After due recrystallization from alcohol curcumin forms yellow crystals, having an odour of vanilla, and exhibiting a fine blue in reflected light. They melt at 165° C. Curcumin is scarcely soluble, even in boiling water, but dissolves readily on addition of an alkali either caustic or carbonate. On acidulating these solutions, a yellow powder of curcumin is precipitated. Curcumin is not abundantly dissolved by ether, very sparingly by benzol or bisulphide of carbon. It is not volatile; heated with zinc dust it yields an oil boiling at 290°; fused with caustic potash, curcumin affords protocatechuic acid (page 243).
Paper tinged with an alcoholic solution of curcumin displays on addition of an alkali a brownish-red coloration, becoming violet on drying. Boracic acid produces an orange tint, turning blue by addition of an alkaline solution.[2372] This behaviour of (impure) curcumin was pointed out by Vogel as early as 1815, and has since that time been utilized as a chemical test.
Borax added to an alcoholic solution of curcumin gives rise to a crystallizable substance, which Ivanow-Gajewsky (1870) isolated by heating an alcoholic extract of turmeric with boracic and sulphuric acids. It forms a purple crystalline powder with a metallic green lustre, insoluble in water, but soluble in alcohol. Its solution is coloured dark blue by an alkali.
According to the same chemist there also exists in curcuma an alkaloid in very small quantity. Kachler (1870) found in the aqueous decoction an abundance of bioxalate of potassium.
Commerce—In the year 1869 there were imported into the United Kingdom 64,280 cwt. of turmeric; in 1870, 44,900 cwt.,—a very large proportion being furnished by Bengal and Pegu. The export from Calcutta[2373] in the year 1870-71 was 59,352 cwt.
Bombay exported in the year 1871-72, 29,780 cwt., of which the greater portion was shipped to Sind and the Persian Gulf, and only 910 cwt. to Europe.[2374]
Uses—Turmeric is employed as a condiment in the shape of curry powder, and as such is often sold by druggists; but as a medicine it is obsolete. It is largely consumed in dyeing.
Substitute—Cochin Turmeric is the produce of some other species of Curcuma than C. longa. It consists exclusively of a bulb-shaped rhizome of large dimensions, cut transversely or longitudinally into slices or segments. The cortical part is dull brown; the inner substance is horny and of a deep orange-brown, or when in thin shavings of a brilliant yellow. Mr. A. Forbes Sealy of Cochin has been good enough to send us (1873) living rhizomes of this Curcuma, which he states is mostly grown at Alwaye, north-east of Cochin, and is never used in the country as turmeric, though its starchy tubers are employed for making arrowroot. The rhizomes sent are thick, short, conical, and of enormous size, some attaining as much as 2½ inches in diameter. Internally they are of a bright orange-yellow.
The beautiful figures of Roscoe[2375] show several species of Curcuma and Zingiber provided with yellow tubers or rhizomes, all probably containing curcumin.