Holes on texture moulds

Holes are blown through single and even double layers on texture moulds when the temperature is taken too high.

Image credit: Debi Scoggins Johnson

This piece was taken to a full fuse at 804°C/1480°F. The bubbles were blown near the edge, causing the glass to fall over the edge of the mould, rather than the blank being too big to begin with. At full fuse temperature the glass viscosity is too low to resist the air pressure from underneath. The air could not escape from the edge as the glass had conformed tightly to the mould surface.

Image credit: Debi Scoggins Johnson

Although it may be possible that the glass draped over the edge of this texture mould during the same full fuse, it is possible the air gathered closer to the centre, again because the full fuse sealed the edges, restricting air flow out to the edges.

These two examples reinforce the requirement that all texture mould schedules should use slow ramp rates, long bubble squeezes, low temperatures and long hold/soaks at the top temperature. This keeps the viscosity of the glass high, helping to resist bubble formation. This post gives information about the rate, bubble squeeze and top temperature.

Be patient. Let the glass have a long time to conform to the mould.

Many bubbles are blown simply because the top temperature is too high and reached too fast.

A Wave with a Bubble Risk

Apologies. I have lost the credit details for this layup.

Difficulties with the layup

At the desired contour fuse this layup will develop a bubble in the centre. 

• This is a single base layer with multiple layers of strips to form the wave image. 
• The circle rim is almost completely covered in the strips.  This edge with only a small uncovered space reduces the places where the air can migrate outwards from under the base. 
• A contour fuse is desired by the maker which will soften the clear enough that there will be little resistance to blowing one or more big bubbles.

Alternative strategy

There is a different firing strategy that can achieve an almost equal profile – sintering, sometimes called fuse to stick. This uses the concept of “heat work”.  Using this concept you fire slowly to a low temperature (ca. 720°C/1328°F) with a long soak (around 6 hours). 

 You will need to check at least every hour until the desired profile is achieved and then skip to the anneal soak, which needs to be for 2.5 times the thickest part (e.g., if 10mm, anneal as for 25mm). 

The speed of the ramp from the bubble squeeze to top temperature should be about 100°C/180°F per hour. The long soak allows the glass to bond together at the molecular level.

The additional requirement is that the whole layup needs to be on fibre paper overlaid with shelf paper.  

Reasons:

  

• - This method keeps the viscosity high, creating more resistance to allowing 
• - The fibre paper allows air to migrate from under the centre of the layup.
• - The shelf paper provides a smoother back than the fibre paper.
• - Any layup that has a thicker rim/outer border will trap air, even with the gap provided as in the image above.
• - A lamination/sinter firing will provide secure attachments between the glass pieces with extremely long soaks, which gives time for the molecules to attach to each other. Experience has shown that the larger the pieces are, the longer time is needed.
• - It is also possible to use a low temperature tack fuse with a long (maybe 1-2 hours) soak, but everything must be really clean to avoid devitrification

Slumping Temperature and Times

There are two main views on slumping ramp rates and soak times. One advocates relatively quick rates, higher top temperature, and short soaks to avoid excessive marking on the mould side. The other advocates slower ramp rates to a low temperature with a long soak.

I am on the "slow, low, long" side. Bob Leatherbarrow even places the shiny side of the prepared glass down towards the mould, with low temperature and long soaks, checking every 15 minutes on the progress and skipping to the anneal when the slump is complete. This gives two shiny sides.

If a slump can be achieved in 13 minutes, the temperature is higher than necessary. An alternative is to use a lower temperature for 30 minutes or more. Peeking at 15 minute intervals will be necessary to determine when the slump is complete.

The anneal and cool for a tack fused piece should be as for twice the thickest part of the piece at minimum. My experience has shown that annealing a slump for one layer thicker than the calculated thickness produces a piece with less stress than just the simple twice the thickness does.

Domed drapes

 

Apologies.  I failed to keep the name of the maker.

A draped bowl didn't end up with a flat bottom. What schedule would flatten it out again and what to fix to get a flat bottom?

