Substitutes for the harder natural resins made by combining the acid resins with alcohols, such as glycerine. They are soluble in oil and turpentine, have acid values as low as from 2 to 20, and are stated to be much more suitable than ordinary resin for use in making varnishes and enamels. To prepare them, the softer parts of the resin are removed by distillation in a vacuum or a current of superheated steam, and the residue is heated with an equivalent proportion of glycerine, phenol, or naphtnol, to a high temperature with a dehydrating agent. Resin esters can be formed with glycerine by heating together at from 280 degrees to 300 degrees C., and passing a current of hydrochloric acid gas, cai bon dioxide, or air through the mixture. Ester gums are used in making enamel paints, and more particularly for waterproof varnishes for boats, yachts, etc.

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Definition of  ESTER GUMS