LICHENS, Colouring matters of. The lichens or liverworts, as they arc frequently termed, constitute a large class of perennial pleats. They are common in nearly all growing on rocks, stones, trunks of trees, &e., and in appearance are insignificant. They vary considerably in colour and size, but generally so much resemble the bark or earth to which they are attached, and over which they spread, that, a somewhat close inspection is necessary before they are observed. Unimportant as they appear to be, however, it is greatly to their agency that we owe those magnificent lilac, blue, violet, purple, and many intermediate and compound colours with which wool and silk are dyed.
The English lichens are not much used for the manufacture of dyes, though were a system for their collection organised, it would probably be found that many varieties might be economically employed. Foreign lichens to the extent of 00,0001. worth are annually imported into this country from the south of Europe and west coast of America, but chiefly front Canary Islands. They are principally collected from rocks in the-neighbourhood of the sea.
Lichens do not contain in their natural state any colouring matter ready formed, but under the influence of air, water, ammonia, and a moderate temperature, certain acids and a neutral body existing in them are converted into a deep richly-coloured substance called orcein. Orcein is the dark purple or red matter that is finally produced under the above circumstances, but its formation ie sufficiently gradual to admit of the process being arrested at any desired stage, and the tint ii the actual colour of the product thus varied at will. On these conditions, and on the dexterous mixture of the products with each other depends the shade of the resulting pigment, and not on the use of any particular variety of lichen. A skilful manufacturer will produce a given colour from any or all of the lichens, his only anxiety belag to use that variety, which contains the largest percentage of colorific material.
The lichen colours are sent out by the maker either in the dry state, lump or powder, as a strong solution, or as a paste, conditions severally best suited to the uses to which they are applied. The details necessary to be observed in the production of these various forme of the colour are given in the respective artioles on each. See ARCuIL; CUDBEAR ; LITMUS.
:Estimation of the value of lichens.—Lichens vary considerably in the
amount of colour.producing substance contained in them. The fol lowing process, proposed by Stenhonae, is very easy of application, and gives accurate relative results. It is founded on the reaction of chloride of lime upon, the bo]orific agent, a fugitive red colour being thereby produced. A. given weight of the lichen reduced to powder in a mortar, or, to a pulp with water, is digested in thin of lime, and the mixture filtered ; a standard solution of chloride of lime is now added in small quantities at a time, fresh portions being poured in as soon as the red colour produced by the previous portion has disappeared, the addition being discontinued when no more colour is produced. The relative value of several samples can thus be very qdickly determined, or the strength of the standard solution being known, the absolute amount of colouring matter is clearly indicated.
As already stated, orcein is the colouring substance ultimately resulting from the treatment of lichens for the preparation of archil, &c. Now the body from which orcein is derived is a colourless matter, named orcin. Orcin exists ready formed in some of the lichens, but its chief source is certain acid matters contained in all varieties of the plant. Under the influence of alkalies these acids are converted irta orcin, and into secondary acids. The primary lichen acids admit of being readily extracted, and Stenhouse has proposed that the opera tion be performed upon the lichens as soon as gathered, the storage, freightage, &e., of a great quantity useless material being thereby obviated. The process recommended by that chemist consists in placing the lichens in wooden barrels, which are to be filled up with water containing a quantity of slaked lime, allowing the whole to stand a short time, then drawing off the liquor and saturating it with common hydrochloric acid. in this way the lichen acids are dissolved out by the lime, but are re-precipitated by the hydrochloric acid, and after collecting on a filter may be drained, dried, and packed in casks for transit. This however, has not been adopted, or if adopted has net been persisted in ; perhaps the ready-formed orcin occurs to a larger extent in the lichens than has been supposed, and not being precipitated from solution by hydrochloric acid, would be lost if the whole lichen were not acted upon.