flow needed.
• Observe the progress as you near the top temperature.
• Extend the soak or advance to the next segment when the surface is smooth.
• Anneal soak for the calculated thickness.
• Use a three-stage cool – as outlined in the Bullseye chart for annealing thick slabs - to ensure no temporary contraction stresses are created.
• Accept there will be devitrification.

The Mechanism of Sintering

 "Do glass molecules actually migrate when they are sintered together? "

Sintering occurs at the atomic level, where the atoms at the edge of the particles attach to others in other particles. An analogy occurs to me of Scottish country dancing. In big gatherings, small groups are formed to perform the dance, say an eightsome reel. As the dance goes on the groups become more coordinated and eventually form one large group, held together by the people on the edges of each group.

A more scientific description comes from Wikipedia:

“Sintering … is the process of compacting and forming a solid mass of material by heat or pressure without melting it. … The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum.”

Applied to glass this means that you can make a solid piece of glass out of multiple touching or overlapping pieces that do not change their shape. This uses low temperatures and very long soaks.

 Schematic-diagram-for-the-sintering-and-fusion-reaction-of-the-glass-frits-on-a-substrate.
Credit: ResearchGate

The usual process is to take the glass at a moderate rate up to the lower strain point. The rate of advance is slowed to 50°C or less to a temperature between slumping and the bottom of the tack fuse range.

The slow rate of advance allows a lot of heat work to be put into the glass. This, combined with a long soak (hours), gives the atoms of the molecules time to combine with their neighbours in other particles.

Sintering occurs in the range of 610°C to 700°C (1130°F to 1275°F). The lower limit is determined by the strain point of the glass and by practicality. The length of time required at the strain point - 540°C/1005°F - is so long (days) that it is impractical.

The upper limit is determined by the onset of devitrification. This has been determined by the scientific studies of sintered glass as a structure for growing bone transplants. Devitrification reduces the strength of the bonds of the particles at the molecular level. The process of crystallisation breaks the bonds already formed between the atomic structures of the molecules. These studies showed that the onset of devitrification is at 650°C/1204°F and is visibly apparent at 700°C/1292°F regardless of the glass used.

The lowest practical temperature for sintering is 650°C/1203°F. Indications are that at least an additional two hours are needed for the sinter soak for each 10°C/18°F reduction below 650°C/1203°F. This would make for a 12-hour soak at 610°C/1131°F. For me this is not practical.

More information on the kilnforming processes and sintering experimentation is available in this eBook: Low Temperature Kiln Forming.

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

Achieving Multiple Profiles in One Piece

People ask about whether it is possible to tack fuse additional elements without affecting the profile of the existing piece.

It is as though glass has a memory of the heat it has been subjected to.  For example, a sharp tack will become a slightly rounded tack, even though refired to a sharp tack again.  So, it is impossible to refire a piece to the same temperature or higher without affecting the existing profile.  But it is possible to fire a piece with differing profiles if you plan the sequence of firings.

 

Tack fuse onto existing profile

 

It is possible to add pieces to be tack fused with little distortion to the existing piece through careful scheduling and observation.  There are several requirements.

 •     A moderate rate of advance to the working temperature is required, rather than a fast one. This is because the piece is a single thicker piece with uneven thicknesses.  Also, a slow rise in temperature allows completion of the work to at a lower temperature.  This means there will be less change to the existing profile.

•     A minimal bubble squeeze - or none at all - is required on this second firing.  The added pieces generally will be small, so if possible, eliminate the bubble squeeze.  The requirement is to add as little heat work as possible.

 •     The working temperature should be to a low tack fuse temperature with a long soak. 

 •     Observation is required from the time the working temperature is achieved.  Peeking at 5-minute intervals is needed.  This to be certain that the current tack fuse can be achieved without much affecting the existing profile.  It will be a compromise that you will be able to choose during the firing.  The decision will be whether to retain existing profile and have a sharp tack.  Or a slightly rounded tack and more rounded profile on the original piece.

Planning for multiple levels of tack

It is possible to design a piece with multiple profiles within the completed piece.  You need to plan out the levels and degrees of tack you want before you start firing. 

To do this planning, you need to remember that all heat work is cumulative. In simple terms it means that on a second firing you will start where you left off with the first one. The texture in the first firing will become softer, rounded, or flatter than the second or even the third firing.

Three degrees of tack can be achieved with a little planning.  It works similarly to paint firings.  Some paints fire higher than enamels, and enamels hotter than stain.  You have to plan to fire all the tracing and shading first.  Then you add the opaque enamels, followed by the transparent enamels.  Finally, you add the silver stain.  This is unlike painting on canvas where you build up the image all together.

The same principle is true of a multiple level tack fuse piece.  When creating various profiles in glass, you proceed from firing the areas that will be the flattest first. Then proceed to the areas which will have the least tack last.  This is a consequence of the cumulative effect of heat on re-fired glass.

