It’s actually kind of interesting, they used wax tablets as scratchpads back then. This practice actually continued well into the early modern era because paper, parchment, and vellum were quite expensive and people wanted something that could easily be erased. So most likely they would have written the calculations in wax and only put the final results in clay. By the way, cuneiform tablets are essentially pottery, not rocks. They were basically clay versions of the wax tablets I linked that got fired in kilns for preservation.