TEA
Tea is adulterated by the substitution of inferior grades for those of better quality, by the addition of exhausted leaves and foreign leaves, by the use of coloring matter or “facing” such as Prussian blue, indigo, or turmeric to color green tea, and sometimes graphite to color black tea. Foreign astringents (generally catechu) are added to conceal the presence of exhausted leaves. An imitation tea, “lie tea,” is made of the stems and dust with mineral matter, and some starch or gum to hold these together.
Foreign Leaves
Though there are several chemical tests for foreign leaves, none are as satisfactory as a microscopical examination. Soften the leaves by soaking in hot water, unroll carefully and examine with a hand lens or low power of the microscope. Compare with a genuine leaf—the shape, margin, and venation.
Exhausted Tea Leaves
Sometimes such leaves may be detected by a physical examination. They are often more or less unrolled and broken on the edges. But the only certain way of ascertaining their presence is to determine the soluble ash which is from 2.5 to 4 per cent in pure tea and usually less than 0.8 per cent in exhausted tea.
Lie Tea
This imitation tea is easily detected by pouring hot water over the leaves. If they are artificial, they will break down into the fragments of which they were made.
Facing
Organic coloring matter may be detected by the same method used for detecting such colors in coffee.
Catechu
Hager’s Test.—Boil a little of the tea in water, and add to the extract an excess of lead monoxid. If the tea is pure the addition of a solution of silver nitrate produces only a slight grayish precipitate, but when catechu is present a yellow flocculent precipitate forms.