I order from a clay company that shall remain nameless (cough cough.. columbus clay). Every few weeks or so we will find a rubber or plastic chunk in the clay we receive. A few times in the past few years we have even had potters "find" metal shavings in the clay while they are throwing. . Once or twice this has resulted in a minor cut. We took precautions to minimize the possibility that contaminates come from us. Moved all unrelated work out of the clay processing area, checked that the clay processing tools were not coming off in the clay, created clear signage for what does or doesn't be