The sheet gelatine sold by grocers for cooking makes an excellent size. Six of the thin sheets to a pint of water is a good strength.[3] The gelatine is dissolved in hot water, but should not be boiled, as that partially destroys the size. When dissolved, a little powdered alum is also stirred in, about as much as will lie on a shilling to a pint of water. The addition of the alum is important, as it acts as a mordant and helps to make a better colour impression.