SACKING AND SACK MANUFACTURE. Sacking is a heavy closely-woven fabric, originally made of flax, but now almost exclusively made of jute or of hemp. The more expensive kinds, such as are used for coal sacks for government and other vessels, are made of hemp, but the jute fibre is extensively used for the same purpose, and almost entirely for coal sacks for local house supplies. The same type of fabric is used for wool sacks, cement bags, ore bags, pea sacks and for any heavy sub stance; it is also made up into a special form of bag for packing cops and rolls of jute and flax yarns for delivery from spinners to manufacturers. Proper sacking is essentially a twilled fabric, in which the number of warp threads per inch greatly exceeds the number per inch of weft. The illustration shows a typical kind of three-leaf twill, double warp sacking. All three-leaf twill sack ings are double in the warp, but four-leaf sackings are usually single; special kinds may be double in the warp. In all cases the warp is comparatively thin, say 61b. to 81b. per spyndle, whereas the weft is thick and single from 161b. to 481b. per spyndle. Cloths are usually 27in. wide, but other widths are made.
The lower part of the illustration shows four repeats of the three-leaf twill, while the lines drawn to the plan of the fabric show that each line of the design is reproduced in the cloth by two warp threads. Large quantities of cotton sacks are made for flour, sugar and similar produce; these sacks are usually plain cloth, some woven circular in the loom, others made from the piece.
Large quantities of seamless bags or sacks for light substances are woven in the loom, but these are almost invariably made with what is termed the double plain weave; i.e., the cloth, although circular except at the end, is perfectly plain on both sides, and similar in structure to, but wider than hose-pipe (q.v.). Circular bags have been made both with three-leaf and four-leaf twills, but it is found much more convenient and economical to make the cloth for these kinds, and in most cases, for all other types, in the piece, and then to make it up into sacks by one or other of the many types of sewing machines. The pieces are first cut up into definite lengths by special machinery, which may be perfectly automatic or semi-automatic—usually the latter, as many thick nesses may be cut at the same time, each of the exact length. The lengths of cloth are then separately doubled up, the sides sewn by special sewing machines.
The chief centres for jute bags are Dundee and Calcutta, all varieties of sacks and bags being made in and around the former city. For paper sacks see PAPER AND PAPER MANUFACTURE.