Method of Detecting the Adulterations of Tea
The adulteration of tea may be evinced by comparing the botanical characters of the leaves of the two respective trees, and by submitting them to the action of a few chemical tests.
The shape of the tea-leaf is slender and narrow, as shewn in this sketch, the edges are deeply serrated, and the end or extremity is acutely pointed. The texture of the leaf is very delicate, its surface smooth and glossy, and its colour is a lively pale green.
The sloe-leaf (and also the white-thorn leaf,) as shewn in this sketch, is more rounded, and the leaf is obtusely pointed. The serratures or jags on the edges are not so deep, the surface of the leaf is more uneven, the texture not so delicate, and the colour is a dark olive green.
These characters of course can be observed only after the dried leaves have been suffered to macerate in water for about twenty-four hours.
The leaves of some sorts of tea may differ in size, but the shape is the same in all of them; because all the different kinds of tea imported from China, are the produce of one species of plant, and the difference between the green and souchong, or black tea, depends chiefly upon the climate, soil, culture, age, and mode of drying the leaves.
Spurious black tea,86 slightly moistened, when rubbed on a sheet of white paper, immediately produces a blueish-black stain; and speedily affords, when thrown into cold water, a blueish-black tincture, which instantly becomes reddened by letting fall into it, a drop or two of sulphuric acid.
Two ounces of the suspected leaves, should be infused in half-a-pint of cold, soft water, and suffered to stand for about an hour. Genuine tea produces an amber-coloured infusion, which does not become reddened by sulphuric acid.
All the samples of spurious green tea (nineteen in number) which I have examined, were coloured with carbonate of copper (a poisonous substance,) and not by means of verdigris, or copperas.87 The latter substances would instantly turn the tea black; because both these metallic salts being soluble in water, are acted on by the astringent matter of the leaves, whether genuine or spurious, and convert the infusion into ink.
Tea, rendered poisonous by carbonate of copper, speedily imparts to liquid ammonia a fine sapphire blue tinge. It is only necessary to shake up in a stopped vial, for a few minutes, a tea-spoonful of the suspected leaves, with about two table-spoonsful of liquid ammonia, diluted with half its bulk of water. The supernatant liquid will exhibit a fine blue colour, if the minutest quantity of copper be present.
Green tea, coloured with carbonate of copper, when thrown into water impregnated with sulphuretted hydrogen gas, immediately acquires a black colour. Genuine green tea suffers no change from the action of these tests.
The presence of copper may be further rendered obvious, by mixing one part of the suspected tea-leaves, reduced to powder, with two or three parts of nitrate of potash, (or with two parts of chlorate of potash,) and projecting this mixture by small portions at a time, into a platina, or porcelain-ware crucible, kept red-hot in a coal fire; the whole vegetable matter of the tea leaves will thus become destroyed, and the oxide of copper left behind, in combination with the potash, of the nitrate of potash (or salt-petre,) or with the muriate of potash, if chlorate of potash has been employed.
If water, acidulated with nitric acid, be then poured into the crucible to dissolve the mass, the presence of the copper may be rendered manifest by adding to the solution, liquid ammonia, in such quantity that the pungent odour of it predominates.
Footnotes
85 Also, 2 Geo. I, c. 30, § 5; and 4 Geo. II, c. 14, § 11.
86 The examination of twenty-seven samples of imitation tea of different qualities, from the most costly, to the most common, which it fell to my lot to undertake, induces me to point out the marks of sophistications here detailed, as the most simple and expeditious.
87 Mr. Twining, an eminent tea-merchant, asserts, that “the leaves of spurious tea are boiled in a copper, with copperas and sheep’s dung.”—See Encyclop. Britan. vol. xviii. p. 331. 1797. See also the History of the Tea Plant, p. 48; and p. 167 of this Treatise.