KX PANSloX ON SHKINKAGK, KTC. 393
massive castings, and even when castings are properly proportioned the portion around the "gates'* which convey the metal to the mould is often very likely to he porous or to exhibit shrink-holes, due to the circumstance that the metal solidifies last at these points, and to the attraction of solidifying particles to the already solid mass. This hypothesis explains also the fact that, in heavy castings, poured tl hot," shrinkage is not often exhibited in the 4t feeding-heads" until long after the pouring and that when it does commence (which is not before some expansion has taken place, due to parts solidifying) it is often so rapid as to require, for a short period, constant additions of molten metal.
Expansion at the moment of solidification being thus one cause of shrink-holes in castings, the practice (not uncommon among moulders) of placing " risers/' not much larger than lead-pencils, so to speak, on massive castings, thinking thereby to make them solid, is to be discouraged as useless. It follows, moreover, that a easting should be t4 fed" until expansion is ended. It is not while a metal looks " hot " or fluid in a " feeding -head " that attention is specially necessary to secure a solid interior; it is when the metal is thickening or tfc freezing *' in the li feeding-heads " that the greatest attention should be paid to the " feeding." It is a general practice among moulders, at present, to let their *l feeding-heads " 4t bung up " at a time when the greatest effort should be made to keep them open, so as to insure a solid easting. It is at this time that expansion is taking place, to enlarge the surface area, and consequently the interior volume uf a easting, thereby causing the hottest or most fluidhe interior ofasonable to say, isU Citut