Attention has already been called to the fact that one of the methods of adulterating tea is the use of spent or exhausted leaves. These may be skilfully treated, faced, and brought into a condition which resembles very much unexhausted leaves. The microscope is not of much value for detecting exhausted leaves, because the microscopic appearance of the exhausted leaf is usually made to resemble very closely that of the natural leaf.

1 Report of George F. Mitchell Fiscal year ended June 30, 1918.

The best method of detecting the adulteration is, by chemical examination. A considerable addition of exhausted leaves to a tea will result in a diminution of the chemical constituents which are characteristic of teas. Happily this form of adulteration is not often found. Also the use of foreign leaves, a form of adulteration easily detected, has lost its vogue.

There is a difficulty attending the chemical method of detection to which attention should be called. While the average constitution of a tea is very well known, there are many very wide variations from that average. For instance, the tannin itself may sometimes reach as much as 15 or 20 percent and sometimes fall below 7 or 8. It follows, therefore, that a tea from a leaf which originally contained 20 percent of tannin may be well exhausted, and still contain as much tannin as an unexhausted leaf of another kind. Generally, however, the best way is to rely upon the total extracted matter. If that is far below the average for tea, the use of exhausted leaves may be suspected, and a more careful examination made.

An important thing also in this matter is the careful study of the leaves themselves. There are certain physical changes which take place in an exhausted leaf which, with a little experience, would indicate to the expert whether or not this form of adulteration had been practised. If the leaves are very much broken, frayed, or partially unrolled, it is evident that it has once been extracted. The percentage of ash is also another means of aiding in the detection.