Slumping Problem

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    I use 96 coe glass. When slumping rectangular projects, the sides slide down and lose shape. How can I get them to stay in shape


    It has nothing to do with the glass being 96 COE.

    It sounds like the glass rim is being pulled into the mold instead of staying in place while the sides stretching.  Slowing down your firing so that the glass softens more evenly may help – but it is unlikely to fully correct the problem which is typical for square molds.

    Take a look at this Bullseye mold:

    If you click the middle thumbnail, you’ll see that even the sample piece has edges that curve inward.  It is the nature of the beast.

    Helios Kiln Glass Studio



    About 50% of my slump firings have resulted with a large dome shape in the bottom of the glass.

    It’s a large bubble shape where it should be flat. I just fired 2 glass pieces in identical molds at the same time, one was perfect and flat in the middle, the other had a large dome in the center. The holes are clear, so that’s not it. HELP!!

    Stephen Richard

    The cause is usually a combination of heat and time, usually too hot for too long.  You do not give details so speculation is required on my part.  I suspect on piece was darker or heavier than the other.  Steep moulds can also cause these uprisings.  A blog post of some time ago may have some useful information:

    Stephen Richard

    blogs at: and

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