Bubbles in texture moulds

People often assert that moisture is a cause of large bubbles on texture moulds.

 Let's think about this.

• Water evaporates by 100°C/212°F.
• Glass is not sticky until around 540°C/1000°F.
• Glass does not begin to slump until about 610°C/1130°F. 

Therefore, with a reasonable schedule the mould, shelf, etc., is dry before air can be sealed in.

 

Damp moulds are not the cause of bubbles in texture mould work.

This observation means you must seek other causes. Three related to glass are:

•        Air pockets: Texture moulds have lots of possible air pockets. Excellent, slow bubble squeezes are required to avoid creating bubbles. These bubble squeezes start about 50°C/90°F below slumping temperatures. The ramp rate for these can be as low as 25°C/45°F per hour, but more commonly are 50°C/90°F per hour. This rate applies to the slump temperature with a soak of 30 or more minutes. The deeper the texture, the longer the soak needs to be.

•        High temperatures: To resist bubble formation low temperatures are required.  The higher the temperature, the lower the viscosity. Low viscosity is less able to resist air pressure from below than glass at a lower temperature with greater viscosity. My testing and research show that 740°C/1365°F is hot enough to form the glass to the mould. Of course, low temperatures require long soaks. The soaks might be as much as 2.5 hours for a single sheet, or as little as 1.5 hours for three layers.

•        Thin glass: there is little weight or mass in a single sheet to resist bubble formation. Using low temperatures becomes even more important. The stiffer (higher viscosity) the glass is the more resistant it is to the pressure of trapped air. Using even lower temperatures will help resist bubbles, but much longer soaks must be used.

 

The moulds can be a cause of course. There may be boundary walls on the mould. There may be vertical sides to the relief. There may recesses that are deeper than the rest of the mould.

In extreme cases, pin sized holes may have to be drilled where a lot of air is trapped. Only experience will tell you where these low spots are holding sufficient air to create bubbles.

Most bubbles created in firing texture moulds come from the scheduling, the thickness, and the nature of the mould.

 

More information is available in the e-book Low Temperature Kilnforming, an Evidence-Based Approachto Scheduling.