Bubble Squeeze Temperature

“My bubble squeeze temperature is higher than my slumping temperature.“ The writer goes on to say that their bubble squeeze is at 1250°F/676°C vs. a slumping range of 1150°F/620°C - 1175°F/634°C.

I applaud the writer for doing the slumping at the lower range of slumping temperatures. This allows the glass to relax into the mould with fewer marks being picked up.  The temperatures might require significantly long soaks depending on the span, depth, shape, weight of glass, etc.   But it is a good practice to get work done at as low a temperature as practical.

 But...

There does seem to be a misunderstanding on how a bubble squeeze works. Like most things with glass, any process works over a range of temperatures.  Bullseye glass begins to soften about 540°C. This continues to about 680°C where the transformation range begins – that is, the glass is behaving more like a viscous liquid than a softening solid.  A bubble squeeze or a slump can begin anywhere in this 540°C to 680° range.  At the lower end of the range, any slump will take “forever”.  At the top end, some slumps may occur too quickly and have mould and stretch marks on the bottom. 

Credit: Fusedglass.org

However…

This note is about the relation of bubble squeeze to slumping temperatures.  If you can slump an item at 620°C, you can also perform a bubble squeeze at that temperature.  Both processes rely on the glass becoming “soft” enough to relax into the shape below it.  It may be that you will need a very long soak to press out air in a bubble squeeze at 620°C, but it can be done if you are willing to wait a long time.  

Many people begin their bubble squeeze at 620°C for fusing glass with a soak.  I am not sure that a soak is required at this point, as slowing the rate of advance over the next 50°C will have the effect of increasing the heat work the glass receives without the need of a soak at the beginning of the bubble squeeze ramp – unless you have a rapid rate of advance toward the bubble squeeze.  

They then progress slowly (maybe 50°C or less, depending on thickness) for the next 55°C to 60°C and soak at that higher temperature for half an hour, or more for difficult shapes.  This additional heat work allows the glass to gradually become more plastic and deform more slowly than at a higher temperature bubble squeeze.  This is often called a cautious bubble squeeze, since it starts at a lower temperature and moves gradually to the top of the bubble squeeze range.  It removes the single shot bubble squeeze at a higher temperature, when air might already be trapped. 

In general terms, the slump can be carried out at or below the softening point of the glass.  This softening point is also the maximum temperature for a bubble squeeze. For example, float glass has a softening point of about 720°C, so a bubble squeeze and slumping can be in the 660°C to 720°C range.  Some glasses have even higher softening points, and others have much lower softening points than Bullseye or Oceanside.

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