Determining the Count of Warp and Filling Threads. Every fabric must contain a certain count of warp and filling threads—a definite number within a certain space for each strength of yarn employed. A fabric is not up to the standard of density when less than the requisite number of warp or filling threads per inch is found. For example, if a buyer was told that a fabric is 80 square, that is, eighty warp threads and eighty filling threads to the inch, and on examination found only 72 square, he would immediately reject the goods.
The count of warp and filling is determined by means of a pick-glass—a small mounted magnifying glass—the base of which contains an opening of one-half inch by one quarter inch, or one quarter inch by one quarter inch. If the pick-glass is placed on the fabric the number of warp and filling threads may be counted, and the result multiplied by either two or four, so as to give the number of threads to the inch. For example, if I count twenty picks and twenty threads on a one quarter-inch edge, there are eighty picks and eighty threads to the inch. A more accurate result can be obtained by using a pick-glass with a one-inch opening.
Determination of Shrinkage. A very important factor in the value of a fabric is the shrinkage. The extent of this may be determined by pouring hot water over a sample of about twelve by twenty inches, and leaving the fabric immersed over night, then drying it at a moderate temperature without stretching. The difference in length gives the shrinkage, which is usually expressed in percentage.
Determination of Weight. Buyers and sellers of dry goods, when traveling, are anxious to determine the weight of fabrics they examine. This may be done by means of small pocket balances so constructed as to give the number of ounces to the yard of a fabric.
Testing the Constituents of the Warp and Filling. Take a sample piece of the cloth to be examined—the piece must be large enough to contain specimens of all the different kinds of yarn present in the material—and separate all the filling and warp threads. Be sure that all double threads are untwisted.
Combustion Test; Test for Vegetable and Animal Fibers. Burn separately a sample of the untwisted warp and filling threads. If one or both burn quickly without a greasy odor, they are vegetable fibers, cotton or linen. If one or both burn slowly and give off a greasy odor, they are animal fibers, wool or silk. This test is not conclusive, and further chemical examination—acid test—must be made to ascertain whether wool is pure or mixed with cotton.
Acid Test. The vegetable fibers, cotton and linen, are distinguished from those of animal origin by their behavior in the presence of acids and alkalies. The vegetable are insoluble when boiled with a 4 per cent sodium hydrate solution, but readily clear or carbonize when saturated with a 3 per cent sulphuric acid solution and allowed to dry at a high temperature in a hot closet. Wool on the other hand is not affected by the action of weak sulphuric acid.
Cotton Distinguished from Linen. If the fibers are vegetable, cotton may be distinguished from linen by staining the fibers with fuchsine. If the fibers turn red, and this coloration disappears on the addition of ammonia, they are cotton, if the red color remains the fibers are linen. Whenever cotton yarn is used to adulterate other fabrics, it wears shabby and loses its brightness. When it is used to adulterate linen, it becomes fuzzy through wear. One may detect it in linen by rolling the goods between thumb and finger. Linen is a heavier fabric, and wrinkles much more readily than cotton. It wears better, and has an exquisite freshness that is not noticed in cotton fabrics.
Silk Distinguished from Wool. Place the fabric or threads containing animal fibers in cold, concentrated hydrochloric acid. If silk is present it will dissolve, while wool merely swells.
Artificial Silk from Silk. On account of the low value of the artificial and the high value of genuine silk, there is a tendency to offer the artificial instead of the pure article. Test: When artificial silk is boiled in 4 per cent potassium hydrate solution it produces a yellow solution, while pure silk gives a colorless solution.