120 MARINE PRODUCTS OF COMMERCE 



Nonaqueous Suspensions and Lacquers. Pearl essence is often marketed in the 

 form of suspensions in acetone and amyl acetate. It is also sold in the form of 

 lacquers which contain a suspension of guanin crystals in a cellulose ester solu- 

 tion. The methods of preparing these lacquers are either trade secrets or, in some 

 instances, patented processes. Taylor's process of preparing such a lacquer, how- 

 ever, is not patented.* It has been described as follows: 



The scales (they need not be clean) are washed free of adhering lustrous 

 matter and the water is drained off. This suspension in water is allowed to subside 

 and the supernatant water (and blood) decanted off. The remaining thick fluid, 

 containing lustrous particles of guanin, is treated with a digestion mixture of 

 pepsin, 2M gm, and acetic acid (glacial), 30 cc to each liter of fluid, and allowed 

 to stand for 48 hours. This mixture completely digests and removes all proteina- 

 ceous matter from the guanin crystals. Hydrochloric or other mineral acids cannot 

 be used to activate the pepsin because these readily dissolve the guanin crystals, 

 while acetic acid does not. 



After 48 hours' digestion in the pepsin-acetic acid mixture the surfaces of the 

 guanin crystals are clean. Since they now possess the peculiar property of being 

 more readily wetted by ether and certain other nonaqueous liquids than by water, 

 they are separated from the fluid mass, as follows: Ether is poured into the vessel 

 containing the digestion mixture and guanin crystals and allowed to form a 

 separate layer on top. The vessel is then gently rocked. The lustrous crystals of 

 guanin, being more readily wetted by the ether, leave the water and pass into the 

 ether layer, leaving behind all dirt, organic matter, and the like. Fatty substances 

 also pass into the ether, but the crystals are allowed to settle out. The oily ether 

 is decanted and replaced. The ether (which should now be strictly anhydrous, as 

 by treatment with calcium carbide) is replaced once or twice, removing all the 

 oil or fat. The lustrous material, now clean, can be kept indefinitely in ether, or 

 easily transferred to ethyl or amyl acetate. High grade nitrocellulose (celloidin 

 or "Parlodion") is dissolved in the ethyl or amyl acetate, forming a pearl lacquer 

 which is applied directly to the bead. Ethyl acetate is preferable because it 

 evaporates more quickly. Large quantities of pearl "essence" or lacquer can be 

 readily produced by this process. 



The particles of guanin should be neither too coarse nor too fine. The finest 

 particles, consisting of minute fragments of crystals when separated from the 

 coarser ones, appear chalky white or yellowish; if too small they make dull pearls. 

 The larger crystals have a grainy coating and are likely to be too brilliant. Crystals 

 of intermediate size produce the more desirable pearly luster. Large crystals can 

 be broken in a pebble mill containing 5 million glass beads; the grinding is best 

 done in a concentrated suspension of the crystals in amyl acetate. 



Manufacture of Imitation Pearls. Imitation pearls are of four major types: 



( 1 ) Those made by coating the inside of hollow glass spheres with pearl essence. 



(2) Those made by coating the inside of hollow glass spheres with pearl 

 essence and then filling the spheres with wax or some other material to give weight 

 to the pearl. (3) Those made by coating the exterior of solid glass (known in the 

 trade as alabaster) spheres with pearl essence. (4) Those made from plastic beads; 

 either a naturally lustrous plastic or a plastic with a coating containing mother-of- 

 pearl may be used. 



* Taylor, 1925; also the 1923 edition of "Marine Products of Commerce." 