Drape schedule:

• 167°C/300°F – 527°C/980°F, 30’
• 111°C/200°F – 710°C/1310°F, 10’
• AFAP – 472/880°F, 1:00

Scheduling Factors

Ten minutes soak time at this top temperature is not enough time to allow the glass over the top to relax and become flat. To understand the dome formation on the top of the bowl drape mould, visualise the way the glass moves during the drape. I will attempt to describe it, but peeking at intervals from 55°C/100°F below the top temperature will show better than I can explain, of course. At the beginning of the drape, the weight of the glass at the edge bends the glass into a domed shape because it is not yet at a low enough viscosity to conform to the flat base. It is not until it has absorbed enough heat that it can begin to stretch and sink towards the mould surface at what will be the base of the glass piece.

At the rate used, the glass needs more time to absorb the heat to allow the glass to relax to the central surface. The extra time may lead to distorted edges, of course. So, a slower rate up to the top temperature may be the best solution. It is a curious fact that clear glass does not react as coloured glass does. It seems the infrared radiation goes through the clear in a way that it does not with coloured glass. This lower viscosity of the clear than coloured glass may be an additional factor in in allowing the dome to form.

The possibility of trapped air holding the glass up is not likely. At 710°C/1310°F the glass will not have made a seal with the upper edge of the mould. 

Drilling a hole in the supporting mould will not solve the problem, but it won’t create another either. Just as in draping a votive, a hole is not necessary, but of course the draping is done at a higher temperature. The amount of heat work the glass receives is important to the glass being able to relax to the surface of the mould.

The drape schedule

• 167°C/300°F – 527°C/980°F, 30’
• 111°C/200°F – 710°C/1310°F, 10’
• AFAP – 472°C/880°F, 1:00

This schedule is suspect for two main reasons. The first segment might be best maintained to 621°C/1150°F. The second segment ramp rate might be reduced to 33°C/50°F to 740°C/1364°F for up to 30 minutes to allow the glass to relax to the base part of the mould while the rest of the glass continues to drape. Observation, by peeking at intervals, from 720°C/1328°F until the glass has fully formed flat at the base and is fully draped, will indicate when the segment should be skipped to the anneal. 

Anneal

And, although the annealing temperature is not the cause of the dome, the anneal temperature used is not one normal for either Oceanside or Bullseye and could be responsible for a later break due to inadequate annealing.  

If not a typo, 472°C is below the accepted annealing temperature for any of the fusing glasses, and is so near the strain point that it will be difficult to anneal at all. If the glass is Oceanside, the anneal temperature is 510°C/960°F; if Bullseye the anneal temperature is 482°C/900°F. The soak time indicates the glass is being annealed as for two layers. The associated cool rate for 6mm is:

• 83°C/150°F to 427°C/800°F, 0 minutes, 

• 150°C/270°F to 371°C/700°F, or to room temperature.

However, this is a tack fused piece of a single layer plus 3 to 6 mm of decorative elements. So, it should be fired as though twice the 9mm thickest part, or 19mm. This profile of glass needs three hours anneal soak, and cooling rates of:

• 25°C/45°F to 427°C/800°F

• 45°C/81°F to 

• 371°C/700°F

• 180°C/270°F to room temperature

Note that this tack fuse profile needs cooling rates all the way to room temperature. If your kiln cools at a quicker rate from 371°C/700°F than 180°C/270°F, the schedule should include that final segment to room temperature.

Anneal Cooling

In the original schedule, there were no cooling rates given, as though the kiln will cool slowly enough (the natural cooling rate)to achieve a good anneal. This is rarely the case for any thing being fired for more than 3mm. The purpose of controlled cooling is to initially maintain the small difference in temperature across the whole piece to avoid it increasing. As the glass temperature falls below the strain point, it can withstand greater temperature differentials, but the temperature fall still needs to be controlled enough to avoid large contraction differentials. For thicker glass, the rate to room temperature also needs to be controlled.    

This piece is being fired as for 19mm and does need controlled cooling to avoid inadequate annealing and possible fractures from too quick cooling.

Layup

One thing that may have helped the formation of a flat bottom would be to have a tack fused a disc of clear, the same size as the base of the former in the first fuse. This would give additional weight to the glass and help to flatten the base sooner. The bending of the outer edges of the draping glass, would not pull the glass on the base into such a large dome, and possibly not at all. Think about weight when draping, as the heaviest will tend to fall first.

Glass Size for a Drape

 How big can a square be for a drape former?

The diagonal of the square for a former can not be more than twice the height of the former plus the base diameter. Of course it is better to be slightly smaller to allow for the glass stretching during the drape. But twice the height plus the base diameter is the maximum.

