problems with glass cracking when slumping

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    I have had my last two projects crack when I slumped them and am getting somewhat frustrated and need to find out why to avoid future problems.

    The last project was made up of about 500 small dots that I made and tack fused together, then filled in around the dots with frit and did a full fuse of the whole piece.

    After doing some cold work to smooth out the edges I put the round plate into a wave bowl mold to slup. The slumping program is as follows:

    Segment 1 – 300/hr to 300 hold 10 miin

    Segment 2 – 600/hr to 1225 hold 5 min

    Segment 3 – 9999/hr to 950 hold 30 min

    Segment 4 – 250/hr to 500 hold 10 min

    Segment 5 – 9999/hr to 255 hold 0


    Any suggestions are appreciated.


    Try this schedule. I’m assuming you’re using COE90 glass. It looks like your second segment is too fast.  New anneal temp is 900.  Should work for 1/4″ to 3/8″ thick piece.

    200/hr  900deg   30min

    200/hr   1225deg  5min

    9999     900deg   60min

    100/hr    700deg   0min

    200/hr    100deg   0min

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