I was a potter in the early 70's, it was one of the most
exciting hobbies I have ever had (not counting gardening with Hostas). I
remember reading about testing clay for impurities, using muratic acid. If
there are too many impurities such as fine gravel or dirt the firing will fail.
we did not use red clay, only the grey, stoneware quality, firing to cone 12.
The clay was purchased in large heavy blocks from wholesalers, there was no need
to question their qualilty.