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kiln disasters!!

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  • kiln disasters!!



    Im sort of new to ceramics so when my white stoneware pieces exploded/ shattered in the kiln I dont really know what Ive done wrong. any help is very welcome.
    I fired to 1250 with dwell on. the pieces arent big but they are two pieces joined together, I thought they were completley dry, well they looked it ! could they have been still wet maybe in the joint or could it be air still in the clay. Help I cant waste any more clay. thanks

  • #2
    It does sound as if not fully dry, with the damp weather can play havoc with greenware and when heated the steam will expand and cause the clay to explode. One way I fired was pre-heating starting at a lower temperature for say an hour then turn the kiln up, repeat then to the required temperature. Are you using a kiln sitter? that way will not shut off before the correct firing time.

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    • #3
      Thanks Caroline, no I dont have a kiln sitter at least I dont think I do. After a bit of research I think it does sound like the clay not being completley dry and your right the weather isnt helping. thanks again

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      • #4
        It does sound like not dry clay, it could also happen if the clay isn't wedged properly - I take it you did wedge it? I always take up temperatures slowly for a bisque firing.
        Daesul

        http://www.clairemanwani.com
        http://www.folksy.com/shops/clairemanwani
        http://www.etsy.com/uk/shop/ClaireManwaniPottery

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        • #5
          The biggest cause of firing explosions is air trapped in the clay, even a small bubble will expand in the heat and crack the piece.

          It's really important to wedge properly before making anything. Either bang the clay down really hard several times, slicing in two and then continue banging it together, or use the 'Bulls Head' method (most beginners' pottery books will show how to do this). When the clay is sliced in two there should be no evidence of any air bubbles. If they are still there you have to continue until they're not (it really burns up the calories!!)

          You can also easily trap air when you join two sections, so be very careful to score each surface and slather it with slip. Wriggle the two sections together before smoothing the join to release any trapped air.

          Another cause of breakage is firing too fast in the early (first) firing stage. The kiln needs to go up really slowly until it reaches 300deg.

          Wish you all the best with it.
          Last edited by Cuckoos Nest; 24-06-2011, 10:27 PM.
          Kate
          www.cuckoos-nest-fairs.co.uk

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          • #6
            I'm going to disagree alittle ref the air pocket debate. Pieces blow up due to water turning into steam, this much is definite. Ref air pockets what I believe is that water vapour/moisture collects in the air pockets and the clay on the outside looks dry. Then we pop it in the kiln and hey presto explosion. I have fired items that have air pockets with no explosions or cracks. However I have made sure that the climb rate was vey slow 50C/hour (to at least 600C) and I was using a coarse crank clay.
            Good luck with your future firings
            www.toppotsupplies.co.uk

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            • #7
              Trapped Air / Steam

              Also, if you are making hollow, enclosed forms make sure that there is a small hole in them somewhere to let the air / steam out. (Usually in the base) It only needs to be 2/3 mm.

              Plus follow the advice about wedging and slow initial firing in biscuit firings.

              Good luck!

              Terrorcotta
              We all sit on the banks of the same river.

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