Plan out the areas that you want to have the least profile.  Assemble the glass for those areas. I suggest that a 6mm base is the initial requirement for anything that is going to be fired multiple times.  Add the initial pieces that will become a contour fuse or a very rounded tack. 

First firing

Put this assembly in the kiln and schedule.  Do not fire to the contour profile temperature.  Instead, you will be scheduling for a sinter or sharp tack. This depends on how many textures you plan to incorporate.  Start with a sharp tack.  Fire at the appropriate rate with a bubble squeeze to about 740°C for 10 minutes and proceed to the anneal cool.   Different kilns will need other temperatures to achieve a sharp tack.

You do not fire to the contour fuse temperature, because the base will be subject to more firings.  Each of these firings will soften the base layers more than the previous one.  This is the application of the principle of cumulative heat work.  When you fire a piece for a second time, there will be little effect until the softening point of the glass is reached. Once there, the glass further softens, giving the effect of a contour fuse.

Any glass that had already achieved contour profile from the first firing will flatten further.  This can be used in cases where the working temperature was not high enough.  Just fire again to the original schedule’s temperature.  Take account of the need for a slower ramp rate to the softening point.

Second firing

Once cool and cleaned, you can add your next profile level of tack fusing to the base.  Note that “level of tack” does not refer to thickness being built up.  It is about the amount of roundness you want to impart to the pieces.  You may be placing this second - sharper – level of tack in the spaces left during the first firing.  Again, schedule to the original approximate 740°C. But remember the base is now a single piece.  You need to slow the ramp rate to the softening point, after which the speed can be increased.  You will not need to retain the bubble squeeze unless you are adding large pieces, or into low areas. 

The second firing will show the pieces added for the second firing to have the profile of the original pieces.  Those pieces having their first firing will have a sharper appearance.

 

credit: vitreus-art.co.uk 

This is a piece where the flower petals and leaves could have been placed for the second firing to give a softer background with less rounded flower details.

 

Third firing

Clean well and add the pieces for the final level of tack.  Schedule the initial rate of advance a little slower than the second firing.  The piece is growing in thickness and complexity.  Once the softening point is reached, the original rate of advance can once again be used up to original temperature. 

Final firing

Clean well and add the pieces for the final level of tack.  Schedule the initial rate of advance a little slower than the second firing.  The piece is growing in thickness and complexity.  Once the softening point is reached, the original rate of advance can once again be used up to original temperature. 

Further notes on multiple firings

It is a good idea to observe the firing, once the working temperature is achieved.  This is to ensure enough roundness is being given to the final pieces being tacked to the whole.  Be prepared to extend the soak if the final pieces are not rounded enough.   Although you should have a good idea of the degree of tack for the final pieces from the previous two firings.

You may need to experiment a little with the temperature and length of soaks at the working temperature.  For example, if the degree of tack is too sharp in the first firing, you can extend the soak or increase the temperature for the next ones. 

If you are firing at 740°C, you may feel you can afford to extend the soak for the subsequent firings, because you are in the lower part of the devitrification range. Consider the risk of devitrification increases with the number of firings of the glass.  The preference is to increase the temperature a bit for subsequent firings to ensure you are not spending a cumulatively long time in the devitrification range but still be able to get the final tack level desired. 

The preference is to increase the temperature a bit for subsequent firings to ensure you are not spending a cumulatively long time in the devitrification range but still be able to get the final tack level desired. 

Because most of your heat work is happening in the low end of the devitrification range, the cleaning regime must be very thorough.  Any chemicals or soaps used must be completely washed off with clean water.  The piece must be polished dry to ensure there are no water marks left on the glass.

You can, of course, have more levels of tack.  One approach would be to start with a sinter, or tack to stick, firing. And repeat that four or more times.  Another is to increase the working temperature and reduce the length of time soaked there.  The shorter time means there is less rounding of each level, allowing the build-up of many levels of tack.  All of these require some experimentation. 

More information is available in the ebook Low Temperature Kilnforming.

Three firings to the same sharp tack profile will give multiple profiles in the finished piece. 

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

Sintering

This is a process used in glass to stick glass together without any change in appearance of the separate pieces.  It has various names - fuse to stick and lamination are two.

General description
“Sintering or frittage is the process of compacting and forming a solid mass of material by heat or pressure without melting it…. Sintering happens naturally in mineral deposits [and] as a manufacturing process used with metals, ceramics, plastics, and other materials.

“The atoms in the materials diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. Because the sintering temperature does not have to reach the melting point of the material, sintering is often chosen as the shaping process for materials with extremely high melting points such as tungsten and molybdenum….
 
“An example of sintering can be observed when ice cubes in a glass of water adhere to each other, which is driven by the temperature difference between the water and the ice.”
https://en.wikipedia.org/wiki/Sintering
 
Applied to glass this means that you can make a solid piece out of multiple touching or overlapping pieces that do not change their shape.  This is done by using low temperatures and very long soaks. 
 
