Leaves used in the adulteration of tea—the sloe, willow, oak, beech, elder, and hawthorn, have been nature-printed and then lithographed. The drawings of the chloranthus inconspicuus and the camellia sasanqua, which are said to be used by the Chinese, are copied from Hassall. The leaves of the elm, poplar, and plane are said to be sometimes used in England. Falsification with any kind of leaf is, however, now decidedly uncommon in this country.
3. Adulterants for imparting a fictitious strength. (a) Extraneous tannin matters, such as catechu, &c., are detected by an unusually high percentage of tannin, as indicated by the lead process. Tea, adulterated with catechu, gives an infusion which quickly becomes muddy on cooling. 1 gram of the sample and 1 gram of pure tea are each infused in 100 c.c. of water, and the solutions poured
off from the leaves are precipitated, while boiling, with a slight excess of neutral lead acetate, filtered, and tested as follows:—About 20 c.c. of the pure tea infusion, when gently heated with a few drops of silver nitrate, gives a slight cloudiness only; while tea, containing catechu, gives a copious brownish precipitate, and the liquid acquires a distinct yellow tinge. One drop of ferric chloride gives a light green colour if catechu is present, and a greyish-green precipitate on standing; the solution from pure tea gives a reddish colour with ferric chloride, due to acetate, and no precipitate on standing. These tests are applicable only when catechu is present in tolerably large quantities.
(b) Lie tea, when thrown into hot water, falls to powder, because the gum or starch used to keep it in a compact form is dissolved. The liquid may be acidified with sulphuric acid, decolourised with permanganate, and tested for starch. The ash of lie tea is often as high as 30 or 40 per cent.
(c) Caper tea is made into little glossy masses by the aid of gum or starch; it is usually much adulterated. The insoluble matter is usually much less than in genuine tea; the gum amounts to 15 or 20 per cent. The soluble ash often falls below 2 per cent.
(d) Soluble iron salts are added to give an appearance of strength by the formation of tannate of iron. They are detected by shaking the powdered leaves with cold dilute acetic acid, filtering, and testing for iron, in the filtrate.
(e) Alkaline carbonates are sometimes added to tea. The soluble ash gives the yellow sodium flame, if sodium salts have been added; the alkalinity may also be determined in the soluble ash. The average amount of potash (K2O) in tea is about 1·62 per cent.
4. Facing and colouring materials. These may be detected under the microscope, or the leaves may be washed with warm water, the colouring matter collected and examined. Indigo is best detected by the microscope; Prussian blue, by boiling with caustic alkali, filtering, and testing for ferrocyanide by ferric chloride. The residue, insoluble in alkali, is fused with alkaline carbonate, evaporated to dryness with hydrochloric acid; the residue tested for silica, and the filtrate tested for lime and magnesia.
Moisture varies from 6 to 8 per cent.