during use, either in slip casting or plastic making, plastic moulds absorb water. this water must be removes, or the mould will become saturated and will no longer perfume its function. mould drying is usually done overnight at a gentle heat, and must again be carefully controlled to avoid removal of water of crystallisation which would lead to cracking of the mould. with the water absorbed from plastic body or slip, mould will absorb deflocculant and soluble salts. insoluble calcium carbonate and calcium silicate, and sodium sulphate is carried by water to the surface of the mould, where the water evaporates and leaves a deposit of sodium sulphate behind (a process known as efflorescence). the use of organic deflocculants can educe the attack on the mould, and prolong the mould's working life.