Orange, Red and Yellow Colors from Coal Tar.

Scientific American 24, 15.6.1861

The following is the substance of a patent lately taken out in England by C. Cowper, of London. It relates to a new method of extracting colors from coal tar. The patentee takes a quantity of the solid pitch obtained from coal tar, which is placed in a clay retort and heated until the retort is red. In conducting this operation, a quantity of red-orange and resinous matter distills over toward the end of the operation. This resinous matter is then treated for 24 hours with cold fuming sulphuric acid, which dissolves it. It is now diluted with water, the excess of acid neutralized with chalk and the clear liquor filtered. This solution slightly acidulated and heated colors silk and wool a red-brown.

A beautiful yellow color is also obtained from the coal tar as follows: - Sulphuric acid, as free as possible from nitrous vapors and sulphate of iron, is heated in a water bath or in a glss or earthenware vessel, to about 190° Fah. The orange-red matter is then added gradually to the extent of about one-ninth the weight of the acid - i. e., nine parts of sulphuric acid to one of hte orange-red matter. When it is found that, by throwing a small quantity of this mixture into water, it is dissolved, the heat must be removed to promote the action of the acid on the coloring matter, teh mixture should be continually stirred during the operation by means of a glass spatula. If neutralized by means of carbonate of soda, a yellow dye is obtained principally for dyeing silks, which is purified in the following manner: - The mixture of the coloring matter with sulphuric acid is diluted with water; it is then neutralized by means of carbonate of lime. After having removed the sulphate of lime again by washing and filtering, the yellow solution is heated to boiling point, and small quantities of hydrate of lime are gradually added, until it is found that, by pouring a small quantity of the yellow solution into a solution of protochloride of tin, a brown powder becomes precipitated. The yellow solution is allowed to cool completely. After separating from it the brown precipitate, by filtering and washing, the yellow solution is again heated to boiling point, and is acidulated with pure hydrochloric acid (muriatic acid). A solution of albumen or gelatine is then added, in small quantities, until it is found that the yellow solution, when filtered and heated to boiling point, colors silks a pure yellow.

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