Texture moulds

"I could use some help here please. I’ve tried this sun mould 3x and as you can see all 3x I get a hole.  If you could tell me what I’ve done wrong I would greatly appreciate. They were all full fused to 1430F (776C)."

Example of the problem

There are a range of views that have been given on how to make texture moulds work without the glass developing bubbles.

closer view of one example

These are a summary of the suggestions made to the enquirer.

Not enough glass thickness. The view is that glass needs to be 6mm thick to be used on texture moulds, as the viscosity of glass tends to draw glass to that thickness, robbing from other areas making them thin and prone to bubbles.

Glass always wants to go to 6mm.  Not always.  It depends on temperature.  The kiln forming temperatures we use results in a viscosity that tends to equalise the forces at 6 – 7 mm.  Hotter glass will flow out more thinly, until at about 1200C, the glass is 1mm or less thick.

Full fuse two sheets first.  The object is to avoid placing two separate sheets on top of the mould, creating the potential for more bubbles between the sheets, as they may slump into the mould at different rates.

Too hot. As the glass increases in temperature the viscosity is reduced and can no longer resist the air pressure underneath the glass.

Use a lower temperature. The idea is to keep the glass relatively stiff to resist bubble formation.

Bubble squeeze needed to avoid trapped air.  Another way to reduce the amount of air under the glass is to allow the glass to relax slowly at a temperature below which the glass becomes sticky.

Elevate the mould.  The idea is that hot air circulating under the mould will help equalise the temperature of the mould and the glass.

Drill holes at low points. This gives air escape routes under the mould, assuming the mould is slightly elevated.

Go lower and slower.  Use a slower rate of advance toward a lower top temperature with longer soaks to avoid reducing the viscosity, but still get the impression from the mould.

Now for a different viewpoint.

None of the views given above are wrong, but they all (except in one case) fail to consider the fundamentals of obtaining texture from such a mould.

It is apparent that the temperature used was too high because the glass had low enough viscosity to allow the air underneath to blow the bubble.  The suggestions of thicker glass, bubble squeezes, lower temperatures, drilling holes and elevation of the mould are ways of reducing the amount of air or resisting the air pressure.  They are not wrong, but miss the fundamental point.

That fundamental point is that you need to raise the temperature slowly on these texture moulds to allow the glass to fully heat throughout. By doing this most of the air has a chance to filter out from under the glass before it conforms to the edges of the mould.  It is simpler to use the slow advance rather than a quick one with a slow-down for a bubble squeeze.  The glass is more certain to be the same temperature throughout by using a slow rate of advance.  Glass with an even temperature can conform more easily to the undulations and textures of the mould.

Mostly, the recommendations given are to use two layers, or 6mm of glass that has already been fused together.  This gives greater resistance to bubble formation and reduces the dogboning and needling of the edges.

However, you can form in these moulds with single layers.  There are of course certain conditions:

• You must advance the temperature slowly.  A rate of 100C per hour will be fast enough.
• You can add a bubble squeeze soak of 30 minutes at about 630C as additional assurance of removing most of the air.  The bubble squeeze is done at a lower temperature than usual, as the glass is less viscous because the slow rate of advance has put more heat work into the glass.
• The top temperature should not go beyond 720C. Beyond that temperature the viscosity of the glass drops quickly and so becomes subject to bubble formation.

The soak at the forming temperature will need to be long and observation will be needed to determine when the glass has fully conformed to the mould. Quick peeks at intervals will show when the design is visible on the top of the glass. The time will vary by:

• Mould texture complexity 
• Type of glass (opalescent or transparent),
• Heat forming characteristics of the glass,
• Viscosity of the glass or colour,
• Etc. 

Be knowledgeable about how to extend the soak or to advance to the next segment of the schedule to take advantage of your observations.

Your observation may show that you can do the texture formation at a lower temperature in future. This will provide results with less separator pickup and better conformation to the mould without excessive marking. 

You will need a long soak in either circumstance. This will be in terms of hours not minutes.  If you do these texture moulds at slumping temperatures, you will probably need at least twice your normal soak.

You can do a lot to fool the single layer glass into doing what you want by using low temperatures and long soaks. See Bob Leatherbarrows's book on Firing Schedules.  He gives a lot of information on how to manipulate glass through heat work - the combination of temperature and time.  You might also consider obtaining my book - Low Temperature Kilnforming.

Most of the search for the right temperature, fails to note that the important element is how you get to the temperature. You can get the same result at different temperatures by using different rates of advance.

Kilnforming is more than temperature, it is also about time and the rate of getting to the temperature. By concentrating on temperature, we miss out on controlling the speed and the soak times. You can do so much more to control the behaviour of the glass at slow rates, significantly long soaks, and low temperatures.