146 ADULTERATION AND DETECTION. 



Chinese as " Bastard tea," and is rarely sold alone, being 

 used principally for mixing or blending with pure 

 teas. 



The presence of spent or exhausted leaves in either 

 Green or Black tea is best determined by estimating the 

 amount of tannin contained in the liquor after infusion, 

 and for which experiment various tests are in use. A 

 large proportion of the tea-extract is found to consist of 

 tannin (tannic acid), there being much more in Green 

 than in Black tea, the larger portion of that originally 

 existing in the latter being dissipated by the extra fer- 

 mentation to which this variety is subjected in curing 

 and firing. Green teas contain on an average about 15 

 per cent, of tannin, Black teas never exceeding 10 per 

 cent. This rate, however, varies considerably from differ- 

 ent causes, such as age, quality, soil and climatic condi- 

 tion of the districts of growth, the main average being 

 12 and 9 per cent, respectively in pure teas. Spent or 

 exhausted leaves, on the other hand, contain only 2 per 

 cent, on an average at the highest estimate, a difference 

 of 7 to 10 per cent, of tannin, as will be observed, in 

 favor of pure teas. 



For the purpose of estimating the percentage of 

 tannin contained in tea, the simplest method is to make 

 an infusion of the leaves and pour it into a cup or glass 

 and add to it a small quantity of a standard solution of 

 plumbic acetate. The acetate will cause the tannin to 

 form a precipitate, which must be removed and weighed 

 in a small scale fitted for the purpose ; then by taking 

 the nominal percentage of tannin contained in pure teas 

 at 12 in Green and 10 in Black and 2 per cent, in Spent 

 or exhausted leaves, the difference will be the percentage 

 of adulteration, the extent being indicated by the lessened 

 proportion of tannin in the same ratio. The presence of 



