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I understand there are many different kinds of clay, so maybe you could offer advice for clays that will be typical to beginners, or other otherwise buying from common arts stores.

I imagine that red and grey clays are different in this regard, but maybe not? I know that bubbles are an obvious issue, as is insufficient mixing of add-ins, and large inclusions.

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Yes. Look out for "veins" in the clay as it is altered and manipulated from its original resting state.

Veins might also look like threads or mini cracks.

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There are no tell-tale signs that a piece will crack, but there are definitely certain things that can lead to a piece cracking when fired.

  1. Significant variation in thickness from the thickest to thinnest areas of a piece (a ballpark for this might be if the thickest area is more than twice as thick as the thinnest area). Also, cracks can propagate from thin/sharp edges, so be careful of those.
  2. Wide flat pieces tend to crack due in part to friction with the kiln shelf.
  3. Pieces that weren't compressed adequately (no visual indicator for this, though) will often get s-cracks in the bases.
  4. Large or thick pieces that are dried too quickly or not completely. If a piece feels cold to the touch, then it is still evaporating water and needs to sit longer before going in the kiln.

Ironically, bubbles are probably the least of your concerns, provided you don't fall into the categories I've listed above (in particular the last one).

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  • all of Ken's items are correct .. i would only add that hard corners, like the 4 corners of a ceramic "box", will dry out faster (source of stress in the clay) and have inside corners that are always prone to splitting. Also area of sharp transitions in thickness are weak points
    – vulcan_
    May 25, 2018 at 16:43

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