Is Pate de Verre Watertight?

Clear frit sintered at 690C, 670C, and 650C (left to right)

"This is fascinating. I had no idea about the water leaving the glass at different temperatures."

 This comment was made in relation to some of my tests of pate de verre at different temperatures. I sintered glass at 620°C, 650°C and 690°C (1150°F, 1200°F, and 1275°F) to test for the strength of bonds at different temperatures and thicknesses.

 Because the glass appeared porous in some cases, I tested to see if vessels would be watertight at the different sintering temperatures. I found that at 620°C/1150°F the glass leaked water slowly. At 650°C/1200°F the water “sweated” out. At 690°C/1275°F it was watertight.

 It is not that the water leaves the glass. There is no water in glass. The question is whether the sintering is watertight at different temperatures. At the lower range, the sintered glass is porous; mid-range they sweat like unglazed pottery, but at the higher temperature they are watertight.

 Pate de verre is a form of sintering glass – normally in a mould. In pate de verre, a vessel needs to be fired at a higher temperature to be watertight. If a porous wall is acceptable, it can be fired to a lower temperature to preserve the granular appearance on the inside. The outside – which is in contact with the mould – will retain the granular appearance at all these temperatures. If the object is decorative, it can be sintered at a lower temperature which will preserve the brilliance of the colours.

 

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.

Sintering Ramps and Soaks

Sintering (or laminating) is a special form of low temperature kilnforming that requires attention to the ramp rates and the length of soaks. The rates and soak times were determined by the strength of the resulting pieces.

Credit: Researchgate.net

Rate

 The ramp rate has a significant effect on the strength of the resulting piece.

 A moderate rate (150°C/270°F) all the way to the sintering temperature of 690°C/1080°F gives the glass particles time to settle together. It works similarly to a slow ramp rate in slumping.

 A rapid rate (600°C/1275°F) - as used in medicine – to the sintering temperature of 690°C/1080°F is used for float glass particles.

 An alternative to both these is to schedule a rapid rise to the strain point followed by a slow - 50°C/90°F per hour - rate to the sinter temperature.

Soak

The soak time is extremely important in sintering to provide strong results. It is loosely related to the ramp rate, but in an inverse manner. The quicker the ramp, the longer the soak required.

 The moderate rate of 150°C/270°F needs a two-hour soak at the top temperature for maximum strength.

 The rapid rate of 600°C/1275°F requires about six hours of soaking at the top temperature.

 The alternative of a rapid rise to the strain point followed by the slow 50°C/90°F per hour rate requires at least a three-hour soak.

 These results show the ramp rate is important to the strength of the resulting piece. Fast ramp rates require increasingly long soaks at top temperature. Even slowing the ramp rate after reaching the strain point requires longer soaking than a steady rate. This is so even though the steady rate is faster than the two-part schedule to the top temperature.

 These results indicate that heat work is put into the glass throughout the temperature rise. The heat put slowly into the structure below the strain point still has an effect on the sintering of the glass.

 This is shown by the two-part schedule that has a slow ramp rate after the strain point. And even then, the time required is only 0.3hour shorter than for the moderate steady rise and soak. 

There is no time advantage to rapid rises to the strain point followed by a very slow rise to top temperature. The six-hour soak required by fast rises to top temperature show there is a large time disadvantage with rapid rise scheduling of sintering.

More information is available in the ebook Low Temperature Kilnforming and from Bullseye.

Preventing dog boning

Firing a single layer, even with decorative elements on top, is most likely to “dog bone” due to lack of volume.  With a single layer you are always going to have difficulties with volume control. 

Photo credit: Paul Tarlow

Unless you are satisfied with an angular tack fuse at the lower end of the tack fusing range, you will always run the risk of dog boning. All the other variations of tack fusing use increased temperatures causing the glass to begin to pull in along the long sides to a greater or lesser extent (more with contour fuse, less with angular tack fuse).

Dog boning occurs because as the glass softens and the edges begin to round, the viscosity takes over from the solid phase of glass as a major force.  Viscosity can be thought of as an approximation of surface tension. 

Glass is a material with a plastic range over several hundred degrees.  This means that the hotter the glass becomes, the less stiff it becomes, and the viscosity force thickens the glass toward 6-7mm in the kilnforming temperature range. The greater the temperature, the more the glass pulls into a ball shape, or in the case of sheets, thickens at the edges and thins in the middle.  Higher temperatures reduce the viscosity to the extent that it becomes as thin as one millimetre.

Trick the glass

To avoid dog boning on tack and full fusing, you have to trick the glass with some special scheduling.

The trick employs the concept of heat work.  The nature of glass allows you to put a lot of heat work into a piece by soaking for a long time at a low temperature.  You might think of it as a kind of sintering.

