Source, Etc

The logwood tree, Hoematoxylon campechianum, Linne (N.O. Leguminosae), is a tree of moderate size, indigenous to Central America, but naturalised in the West Indian Islands. The use of the wood as a dye was probably known to the Mexicans, for its introduction into Europe followed closely on the conquest of Mexico by Cortes; in 1746 it was introduced into the London Pharmacopoeia as a mild astringent, but is now not much used medicinally.

The wood is exported in the form of billets and logs from which both bark and sapwood have been separated; the heartwood alone contains the colouring and astringent principles.

Description

The logwood of commerce consists of the heartwood of the tree, and is imported in logs and billets from 6 to 15 cm. in diameter and 1 to 2 or more metres in length. Externally these are of a dull dark orange or purplish red colour, internally they are reddish brown; they are hard and heavy, but easily split. The transverse section exhibits under the lens very narrow and closely approximated medullary rays and narrow concentric dark zones alternating with paler ones, a difference due to the colouring matter secreted in the former. The freshly felled trunk wood is pale in colour; the gradual darkening is due to oxidation of the hematoxylin to haematein.

The odour of the chips is faint but pleasant, recalling that of violets; the taste is sweetish and astringent. It imparts a violet colour to dilute aqueous solutions of caustic alkalies.

Logwood is cut by suitable machinery into chips or turnings of a reddish brown colour, and these are usually subjected to a process of fermentation ('ageing'). They are well moistened, heaped together, and exposed to the air for a period of from four to six weeks, the heaps being frequently turned over; they are then dried. By this process the chips darken in colour and exhibit patches of a dark beetle-green lustre. The unfermented chips are alone official. Fermentation of logwood is not much practised, as it has been found that the oxidation of the hematoxylin to haematein that is necessary in the dyeing process can be effected by using an oxidising mordant (potassium dichromate).

The student should observe

(a) The reddish brown colour of the chips,

(b) The violet colour they produce in contact with alkaline solutions.

Constituents

The principal constituent of unfermented logwood is haematoxylin, C16H1406,3H10, of which it contains about 10 per cent. This, when pure, forms colourless crystals which acquire a reddish colour on exposure to the air. It is sparingly soluble in water, but dissolves readily with purple coloration in solutions of the caustic or carbonated alkalies and ammonia. The latter solution absorbs oxygen from the air, forming hematein-ammonia, from which acids separate haematein, C10H12O6, a dark violet crystalline body with green metallic lustre. This change takes place during the fermentation of logwood, the haematoxylin being partially converted into haematein. Haematoxylin is hydroxybrasilin (see below) and haematein hydroxybrasilein.

Logwood contains, further, tannin, resin, quercetin, and a trace of volatile oil. The sweetish taste is produced by the hematoxylin, the astringency by the tannin.

Varieties

Several commercial varieties of the wood are recognised, that from Yucatan (Campeachy) being considered the best, while British Honduras and San Domingo also furnish wood of good quality; Jamaica logwood is less esteemed as it is inferior in colouring power. Bastard logwood is the name given to a variety of the wood of paler colour and much less colouring power than the genuine; it appears to be derived from a variety of H. campechianum.

Uses

Logwood is largely used as a dye and in the manufacture of inks. Much of it is converted for these purposes into an aqueous extract containing about 50 per cent. of haematoxylin and from 10 to 30 per cent. of haematein. Medicinally, it is employed occasionally as a mild astringent.