This section is from the "A Complete Dictionary of Dry Goods" book, by George S. Cole. Also available from Amazon: A complete dictionary of dry goods and history of silk, cotton, linen, wool and other fibrous substances,: Including a full explanation of the modern processes ... together with various useful tables.
Sack-Cloth. A penitential fabric. The ancients, more particularly the Hebrews and Assyrians during the period of great affliction, laid aside the garments best suited to their ease and comfort and put on sack cloth, which was woven of the coarse, short hair of animals, twisted together and used for both warp and weft; this coarse, prickly material was made into garments which fit close like a sack, without trimming or other adornment, and were worn solely to inflict personal discomfort in cases of grief or penance. [See Haircloth]