Once the skin is completely dry, it is thoroughly cleaned and processed into sheets. The number of sheets extracted from the piece of skin depends on the size of the skin and the given dimensions requested by the order. For example, the average calfskin could provide roughly three and a half medium sheets of writing material. This can be doubled when it is folded into two conjoint leaves, also known as a bifolium. Historians have found evidence of manuscripts where the scribe wrote down the medieval instructions now followed by modern membrane makers. The membrane is then rubbed with a round, flat object ("pouncing") to ensure that the ink would adhere to the surface. Even so, ink would gradually flake off of the membrane, especially if it was used in a scroll that was frequently rolled and unrolled.