The short answer is no. Firing clay from other workshops presents significant risks to both individual ceramic pieces and the kiln. In the past, we experienced an incident involving clay from another workshop that resulted in it melting and ruining a kiln shelf. Luckily it didn’t ruin other people’s pieces or the kiln itself. This happened even though the person said it was the correct type of clay and that it could handle 1250°c. To avoid such occurrences, we have a policy against firing external materials. We cannot risk ruining the kiln. Nevertheless, if you have hard proof, such as a receipt or documentation, specifying the type of clay you used, I would be open to reconsidering your request.