Describe Mould Surface Coating?

Mould surfaces are coated (after the pattern is drawn out) with certain materials possessing high refractoriness. It eliminates the possibility of burn-on and enables casting with smooth surface to be obtained. However, the permeability of the mould gets reduced. Therefore, the Coating should not contain gas forming materials. Mould surface coating which are also known as facings, dressings, washes, blackings or whitening, may be applied dry(by dusting) or wet in the form of thin cream. The various mould surface coating materials include: Coal dust, pitch, graphite, china clay, Zircon flour or French chalk (Calcium oxide).