Take sour milk, boil it until the solids settle on the bottom, then strain it through a linen cloth; the liquid runs off and the solids remain behind like groats. Now stand this in a covered vessel for 8 days, after which work it with fat on a plate of some sort into the shape of a blunt cone and put it in a place out of draughts and heat. If it splits at this stage, work it again into its former shape and put it back in the same place to dry – it often takes one or two months for this to happen. At this stage an epidermis forms on it, which is wrinkled or cellular as in a reticulum. This is no more than a fusty layer of old substance that may be scraped off afterwards.