The Colors of Coal Tar. Number 1.

Scientific American 19, 10.5.1862

The art of dyeing has been almost revolutionized within a very few years by the production of several most brilliant colors from artificial compounds derived from certain producs of coal tar. Such colors are triumphs of moder chemical skill. THe chemist takes the offensive coal tar of our gas works and extracts colors therefrom which impart beauty to the finest products of the loom. A description of these colors and their application will be generally useful and interesting. Ailine is the chief basis of these peculiar dyes. This substance was first discovered in coal tar about 841, by Dr. Hoffman, of London, and a pupil of this distinguished chemist, W. A. Perkins by name, first produced aniline purple on a commercial scale.

Aniline is an alkaloid, and is derived from benzole, otherwise called benzine - not the improperly-named benzine of petroleum. The chemical symbol of benzole is C₁₂H₆, It is obtained from purified coal naphtha by careful distillation at a temperature of about 186°. Benzole is a volatile fluid - a carnuret of hydrogen.

The next step in the process of producing aniline is to mix benzole with strong nitric acid in about equal quantities. A violent action ensues with a development of heat, and nitro-benzole is formed. Its chemical symbol is C₁₂H₅NO₄. To convert this substance into aniline, its ozygen must be displaced and two equivalents of hydrogen added. For this purpose equal quantities of it and acetic acid are mingled together, and also an equal quantity of iron filings. A chemical action takes place, the iron is converted into an oxide, two equivalents of hydrogen in the nascent state unite with the nitro-benzine and four equivalents of oxygen are driven off, leaving aniline - the composition f which is  C₁₂H₇N - mixed with the ozide of iron. The latter is removed by two distallations in a retort. A very small quantity of fresh-slacked lime is added before the second distillation. The same result may also be obtained by using zinc instead of iron filings, and sulphuric instead of acetic acid.

Aniline is a colorless fluid; it boils at 359° and has a specific gravity of 1.028. Being of an alkaline character it combines with acids, forming salts. When combined with sulphuric acid it forms the sulphate of aniline, which is the salt used by Mr. Perkin for making his purple. Equal quantities of the sulphate of aniline and the bichromate of potash in solution are mixed together and allowed to stand until their reaction is complete ,when a black precipitate falls down to the bottom of the vessel. This precipitate is placed upon a filter and washed with soft water to free it from any sulphate of potash that is in it, after which it is dried. It now contains some resinous matter which is injurious to its coloring qualities. This resin is removed by digesting it several times in purified naphtha, or until it ceases to give a brown coor to the naphtha. After this it is boiled with alcohol and distilled, so as to drive off the whole of the alcohol. The product left in the retort is a beautiful bronze-colored substance constituting the commercial purple of Perkin, and it produces the same color as that which is called magenta. Other metallic salts, as well as the bicromate of potash, when combined with aniline, will produce purple colors. This color in its dry bronze state is only slightly soluble in water, but it dissolves freely in common alchol, or methylic alcohol (wood spirits made by distilling birch wood, &c., in retorts). This purple color possesses the valuable property of not being affected with light, acids or alkalies. Wool, silk and cotton are dyed with it by the following processes: -

An alcoholic solution of this bronze substance (a very small quantity is required) is placed in a hot-water bath, slightly acidulated with tartaric acid, and the whole thoroughly stirred. The white wool to be dyed is now placed in this and handled rapidly, the temperature being maintained at about the boiling point. In a short periof the desired shade will obtained. Small quantities will color lilac shades, a larger quantity a purple. Silk is dyed in the same manner, only the temperature of the bath is lower. By adding a small quantity of the sulphate of indigo (dyer's chemic) to the bath, a beautiful lavender color is produced. An endless variety of shades can be dyed in this manner on silk and wool, y simply using different quantities of the aniline purple and sulphate of indigo. The process of coloring cotton with aniline purple, is more complicated and requires greater skill than the coloring of silk or wool. The cotton is first handled then steeped in a tannin solution, such as that of an extract of sumac, for about two hours, after which it is handled in a weak solution of the stannate of soda for about one hourm when it is run through a weak "sour" of dilute sulphuric acid and rinsed afterward in cold water. When squeezed or wrung it is ready for the coloring operation, which is performed in a warm bath, exactly as for the silk, only it requires about double the quantity of coloring matter for the same quantity of cotton. Aniline purple may also be dyed on cotton by preparing it first with several semi-soapy solutions, by what is called the Turkey-red process, also by giving it a mordant of the acetate or nitrate of lead.