IDENTIFICATION OF DYES ON OLD TEXTILES
2 PRELIMINARY EXAMINATION
A SMALL SAMPLE of the colored fabric is first boiled in a 1% ammonia solution, in order to remove soil and finishes. Most natural dyes do not run when subjected to this treatment, since they are usually mordant dyes, that is to say they are present in the fabric as insoluble lakes. Indigo is also fast to dilute ammonia. Most of the earlier synthetic dyes made prior to the end of the 19th century run considerably. The same observation is made by textile restorers when they wash carpets with anionic surfactants in ammoniacal solution: if they see that no dye runs they draw conclusions about the presence of natural or synthetic dyes.
Prior to testing the fabric sample is washed with water and then with methanol, pressed between filter papers and set aside to dry.
A small piece of the cleaned dyeing is boiled successively with water, ethanol, glacial acetic acid, and a 20% ammonia solution.1 The material should be thoroughly rinsed in water before being transferred to the ammonia solution. The extents to which the various extracts are colored allow conclusions to be drawn about the types of dyes present. Synthetic acid and direct dyes are extracted by water and by ammonia solution. Synthetic basic dyes are extracted by ethanol and glacial acetic acid. Indigo (natural or synthetic) is extracted by glacial acetic acid.
Most of the natural dyes are extracted only slightly or not at all. There are a few exceptions however, one of which is provided by safflower, whose most important color constituent is carthamic acid. The red carthamic acid loses its color irreversibly when boiled with ammonia, while the safflower yellow that accompanies it goes into solution without change in color. Indigo extract is also extracted to a considerable extent by boiling ammonia. This dye, which has been made by treating indigo with sulfuric acid since 1740, consists principally of indigo-disulfonic acid.
When a synthetic dye is believed to be present because the color is extracted by water and ammonia, or ethanol and glacial acetic acid, simple dyeing tests can be used to indicate whether it is an acid, basic or direct dye. The most strongly colored extract is evaporated to dryness and the residue is taken up in a little water. The solution is divided in two portions; one is acidified with acetic acid, and to part of the other (5 ml) a 5% sodium sulfate solution (1 ml) is added. The solutions are tested on wool and tannin-mordanted cotton.2 if the wool is stained more strongly by the acid solution than the cotton, an acid dye is present. If the cotton is dyed more strongly, a basic dye is present. A direct dye can be recognized by the fact that it easily goes onto unmordanted cotton from the neutral solution containing sodium sulfate.
Two simple tests can be used to confirm the presence of synthetic dyes. If the ammonia extract is strongly colored and the color disappears when the extract is shaken with a little zinc powder at room temperature, the dye is an azo dye. If a few drops of concentrated sulfuric acid are poured over a small portion of a dyed fabric and observed for a few minutes, the acid may develop an intense color (red-violet, blue, or green); and if it does so, this is a certain indication of synthetic dye.