An example. If the former base is 2 and the height is 7, the diagonal can be no longer than 16 (7+7+2). The question then is what size square will that diagonal produce. The geometry formula of the sum of the squares of the two right angle sides equal the square of the hypotenuse (diagonal). That means the square pieces of glass cannot be more than 11.3. This is arrived at by:

• Calculate the square of the diagonal (16*16) = 256

• Divide that in two; half of the square is 128 (the square of one side)

• The square root of 128 is 11.31

• So the sides of the square are 11.3 each.

This is the largest square you can put on on that vase drop mould, because that size square has a 16 diagonal.

Or if you don't like maths:

• Mark a line at a 45 degree angle from the corner of a piece of paper.

• Mark off the length of the diagonal needed for the former on it.

• Draw a right angle from the edge of the paper to the point that is the length of the diagonal. That is a measure of the sides of the square.

This will require some drawing tools.

A no-tools method:

• Fold a piece of paper at the corner until the edges meet.

• Crease the fold to give a 45 degree angle

• Mark the diagonal length along that 45 degree crease

• 
  

  
  
  

• Fold the paper from the top toward the bottom where the 45 degree crease began.

• When the mark for the diagonal is reached, carefully crease the paper to the edge.

• 
  

  
  
  

• That gives you the length of the size of square needed.

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Circle sizes are a easier, because twice the height plus the base equals the diameter of the circle. So in the example above, 7+7+2 = 16 which is the largest diameter that can be used for that former.

Elevation of Moulds

Putting slump and drape moulds on kiln posts is not necessary.  If you think you need air circulation under the mould, pieces of thick fibre paper are all that is needed.

The widely accepted notion that all moulds need to be elevated from the shelf is not backed up by evidence.  There is a lot of anecdotal discussion which supports the idea that it is necessary, though.  The idea is that air circulation under the mould will keep the lower part of the mould the same temperature as the upper part. That may or may not be the case. But it is the glass that needs attention.

Heating glass on moulds

Slumping

The glass is suspended either at the edges in slumps or at the centre in drops. The suspended glass will heat independently of the mould. Any difference in temperature between the glass and slumping mould will only become important at the end of the slump when it touches down onto the mould. If slow ramp rates and long soaks are used, the mould will be heated through by the time the glass touches down to the mould. But there could be an argument, based on the use of cold moulds in glass blowing, that a cool mould will impart less texture on the glass than a hot one.

Draping

In draping, the glass is supported in the centre on the mould and the differential temperature between mould and glass in the early stages of the firing may be a cause of a break. The supporting surface of the mould is being heated through the glass and, to a lesser extent, by conduction from the lower part of the mould. This requires slower heating than a slump mould does, to ensure the heat is not drained from glass by the mould which would allow the temperature differential between the supported and the unsupported parts of the glass to become great enough to break the glass.

Air Circulation

The idea that air circulating under the mould will even out the temperature between the bottom and top of the mould is not supported by two pieces of evidence.

The main effect to heat glass is infrared radiation. This is often described as the glass “seeing” the heat. Glass is only secondarily heated by the ambient temperature of the kiln. How heating the mould in the slumping process will protect from breaks or uneven slumping is not clear, and is supported only by analogies.

The idea that convection will help to heat the glass as well as the mould is countered by the general agreement that there is very little movement of air in the closed kiln. What convection of air there may be from the differential air temperature between the bottom and the top of the kiln is minor. If it were major, there would not be the large air temperature differential experienced – in deep kilns in particular, and to a lesser extent in shallow kilns – between the top and bottom of the chamber.

Temperature Differences

We all agree that there are temperature differentials from top to bottom in kilns.

The evidence is that the temperature difference is much greater between air and glass, than between mould and glass. The average temperature difference (∆T) between raised moulds and on-the-shelf moulds in tests ranged from about 50°C to 40°C (90°F to 72°F) on the rise to top temperature, depending on the ramp rate.  (a ramp rate of 150°C/ 270°F averaged a 50°C/90°F differential vs a 120°C/216°F ramp rate which gave the average 40°C/72°F differntial.)  It is also clear that by the time the slump temperature is reached, the ∆T between the two mould arrangements is about 25% less at the end of a 30 minute slump than at the beginning. So, yes, there is a difference in the temperatures under the mould versus an on-the-shelf mould.   Is it significant?

