Different clays have different shrinkage, require different firing, or stand a greater or less degree of temperature, hence the glaze is a matter of trial. Glazes are coloured by admixture of small quantities of metallic oxides. Common clay vessels are painted over with red-lead, but this glaze is dangerous, as it is affected by acids. Borax will make a glaze, ami is used as a flux. A white earthenware glaze may be made from Cornish stone 35 parts, borax 20, crystals of soda 10, red-lead 20, and blue calx 1/8 part. Calcine and pulverise and grind with 20 lb. of white-lead, 101b. of Cornish stone, and 51b. of flint.