A description of sintering:

The atoms in the [glass] diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. [This can be done by heat at low temperatures with extended soaks.]  Examples of pressure-driven sintering are the compacting of snowfall to a glacier, or the forming of a hard snowball by pressing loose snow together.   https://en.wikipedia.org/wiki/Sintering

By sintering (sometimes called fuse to stick) or - in kilnforming terms - by the use of heat work you can achieve the result you want without dog boning.

By taking the temperature slowly to about 700°C to 720°C and soaking there for two to four hours you can achieve a rounded tack fuse without dog boning.  You will have to experiment with the exact temperature and length of soak to get exactly what you want.

The length of soak time or exact temperature is not vital.  The two in combination will achieve the effect you want.  The importance of observation of your firing is re-enforced in the cases of sintering.  You cannot be sure until you check during the firing whether the edges of the glass are rounded enough for your purpose. That observation will also tell you whether a slightly raised temperature would be useful.  You will learn the time required to achieve the effect by recording the soak time when you advance to the anneal soak and cool.

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

By the use of heat work in kilnforming you can achieve tack fused pieces without dog boning.

Tacking Freeze and Fuse to Base Glass

The question has been asked:

I'm wanting to add some freeze fuse pieces on to float and just fire to a tack fuse … in one firing instead of two …[to avoid] losing the detail on the freeze fuse pieces. The top temperature on freeze and fuse is 720°C versus a … float tack temperature of 787°C. [can this be done?]

My response:

What you are doing with the freeze and fuse process is sintering the glass particles together by holding at a low temperature for a very long time.  This binds the glass together without altering the overall shape of the object. 

Sintering

There is no reason why you cannot sinter the freeze and fuse piece on top of a base glass, if you pay attention to one major thing.  The freeze and fuse object will shade the heat from the base glass.  If you do not slow the rate of advance enough, you will break the base glass by creating too great a temperature differential between the part under the freeze and fuse piece and the uncovered part.

Another element to be considered, is that the frozen object is damp.  This will need to be dried by a slow ramp or it will further complicate the uneven heating problem.

Scheduling the Rate of Ramp

Choosing the rate of increase in temperature is determined by the dimensions of what is being sintered.  One widely practiced method is to double the total height and fire for that dimension.  For example, if the freeze and fuse is 8mm high, add that to the 6mm base and fire for 28mm – (6+8=14)*2 =28mm.

Another slightly less cautious approach is to multiply the total height by 1.5 and use the firing conditions for that thickness.

Determining the rate of advance for the thickness you have calculated – by either method - can be aided by using the Bullseye chart for annealing thick glass.  Look at the final cooling rate in the chart for the nearest thickness. In this case, use the one for 25mm.  The cooling rate is given as 90°C per hour.  If the glass can safely cool at that rate, it should also survive that speed of heating at the start.

If you chose the 1.5 factor, the thickness to schedule for will be 21mm.  This is between the 19mm and 25mm thicknesses given in the Bullseye chart.  The cooling rate given for 19mm is 150°C and and for 25 is 90C. As 21mm is almost the mid point between the two, you can halve the difference in rates (150 and 90) to give 120°C as the rate of advance. Although in both schedules using these rates of advance for the described circumstance, I would add a soak at 250°C for 20 minutes, to be cautious.

Remaining Parts of the Schedule

Sintering Soak

The length of soak for the sintering stage can be the same as the soak for the freeze and fuse, as you will be both sintering the glass pieces together and to the base glass too.

Anneal Cool

The annealing soak and cool should follow the rates given for the calculated thickness - in this case for 21mm or 28mm.

The Bullseye chart Annealing Thick Slabs can be used for all types of soda glass (which includes float glass) to determine the soak times and cooling rates.  You only need to make alterations for the annealing temperatures.  The annealing temperature I use for float glass is 540°C. 

The first two stages of cooling are 55C each, so simple subtraction from the annealing soak will give the temperatures for each stage of the cooling. If we use the calculated 21mm thickness, the soak time will be 3.5 hours at 540°C.  Then the Bullseye chart's displayed cooling rate of 20°C will apply from 540°C to 485°C, and the cooling rate of 36°C will apply from 485°C to 430°C. The final cooling rate of 120°C will be from 430°C to room temperature.  The chart for these adaptations is described in the post about adapting the Bullseye chart for annealing.  The reasons behind these operations are given in the ebook Low Temperature 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.

Making Thin Sheets

The question of how to make thin sheets arises from time to time.  Unless you are a glass manufacturer, it is unlikely you can make large, thin glass sheets.  But you can approximate making thin sheets by two methods that I know.

Sintering

One of these is sintering.  This is firing the glass to a low temperature and soaking for a long time.  The common form of this is powder wafers. 

