Back
An abbreviation for "Draw-back." See DRAW-BACK.
Backing out is the method of producing a pattern or casting, equal in thickness all over, from a carved wooden block or a rough plaster-cast. The backing out of such blocks by the moulder saves much carving, and will ordinarily produce a more regular thickness throughout. The method is as follows: two copes are pinned to fit one nowel, the block is set face up in one of them, and an extra hard impression obtained in the nowel. This impression is then transferred to the same cope; also rammed extra hard, and when lifted laid face up on the floor--after which the block is drawn from the nowal and thicknessed with a suitably prepared layer of clay. The parting is then prepared and the impression taken in the second cope in a proper manner for casting, as this is the mould-surface or back of the intended object to be cast. It only remains to return the block into the first cope, and, after removing the first hard-rammed nowel-mould, return the flask to receive the final impression, which in this instance must be rammed with the customary care, as this forms the front or face. Both cope and nowel are by this means obtained from the first cope, and must as a consequence be a perfect match at the parting, the space formed by the clay answering to the design back and front.
If it is desired to accomplish this by casting the moulds face up, the block is placed in the nowel face up, and a correct parting, made very hard, formed all round it. The first receives the intended mould-impression, after which the second cope is rammed extra hard thereon, so that a hard working-face may be obtained on which to lay the clay thickness. It is then ready for the nowel proper; and when the latter has been duly rammed and the whole reversed, the dummy cope is removed, clay lifted out, mould finished, and the previously rammed first cope placed over it. As in the former case, the partings in both nowel and cope are obtained from one original, and must consequently match. See THICKNESSING; KETTLES; STATUE-FOUNDING.