Spectrum Glaze Thinner is a solution of sodium hexametaphosphate in water. It is a very powerful thinning agent and should be added very sparingly. For example, 2 or 3 drops in a 4 oz. jar should have a significant effect. It is a useful product for thinning glazes, underglazes, etc. when you do not want to dilute the strength of the glaze by adding water.