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    dsw reacted to glazenerd in Large Platter Broke In Half In Bisque Firing. Anything To Do With It?   
    Hi Diane:
    Have several questions, but will comment partly from your pics and partly from making certain assumptions. I will "assume" these plates were bisque fired before they were glazed fired. From that assumption: the cracks you see on the finished product occurred during the bisque firing. You did not see them after bisque firing because they were fissures in the body. I will venture a guess and say the edges of the crack are sharp.  When the kiln dropped below 1300F and down to 400F: the tension created by the clay cooling and the glaze cooling pulled the fissure apart. This also makes me believe that the COE of your clay and your glaze is not aligned properly.
    Let's say I am wrong and you once fired these pieces. The above still applies, however the edges are smooth and not sharp. You blew through the quartz inversion temp (1063F) and blew the pieces apart and the glaze sealed the crack. Two problems causes that type of severe cracking: blowing through the inversion temp during bisque, or during once firing. Secondly, a COE differential between the clay and the glaze.
    FIXES: program a slow ramp of 100F an hour between 1025F up to 1125F to stop fissures (bisque) or severe cracking during once firing. Secondly; double check your glaze calculations to bring the COE of the clay and glaze together. You have a unique issue because you are glazing both sides, so putting dry silica or wadding under the piece is not a viable option. If you are bisque firing, placing silica under the piece to allow it to move ( reduce contact tension with the shelf) will certainly help.
    Simple Test: if you are bisque firing these larger/flat pieces on a regular basis then you need to start doing the "ping" test. Buy a cheap metal glass cutter with a small metal ball at one end. ($3 USA). Hold the bisque piece by the edge and gently tap it with the metal ball (GENTLY). If it "pongs", then the piece is solid and no fissures or cracks are in the body. If it "pings" (hallow sound) then fissures are present rather you can see them or not. If it does ping, you are wasting time and money trying to glaze fire it. As far as the glaze sticking to the shelf: that one is tricky.
    Warping concerns me as well: 1. not properly supported 2. over fired.
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