The ∆T between the raised mould and the air is approximately three times any difference between the on-the-shelf and raised moulds. This is consistent at ramp rates of 120°C, 150°C and 240°C (216°F, 270°F and 432°F). This indicates there is a greater risk to the glass with rapid ramp rates than there is from the difference in mould elevation. Slower ramp rates produce smaller differences between air temperature and mould temperatures. They also produce smaller differences between the mould bottom and top temperatures, making the argument for slow ramp rates up to the slump temperature, regardless of whether the mould is elevated or not.

Ramp Rates

The more rapid ramp rates create significant ∆T between the air and the mould. This much larger ∆T than between moulds raised or not, has the greater potential for causing breaks. The slower the ramp rate the less difference. These two elements on their own make the argument for slower ramp rates clear.

Distance from Elements

The distance between glass and heating elements is important in slumping.
The closer the glass is to the heat source, the more uneven the heat reaching the glass will be. This blog post indicates the distance required to even out the temperature. 

Where the elements are located is significant too. Side fired kilns need to use slower rates in going up in temperature, because the heat is concentrated on the edges of the glass unless baffles are in place. The uneven heating risks breaks, and this can be reduced by using rates for one to two layers thicker than actual thickness.

Moulds and Shelves

It can be argued that care in heating up ceramic moulds is required because of the quartz inversion.  If the ramp rate is fast enough to cause a mould to break, it is also too fast for the safety of the glass. The same caution in relation to the shelf needs to be employed for the glass.  Any large heat differential across the shelf will cause a break. 

Heavy, wet, damp, or large casting moulds must be elevated to keep the shelf from cracking due to a large ∆T between mould and shelf which has the real potential to break the shelf. The remedy for these kind of moulds is to elevate them on short kiln posts to maintain a relatively even temperature across the whole of the shelf.

Caution – the temperature differentials are averages and individual moulds will differ. But the principle remains that slow rises to top temperature are less risky than fast ones.

Data for these figures is available in table form and in graph form

Stress Build Up

Does annealing stress in one firing cause breaks in the next firing?

Stress does not cumulatively build up from one firing to the next. Once the glass has achieved the strain point temperature, the relief of stress begins and is complete by the softening point (usually the slump temperature).

So why does the glass sometimes break on the second firing?

Picture credit:  Emma Lee

There are three general causes of breaks of fused pieces on subsequent firing:

Too rapid initial ramp rates.

Inadequate annealing.

Incompatibility. If there is incompatibility, no amount of extended annealing will cure it. Some of the causes may be:

• cumulative compatibility shift due to extended time at high temperature

• long soaks during multiple firings.

• Normally, hot colours are most subject to these.

How is it possible to be sure the fired piece does not have excessive stress before the next firing?

Stress testing with polarised filters of the cooled glass will indicate the amount and sometimes the cause of stress. This test should be conducted on each piece after it has cooled and before firing again. If there is a large amount of stress, the piece must be warmed up slowly to avoid adding additional stress during the heat up for a more adequate anneal.

This heat-up needs to be a single slow ramp to the strain point.  After the strain point is reached, there is much less chance of a break and the ramp rate can be increased.   It isadvisable to go at least 55C/100F above the annealing temperature and soak for at least 10 minutes before proceeding down to the anneal. The annealing for this new firing needs to be scheduled to be much longer and the cool much slower than the first firing of the piece. The additional length may be for one layer thicker for full fused pieces, but for at least two times thicker for tack fused pieces. Guidance for calculating the thickness for the annealing time and cooling rates are available in this blog post.

Compatibility of Stained Glass

Is there a way to find out if some stained glass is compatible with fusing?

Yes, there is a way to test for compatibility. This strip test is the one I recommend. It is simple, repeatable and provides a reference for future use if labelled and stored carefully.

Determining the Coefficient of Expansion (CoE) is a testing laboratory process, not something we can do at studio level. But compatibility of several glasses can be easily established by this testing process.

Some indicators:

• Stained glass is more likely to be compatible with Oceanside or Wissmach than Bullseye.

• Transparent and streaky glasses are more likely to work in Kilnforming than wispy or opalescent, which tend to devitrify easily and have more compatibility problems.

• It is possible to find colours from one manufacturer that are compatible with each other but with none of the fusing compatible glasses, but it takes a lot of testing.