Archil or Orchil.CHAPTER XVI. Lichens, Lichen acids, and Colouring Matters Derived Therefrom.

The Natural Organic Colouring Matters
By
Arthur George Perkin, F.R.S., F.R.S.E., F.I.C., professor of colour chemistry and dyeing in the University of Leeds
and
Arthur Ernest Everest, D.Sc., Ph.D., F.I.C., of the Wilton Research Laboratories; Late head of the Department of Coal-tar Colour Chemistry; Technical College, Huddersfield
Longmans, Green and Co.
39 Paternoster Row, London
Fourth Avenue & 30th Street, New York
Bombay, Calcutta, and Madras
1918

Kaikki kuvat (kemialliset kaavat) puuttuvat // None of the illustrations (of chemical formulas) included.

Archil or Orchil (Orseille, Fr.; Orseille, Ger.; Oricello, It.) appears in commerce in three forms: (1) as a pasty matter called archil; (2) as a mass of a drier character, named persio; and (3) as a reddish powder called cudbear. It is obtained from various lichens of the genus Roccella, growing on the rocky coasts of the Azores, the Canaries and Cape de Verd Isles, also of the Cape of Good Hope, Madeira, Corsica, Sardinia, etc., and from Ochrolechia tartarea, growing in Sweden and Norway. None of these lichens contains the colouring matters ready formed, but they contain certain colourless acids of the type of lecanoric acid, derivatives of orcin, into which they can be readily converted. Thus, lecanoric acid (1) gives first orsellinic acid (2) and subsequently orcin (3) according to the following scheme (see under LECANORIC ACID): [KUVA PUUTTUU]

Orcin itself, when acted upon by air and ammonia, changes into a purple substance called orcein, which is the name applied to the colouring matters of archil (Robiquet, Ann. Chim. Phys., [2], 47, 338).

Finely powdered orcin is placed in. a thin layer under a bell jar, together with a beaker containing strong ammonia solution. As soon as the substance has become brown coloured, it is removed and exposed to air for some time. It is then dissolved in very dilute ammonia solution, reprecipitated with acetic acid, and dried. According to Gerhardt and Laurent, orcein has the composition C₁₄H₇NO₆ (Ann. Chim. Phys., [3], 24, 315), but more recent researches indicate that it is a mixture of substances. Liebermann, for instance (Ber., 7, 247; 8, 1649), considers that by this reaction three colouring matters are produced, having respectively the formulæ (a) C₁₄H₁₃NO₄; () C₁₄H₁₂N₂O₃; and (c) C₁₄H₁₂N₂O₃.

Zulkowski and Peters (Monatsh., 11, 227) allowed orcin to remain in contact with ammonia for two months, and from the product isolated three substances:

(a) Red orcein, C₂₈H₂₄N2O₇, the main product, which appears to be formed according to the following equation:
4C₇H₈O₂ + 2NH₃ + 6O = C₂₈H₂₄N₂O₇ + 7H₂O
It is a brown crystalline powder, soluble in alcohol with a red colour, and in alkaline solutions with a blue-violet tint.

(b) A crystalline yellow compound, C₂₁H₁₉NO₅, which is accounted for as follows: 3C₇H₈O₂ + NH₃ + 3O = C₂₁H₁₉NO₅ +4H₂O

(c) An amorphous product similar to litmus.

These substances can be prepared much more rapidly by the addition of hydrogen peroxide to an ammoniacal solution of orcin.

There can be no doubt that this reaction proceeds in several stages, and that the character of the product varies according to the duration of the process. This is well known to manufacturers, whocan prepare at will a blue or a red orchil. The constitution of these colouring matters has not yet been determined, but in view of the circumstances by which they are produced, it is most probable that they are members either of the oxazine or oxazone groups.

Orchil was originally prepared from the lichens by means of stale urine, which supplied the necessary ammonia, but ammonia solution is now exclusively employed. The older methods have, however, been greatly improved, and in the place of barrels the operation is carried out in large horizontal or vertical cylinders fitted with stirrers, and suitable openings for the admission of air.

In such an apparatus the weed is digested with about three times its weight of ammonia solution at 60° for from three days to one week, the admission of air being regulated according to the judgment of the manufacturer. The first product of the reaction has a blue colour, and if the process be stopped at this point, there is formed the dyeware known as blue orchil. On the other hand, if the action of the air and ammonia is allowed to proceed further, red orchil is obtained. These orchil pastes when dried and finely ground constitute the product known as cudbear.

Bedford (Ger. Pat. 57612, 1889) blows air or oxygen through the ammoniacal mixture, which, especially in the latter case, materially shortens the process. The apparatus employed is erected vertically, and by an ingenious arrangement of projecting shelves, the edges of which are turned down, a considerable quantity of the air or oxygen is entrapped, and exerts therefore a more powerful oxidising effect.

Orchil liquor is prepared by extracting the lichens with boiling water, concentrating the extract to from 8-10° Tw., and submitting this to the action of air and ammonia; whereas orchil extract is produced by the extraction of orchil paste itself.

In former times archil and cudbear were frequently adulterated with magenta, certain azocolours, extracts of logwood, brazilwood, etc.; but as the importance of these dyestuffs has now very greatly diminished, such a contamination is at the present time of rare occurrence.

Archil and its preparations are substantive colouring matters, which dye well in a neutral bath, but have the useful property of behaving nearly as well under slightly acid or lightly alkaline conditions. Even colours of considerable intensity are produced from it without difficulty, but unfortunately these are not fast to light. Wool is dyed in a neutral bath, or with addition of a trace of sulphuric acid, and silk is dyed in the presence of soap solution, acetic acid being sometimes added. Archil is not applied to cotton.

Archil was at one time employed to a large extent for "bottoming" indigo, that is to say, the fabric was first dyed with archil and subsequently with indigo. The reverse process, known as "topping," has again been considerably in vogue. Cudbear and archil are also used to a limited extent in conjunction with other dyestuffs for the production of compound shades. White wines are sometimes coloured with archil, but its presence can be detected by precipitating with lead acetate and extracting with amyl alcohol, when a red colour indicates the presence of archil or magenta. The addition of a little hydrochloric acid changes the colour to yellow if magenta be present, but does not alter it if archil is the adulterant (Haas, Zeitsch. anal. Chem., 20, 869; J. Soc. Chem. Ind., 1, 119).