Drawing-down
When the cope part of a casting shows an unevenly buckled surface, with shell-scabs in parts, it is technically spoken of as "drawn-down," or "drawn-in." Although this phenomenon is not unknown in loam and dry-sand work, it is chiefly in green-sand copes that it occurs. Very damp copes with no vents are apt to be drawn down as above described, because of the rapid generation of steam, which can find no other means of escape. Especially is this the case when the mould is long in filling. Sand with too much clay is always liable to buckle if subjected to a long-continued heat, as the skin dries-into a cake, expands, and buckles, sometimes falling off in flakes before the mould is full. Silica, or fire-sand, comparatively free from lime, brought up to the requisite degree of consistency by a slight admixture of clay, will invariably hold together under the severest trials, if well vented with a small wire, in order to lead the steam from the surface to the outside. In important cases it is a wise precaution to cover the cope-vents with loose sand. Vents, if left uncovered, permit a too free escape of air from the mould, and the cope surface is to that extent robbed of such support as the compressed air within gives to it. See VENTING.