In filling a mold with metal its surface is subjected to thermal, mechanical, and physicochemical actions.The oxidation products of the metal, reacting with the casting mold material, form low-melting molten materials which lubricate the grains of the quartz sand well.This promotes penetration of the metal into the intergrain spaces and the formation of mechanical pickup which is difficult to remove from the casting surface.In recent years liquid self-hardening mixtures possessing a number of advantages in contrast to sand--clay mixtures have been widely used in foundries for the preparation of molds and cores. However, liquid self-hardening mixtures have high porosity, as much as 50% and more [i]. Therefore, the production of castings in liquid self-hardening molds without pickup and other surface defects is possible only with protection of the surfaces of cores and molds with refractory coatings.The basic requirements for the refractory coatings are minimum porosity, high refractoriness, and weakening of the physicochemical reaction at the metal--coating interface (lubrication, solution, penetration).A liquid self-hardening mixture base casting mold was prepared from type KO 2A quartz sand (94-95%*), ferrochrome slag (5-6%), and a liquid composition (11-13%), adding up to more than 100%.The degree of dispersion of the materials and the fluidity of the coating makes it possible to apply it both with the use of a brush and by spraying.The use of the spraying method requires observance of safety measures in connection with the possible presence in the air of hexavalent chromium ions.The mold with the protective coating applied was dried at 250-300~ for 4-5 h. The compressive strength of the dried mold was 1.0-1.3 MPa and the open porosity 50-55%.The temperature of the carbon steel as cast was 1540-1560~During the first hour of natural cooling the temperature at the mold surface--metal interface dropped to 1300-1400~ An investigation of MgO--AI20~--Cr20~ system mixtures [2] with different ratios of the components and the influence of the products of oxidation of the steel, particularly Fe203 and FeO, on the mechanism and kinetics of the mineral formation in the given system [3] indicates the possibility of use of these mixtures for protective coatings of casting molds. With the use of the results obtained a coating composition preventing penetration of the metal into the casting mold was developed [4].In contrast to known ones, a feature of the coating developed is the addition to it of an MgO+MgCr20~ refractory mixture, the chemical composition and method of production of which is described in [2]. Other refractory coatings widely used under production conditions were tested in parslleloThe compositions and certain properties of the coatings are given in Tables 1 and 2. To obtain unambiguous results, the coating consumption and the experimental results were maintained constant.In particular, the coatings were prepared by the same method. One half of the mold or core was coated by the normal method and the other ...