28 U. S. BUREAU OF FISHERIES 



give a grainy coating. For bead pearls the larger particles give too 

 great a brilliancy, approaching metallic luster, while those too small 

 make dull pearls. Intermediate sizes produce the more desirable 

 soft pearly luster. Lar^e crystals can be broken in a pebble mill 

 with 5-milliuieter glass beads. The grinding is best done in very 

 concentrated suspension in amyl acetate, and its progress should be 

 closely watched so as not to grind the particles too fine. 



CHEMICAL EXAMINATION AND ASSAYING OF PEARL ESSENCE 



There is no standardized procedure for the examination of pearl 

 essence, nor is there any standard of concentration, covering power, 

 or value. An experienced eye is now indispensible in judging this 

 product. Fineness of grain, tint, luster, concentration, and, in the 

 lacquers, viscosity must be taken into consideration. The writer 

 has consulted the literature touching on the estimation of guanin 

 and further, as a result of laboratory examination of numerous speci- 

 mens, has arrived at certain tentative methods that are proposed for 

 evaluating pearl essence. 



QUAIJ^TITATIVE ESTIMATION OF GUANIN 



Balke (1893) devised a volumetric method of estimating guanin 

 by titrating with Fehling's solution in the presence of hydroxylamine 

 hydrochloride. The copper compound C5H5N5OCU2O is formed 

 and the end-point is the yellow-red precipitate of hydrated copper 

 • oxide. The guanin is dissolved in alkali solution, and 1 cubic centi- 

 meter of medium concentration of hydroxylamine hydrochloride is 

 added. The Fehling's solution is admitted slowly from a burette. 

 The quantities found by this method were in every case too low. 

 In a series of five determinations the average deficiency was 9.14 per 

 cent. In 1 per cent solutions of guanin the results were only 1 per 

 cent too low, but in more dilute solutions the results were as much 

 as 14 per cent too low. In the case of concentrated solutions he 

 found that the addition of sodium acetate reduces the error. 



Wulff (1893) describes a method of determining guanin as a 

 picrate. He reported that cold saturated picric-acid solution will 

 precipitate guanin from acid or alkaline solutions as dilute as 1 : 30,000, 

 To the neutral or alkaline solution in which the guanin is to be de- 

 termined is added a cold saturated solution of picric acid. An 

 excess of picric acid is not objectionable, provided the guanin solu- 

 tion does not contain too much acid, in which case some picric acid 

 might be precipitated. The precipitation is best carried out warm, 

 and the reaction mixture should stand 24 hours, since the precipita- 

 tion proceeds slowly. The precipitate is collected on a hard, dense 

 filter paper, thoroughly washed with a 1 per cent picric-acid solution, 

 and drained thoroughly. The felty picrate is placed between watch 

 glasses and dried out gradually with increasing temperature. Finally 

 the one molecule of water of crystallization is expelled by one and 

 one-half hours exposure in the oven at a temperature of 110° C. 

 It now has the formula C5H5N50.CeIl2(N02)30II, and is weighed and 

 calculated as guanin. 



Even though a small amount of picric acid adheres to the crystals 

 the results are still slightly too low, perhaps because of the incom- 