The usual process is to take the glass at a moderate rate up to the lower strain point.  The rate of advance is slowed to 50°C or less to a temperature between slumping and the bottom of the tack fuse range.  The operator must choose the temperature, largely by experimentation. 
 
The slow rate of advance allows a lot of heat work to be put into the glass.  This, combined with a long soak (hours), gives the molecules time to combine with their neighbours in other particles.
 
Sintering can be done in the range of 610°C to 700°C.  The lower limit is determined by the strain point of the glass being used and practicality.  

The upper limit is determined by the onset of devitrification. This  has been determined by the scientific studies of sintered glass as a structure for growing bone transplants.  Devitrification reduces the strength of the bonds of the particles at the molecular level.  These studies showed that the onset of devitrification is at 700°C and is visibly apparent at 750°C regardless of the glass used.  Therefore, the choice was to use 690°C as the top sintering temperature. 
 
For reasons of practicality the lowest temperature tested was 650°C.  Indications were that at least an additional two hours would need to be added to the sinter soak for each 10°C reduction below 650°C.  This would make for a 12-hour soak at 610°C.  For me this was not practical.
 
My recent testing has indicated some guidelines for the sintering process:
 
The ramp rate has significant effects on the strength of the resulting piece. 

• A moderate rate (150°C) all the way to the sintering temperature needs a two-hour soak at the top temperature. 
  
• A rapid rate (600°C) - as used in medicine – to the sintering temperature requires approximately six-hours soaking.
• A rapid rise to the strain point followed by the slow 50°C per hour rate to the sinter temperature requires a three-hour soak.

 
The temperature range of 610°C to 700°C can be used for sintering.  The effects of the temperature used have these effects:

• With the same rates and soak times, lower temperatures produce weaker glass.
• The lower the temperature, the longer the sinter soak needs to be for similar strengths.  Generally, the soak at 650°C needs to be twice that of sintering at 690°C.
• Lower temperatures produce more opaque glass.  In this picture all the glass is clear powder and fine frit in the ratio 1:2, powder:frit.

 

The annealing of sintered objects needs to be very cautious. The particles are largely independent of each other, only joined at the contact points.  The annealing soak needs to be longer and the cool slower than for simple tack fusing. 

• Testing showed that annealing as for 12mm is adequate. 
  
• There was no advantage of annealing as for 25mm as that did not increase the strength.

 
Porosity
Although the structure of the sintered glass appears granular, it is not porous except at or below 650°C.  At the lower temperatures, the glass becomes damp on the outside and weeps water.  At 670° and 690°C the outside became cool to touch but did not leak water.  This observation depends on evenly and firmly packed frits.
 

Grain structure at 650C

Grain structure at 690C

The keys to successful sintering of glass are the use of a heat work through slow ramp rates, and long soaks throughout the whole firing.

Further information is available in the ebook Low Temperature Kiln Forming.

Recovering from Devitrification

An explanation of what devitrification is, can be found in the link.

Mild devitrification is generally a smeary appearance on the surface.  Most often this can be corrected by either removing the surface, adding a flux or putting another surface over the piece.

mild devitrification
photo credit: Bullseye Glass Co.

Removing the devitrified surface

Sandblasting and grinding are two common methods of removing the surface. If you have access to a sandblaster, this is the easiest method of removing the surface.  You can remove the surface with manual methods too.  You can use wet and dry sandpapers, starting with coarse ones and proceed through grades to at least 400grit (0.037mm).  The flexibility of the sandpapers is that they can conform to uneven surfaces that tack fusing provides, to remove devitrification in depressions as well as the high bits. Diamond hand pads and sheets do the job more quickly, but are more expensive.

Acid etching is another surface removal method. There are various etching creams on the market which will remove the surface. You need to apply and leave for a long time to allow the acid to work on the glass surface.  It is best to keep the acid paste damp to enable the acid to work over a long period.  A piece of cling film will work well.

Making a new surface

You can provide a new surface by using devitrification sprays.  There are both commercial products and do it yourself ones that work.  The do it yourself product is a borax solution.  The method for making the solution is given here.

Borax powder

You also can give the devitrified surface a new one by covering it with clear powders.  Powders sifted evenly over the surface until there is a thin covering over all the piece will give a new surface concealing or covering the devitrification.  Fine frit does not work so well, as more needs to be sifted over the surface.  This will not be applicable to tack fused pieces, as the whole piece needs to be taken to a contour or full fuse to make sure the powder or frit is completely smooth.  This will make the tack fused areas flat.

Left to right - devitrified surface, powder covering, fired piece
Photo credit: Bullseye Glass Co.

When dealing with devitrification, the whole of the surface should be treated, not just isolated areas.  Treating isolated areas will most probably leave a difference in appearance between the treated and untreated areas.  It is not worth the risk of having to fire yet again.

Dealing with devitrification usually involves removing the devitrified surface or making a new one.