By using a screen to deposit an even layer of glass powder you can make very thin, but delicate sheets of glass.  The procedure I would use is a screen of about 45 – 60 threads per inch.  This is coarse enough to allow the powder through, but not so fine as to “reject” large amounts of the coarser particles. 

You can screen the powder directly onto a kiln washed shelf, or onto Thinfire or Papyros.  You will not be able to move the unfired powder on a sheet of paper or fibre paper without changing the thickness and shape of the screened powder.  It must be laid down onto the separator directly on the shelf.  You can of course, move the shelf to the kiln if you can get in without tipping it.

Method

Support the screen about 3mm above the surface to allow the powder to fall through.

Make a ridge of powder at one end of the screen.  Using a smooth straight edge wide enough to cover the whole of the screen, lightly spread the powder from the starting end to the other. Then repeat drawing the powder to the starting end.  Make about five repeats of this – that is 10 passes, to get enough powder laid down to form about 0.5 to 1mm sheet.  You will need to experiment with the number of passes to get what you want.

Do not try to press the powder through the screen.  That will only wear the screen out quickly and may tear it.  Each pass should be a light spreading of the powder.  It is heavy enough to fall through the screen without additional force.

You could, of course, just sift the powder over the area you want to cover and judge by eye how even the layer is.  It is possible that your observation is good enough, but it is more likely that you will have thick and thin areas.  Often even at sintering temperatures, the thin is pulled toward the thicker, leaving small or large holes.   By screening the powder, you know you will have an even layer

Firing

The kind of schedule to use to sinter the glass particles together without changing their structure is the following:

220°C to 482°C , soak for 60 mins
55°C to 593°C, 10 minutes
28°C to 665°C for 5 mins
as fast as possible to 482°C for 30 mins
28°C to 427°C, no soak
55°C to 370°C, no soak
110°C to 50°C, no soak

This will work for most fusing glasses.

This slow firing allows enough heat to penetrate the glass grains that they will stick together without changing shape or developing holes.  I admit the anneal cool is very cautious.  You can experiment with quicker cools if you want to speed the process.

  

Pressing

This is a technique of thinning already existing sheets of glass.  Typically, you will have a 6mm or thicker piece of glass that you want to be 3mm or less.  Paul Tarlow has described this kiln pressed glass very well in his books and on the fusedglass.org site.

In essence, you use a pair of kiln shelves.  Kiln wash both shelves.  Place the glass to be thinned on one shelf.  At the outer edges of the shelf put down spacers of the thickness you want the glass to be after pressing.  This will keep the upper shelf from settling down too much and more importantly unevenly.  Place the other shelf, kiln washed side down, on top of the glass.  Be sure the spacers are in places where they can support the upper shelf.

If you are thinning from 6mm to 3mm, normally you do not need any additional weight on top of the upper shelf.  But the thinner you want the glass to be, the greater the weight needs to be.  It could be another shelf, fire bricks or steel weights.

When scheduling the annealing remember you must take account of the mass of the weight on top of the glass.  You will need a much longer temperature equalisation soak and a much slower annealing cool.  

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

Low temperature breaks in flat pieces

The usual advice in looking at the reasons for breaks in your pieces must be considered in relation to the process being used.  Breaks during low temperature processes need to be considered differently to those occurring during fusing.  

The advice for diagnosing breaks normally, is that if the edges are sharp, the break occurred on the way down in temperature. Therefore, the glass must have an annealing fracture or a compatibility break.  It continues to say if the edges are rounded it occurred on the heat up, as it broke while brittle and then rounded with the additional heat.

This is true, but only on rounded tack and fused pieces.

I exclude low temperature tack fuses from the general description of when breaks occur in flat pieces as it is not applicable at low temperatures.  

Low temperature flat work includes sintering, laminating, sharp profile tack fusing, etc.  There are lots of other names used for this "fuse to stick" work.  In all these cases, the finished glass edge will be barely different than when placed in the kiln.  It stands to reason therefore that you cannot know when the break occurred, as the edge will be sharp whether it broke on the way up or the way down.  

Periodic observation during the firing is the only way to be sure when the break occurred. These observations should coincide with the move from the brittle to the plastic stage of the glass.  Therefore, about 540C.  It can be at a bit lower temperature, but not a lot.  If the glass was not broken by that time, you can be fairly certain it broke on the way down.

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

Terminology for degrees of fusing

Can anyone describe what a contour fuse is?

No one can satisfactorily describe, to a high level of acceptance, what a contour fuse is. For me it is just before a full fuse. That will not be acceptable for many, just as describing something as a rounded tack fuse is not a contour fuse for me.  A sharp-edged tack fuse is sintered glass. This will be important to observe as you move to other glass processes such as pate de verre.

There is not yet an accepted terminology and will not be as long as people choose to invent new descriptions for what are essentially the same things.

