Of the many tests which may be used to distinguish one fiber from another, only enough will be given here to serve the purpose of those who wish to analyze, in a simple manner, the fabrics commonly found on the markets. No attempt is made to provide a laboratory guide for a course in textiles.
Two kinds of tests used for identifying fibers are the microscopic and the chemical. Some tests are suitable for quantitative work, others only for qualitative identification. In some instances, notably in the case of linen and cotton in the bleached state, the chemical nature of the two fibers is so nearly identical that the quantitative tests are not very accurate.
In qualitative work, the high power microscope is the unfailing test and is the simplest one to apply. Combined with counting, it serves as a partial quantitative test when threads of different fibers are woven together, but not where two fibers are twisted together in one thread. The method of examination is very simple, as it is not necessary to prepare permanent slides. The untwisted end of a thread is placed on the slide dry, or better with a drop of water, and the cover glass put on. It is sometimes necessary to remove the sizing from a material by boiling, with the addition of a little sodium carbonate, and then washing, before a satisfactory microscopic analysis may be made. For a thorough analysis one must be sure that each kind of thread in the material has been examined. The microscopic characteristics of the fibers have been given before and will not be repeated here.
Often a microscopic test may be made more satisfactory by staining the fibers or by treating them on the slide with a chemical which gives a characteristic test.
Cotton and linen, when steeped in a solution of iodine in potassium iodide and then in sulphuric acid and glycerine, are stained blue. The cellulose, by the action of the sulphuric acid, is converted into amyloid, a starch-like substance which is stained by the iodine. The glycerine prevents the destruction of the fiber by the acid.
Three grams potassium iodide in 60 c.c. water, add 1 gram iodine, dilute before using with 10 parts water.
Three parts concentrated sulphuric acid, 1 part water, and 3 parts glycerol.
Fibers moistened first with the iodine solution, then with sulphuric acid solution.1
Methyl violet in alcoholic solution stains linen and cotton a deep blue and shows up the structure of the fiber very well.
When cotton is treated with an ammoniacal solution of copper oxide, the fiber swells and gradually dissolves. The action is slow and a characteristic structure is visible with the high power microscope. Apparently the cotton cell has an outer and an inner cuticle, and in the swelling the outer one constricts the fiber, giving it the appearance shown in the figure. The inner cuticle is also shown. Linen swells in a somewhat different manner from cotton and dissolves rather more slowly. A faint blue coloration is visible in both linen and cotton. Silk swells and dissolves very rapidly in Schweitzer's reagent, as does artificial silk. Wool is apparently not affected at all.
1 Matthews. Textile Fibers, p. 338.
Fig. 45. Cotton Treated with Schweitzer's Reagent.
(a) The sample is boiled for ten minutes in a 5 per cent solution of sodium or potassium hydroxide. The wool present is entirely dissolved, the cotton remains unaltered.
(b) The sample is steeped in concentrated Schweitzer's reagent.1
Two grams of copper sulphate dissolved in 100 c.c. of water. Precipitate copper oxide with a 10 per cent solution of sodium hydroxide. Wash the precipitate thoroughly, dissolve in smallest amount of ammonia sulphuric acid for two or three minutes; the cotton is then washed out, the wool remains.
1 Proportions from Methods of Textile Chemistry, by Dannerth, p. 5.
These tests may be made quantitative by drying the sample at 100° C. and weighing before treating, and by neutralizing the alkali with acid or the acid with ammonia after treatment, washing, drying, and weighing the residue.
The sample is steeped for two minutes in a concentrated solution of hydrochloric acid. The silk is immediately dissolved, while the wool is hardly affected. If the sample is dried and weighed before and after treatment, the test will be quantitative. Loewe's reagent, given below, also serves to distinguish wool and silk. The silk is dissolved.
The sample is treated with a basic solution of zinc chloride, zinc oxide - very concentrated. The silk is almost immediately dissolved; the cotton dissolves more slowly, first becoming gelatinized.
The sample is treated with Loewe's reagent, an alkaline copper-glycerol solution. Silk dissolves readily, cotton is not affected.
Artificial silk is unaffected by Loewe's1 reagent, even when boiled.
Artificial silk stains blue with iodine and sulphuric acid, dissolves readily in Schweitzer's reagent, and gives other tests similar to cotton.
1 Loewe's Reagent. Ten grams of copper sulphate, dissolved in 100 c.c. of water. Five c.c. of glycerol added, then sodium hydroxide solution, until the precipitate is just dissolved.
The sample is steeped for two minutes in concentrated sulphuric acid. The cotton is converted into a jelly-like mass which may be washed out, the linen just begins to dissolve. The linen may be washed, then steeped in dilute ammonia and weighed to give a rough quantitative test. When the linen is bleached it dissolves quite readily in sulphuric acid, so that the reaction must be stopped at the right moment.
Jute and hemp fibers contain ligno-cellulose with iodine, and in sulphuric acid jute turns dark yellow or brown and hemp becomes yellow or greenish in color. Combined with the microscopic tests, the staining tests are more reliable.
For further tests see:
Dannerth. Methods of Textile Chemistry.
Matthews. Textile Fibres.
Matthews. Laboratory Manual of Dyeing and Textile Chemistry.