French Purple.

Scientific American 24, 14.12.1861

(Translated from Dingler's Polytechnic Journal.)

Pourpre française, or French purple, is the name given by Messrs. Quince, Marras & Bonnet, the wellknown dyers in Lyons, France, to a violet pigment produced by them from lichens, such as Lecanora tartarea, Rocella tinctoria, &c. This pigment, in accordance with its origin and nature, closely resembles the dye known as litmus or orchil. It differs materially from it, however, in two particulars; first, by its much greater ability to be fixed on fabrics, particularly silk and wool, without the use of mordants, and to produce genuine and durable color; and second, from the fact that its violet hue is changed to red only by pretty strong acids, whereas orchil turns red by the action of weak acids.

The process of manufacturing this dye comprises, first, the preparation of the material from which the dye is obtained and which is composed principally of lecanoric, erythric and orsellesic acid, &c.; second, the conversion of this material into dye by the action of ammonia, air and heat; and, third, the preparation of the dye in a solid state.

The several acids of the lichens can be extracted by means of alcohol, hot acetic acid, a mixture of alcohol and ammonia, or any other alkali. If ammonia is used, it is diluted with four or five times its volume of water, and a systematic extraction is effected by exposing parcels of the lichens which have already been extracted, to a greater or smaller extent, and finally, fresh lichens to the action of the same quantity of liquid ammonia, whereby the liquid is completely saturated with acid. The extract is afterward mixed with a surplus of sulphuric or muriatic acid, whereby the acids of the lichens are precipitated, and then collected on a filter and carefully washed and dried. The acids of the lichens can also bo extracted by heating the plants with milk of lime and precipitating them with muriatic acid; or the lichens may be boiled in dilute sulphuric acid and afterward washed with water. In this Case the acids remain in combination with the woody parts of the lichens.

The precipitate obtained by either one of the above methods is now mixed with sufficient ammonia to dissolve it, and this mixture is boiled, whereby a liquid is obtained the color of which soon turns to an orange, and which, if exposed to the atmospheric air at a temperature of from 60° to 70°, soon changes in the desired manner by assuming diffrent colors, one after the other, until at last it becomes a bright red. While in this condition the liquid is put into flat vessels and gradually heated to from 120° to 140°. After a few days the liquid assumes a purple violet color, and is not affected by weak acids, and it will dye silk and wool without the aid of other substances; it can also be easily fixed on cotton by suitable mordants. If, instead of the acids, the lichens themselves, purified by being treated with diluted acid, are employed, the proceeding is substantially the same until the red color is formed, and after this the liquid is separated from the fibrous parts by the aid of a press.

As soon as the acids of the lichens have been converted into the dye, the liquids from which the aside have been obtained are mixed together and saturated with sulphuric or some other acid. By this operation a copious precipitate is obtained which is collected on a filter and carefully washed and dried.

Instead of precipitating the red ammoniac liquid with sulphuric acid, as stated above, chloride of calcium may be used, whereby a precipitate is obtained having the appearance of indigo with a violet color. In this condition it is brought to the market.

[We have had a small sample of this substance in our possession for about two years. It was brought over here by a French chemist who endeavored to introduce it among our dyers. The color is a deep rich purple, somewhat resembling the section of a cube of Bengal indigo, when rubbed with the fingernail. Its use, we understand, has lately been almost superseded in France by new aniline colors.

—Eds.