The closest you can get to a sensible range of descriptors is in the Bullseye document "heat and glass" where the temperature ranges are the important constants.

  

The fourth column of this document gives names for the process. It would be a good idea to adopt these terms, as Bullseye is the company doing the research in the area of kilnforming.

Bullseye terminology gives the following:

A slump or bend occurs in the 540C – 670C range

Fire polishing and sintering occur in the 670C – 730C range

Tack fusing (a rounding of edges) occurs in the 730C – 760C range

A rounded tack fusing that begins to sink into the base glass occurs in the lower end of the 760C – 816C range.

Contour fusing occurs in the middle of the 760C – 816C range.

Full fusing (flat) occurs at the upper portion of the 760C – 816C range.

Bas Relief Moulds

Bas relief moulds that have an image carved into the surface are popular at the moment. They are most often called texture moulds.  The image is “carved” into the back of the glass, creating uneven thicknesses of glass that refract the light to show the image through the smooth plane of the front.

One of the problems with these kinds of moulds is that lots of bubbles are created, often very large ones.  This results from the many places where the air cannot escape from under the glass during the forming process.

Solutions

There are some strategies that can help avoid these bubbles.

Use the 6mm rule

Fuse the glass into a six-millimetre thickness first.  Two layers of glass give more weight to help the glass conform to the texture of the mould.  It also resists bubble formation more than a single layer.

Use the Low and Slow approach

It more important to have low and long bubble squeezes.  The most successful strategy will have a slow rise in temperature to put as much heat work into the glass as you can before the bubble squeeze.  The bubble squeeze is the most important part of firing these texture moulds.  It will start at about 600°C rising at only about 25°C/hr to around 680°C – that is, taking three to four hours. 

Use slow rates of advance

A third element is to rise slowly toward the forming temperature.  Possibly nothing faster than 75°C.  This enables you to keep the forming temperature much lower than a fast rise will.  The usual temperature recommended is about 780°C.

By using a slow rate of advance you can probably reduce the forming temperature by about 20°C.  You will need to peek at intervals to be sure the glass has taken up the required texture. Again, it is about putting as much heat into the glass at as low a temperature as possible.

Use Long soaks

An alternative to the slow rate of advance is to use a long soak at as low temperature as seems suitable.  You will need to peek at intervals to determine when the texture is achieved.  When the appropriate texture is imparted to the glass, you need to advance to the next segment.  This means that you need to know how to get your controller to skip the following segment.  Or, if the texture is not achieved before the end of the scheduled soak, how to extend the soak time.  If you are using 760°C as you target temperature with a rise of 150°C, you may wish to soak for about an hour or more.  Remember that this is in the devitrification range.

Alternative - Frit

A completely different approach is to use fine frit and powder to give a patè de verre appearance by sintering the frit.  This eliminates the bubble problem entirely.

You will need a lot of frit if you are trying to make a sheet of 6mm from the frit.  You could just take the sheets of glass cut to the size of the mould and smash them up to get the required amount of glass.  Or you can use your cullet, by weighing and smashing up enough glass. 

The calculations for weight are best done in the metric system (in cm) as there are easy conversions between volume and weight.  Assume your mould is 20cm square.  The area is 400cm².  The volume is that times 0.6cm or 240cm³.  The specific gravity of glass is approximately 2.5, so you multiply the volume by that and get 600gms of glass required to get a 6mm thick sheet. 

You could full fuse this into a clear sheet, although this would take a much higher temperature and longer soak that would be good for the mould. Better is to sinter the glass.

To sinter the glass, you need slow rises in temperature and long soaks.  A rise of about 75°C to the softening point of the glass (around 600°C) followed by a very slow rise (ca. 25°C per hour) to about 660°C is needed to allow the small grains of glass to settle together.   At the upper end of the bubble squeeze you need a three- to four-hour soak to sinter the glass. The thicker the layer of glass frit, the longer soak needed to ensure all the particles are heated.  The densest glass will be formed by a 50/50 combination of powder and fine frit.

Much better is to have a much thinner sheet formed from the frit.  This will be about two to three millimetres thick.  The weight of powder and or frit can be determined by the formula above, substituting 0.2 or 0.3 for the thickness.  This frit mixture needs to be evenly spread over the mould, with as much on the high points of the mould as the low ones.

If the mould has a lot of variation in height, you can sinter the frit mixture as a flat sheet first.  Then place it over the texture mould and give it a slow rate of advance to the to the top end of the bubble squeeze and soak for an hour or more, as required.  This will ensure you get the same thickness across the whole piece even though there differences in height.

The resulting piece will be very light and translucent.  It will have a fine granular feel to the touch.  It will have the same shape on both sides of the piece, with the upper surface having a slightly more shiny appearance than the bottom.

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