The receptacle being warmed quite possibly contains glue, made from bones, sinews and cartilage. What the two workers are doing is less clear. The one on the right is spreading something on a workpiece. This could be glue used to stick veneer on some inferior kind of wood or gesso which was extensively applied as a kind of glue putty to smooth the surface. Gesso was made by mixing whiting with glue. Harder than plaster, it also sticks better to the wood and makes a good underground for painting or gilding. The other carpenter may be preparing plaster by grinding chalk