PEARLS AND THE PEARL INDUSTRY 117 



But although these imitation pearls were beautiful, the coating was not water- 

 proof. Later the interior of hollow glass beads was coated with the "essence" 

 and then filled with wax. Although these imitation pearls were not damaged 

 by water, they were easily crushed. However, it became possible to make "in- 

 destructible" pearls by coating opal glass or other sohd beads with a suspension 

 of the pearl essence in a waterproof lacquer. This is the most popular type of 

 imitation pearls produced today. 



Most species of fish owe their sheen to a deposit of thin blade-hke crystals of 

 guanin in the epidermis of their scales. When the epidermis is rubbed off under 

 water, it is partially disintegrated and lustrous particles are suspended. If this 

 suspension of guanin crystals is strained through cheesecloth to eliminate coarse 

 particles, crude pearl essence settles to the bottom. 



Guanin, chemically speaking, is 2-amino-6-oxypurin (CeHgNsO), a derivative 

 of purin, and belongs to the group known as the purin bases. In the animal body 

 it occurs with other groups in nucleic acids, which in turn are combined in the 

 complex proteins found in cell nuclei. Guanin is insoluble in water, alcohol, ether, 

 chloroform, ethyl acetate, amyl acetate, acetaldehyde, acetic acid, acetic anhy- 

 dride, formic acid, lactic acid, or solutions of citric or salicylic acid. It is very 

 shghtly soluble in cold ammonium hydroxide and more soluble in hot ammonia 

 solutions. It is soluble in dilute mineral acids and in solutions of potassium and 

 sodium hydroxide. 



American Source of Pearl Essence. The principal source of pearl essence in 

 America is the herring {Clwpea harengus) which is caught in great nvimbers 

 along the North Atlantic Coast from Sandy Hook to Labrador. Several plants for 

 the manufacture of pearl essence from herring are located on both the Canadian 

 and U. S. sides of the Bay of Fundy. Some is also manufactured from the scales 

 of the alewife {Pomolobus pseudoharengus) in Massachusetts and Virginia. 



Other potential sources are shad, hickory shad, Cahfornia sardine or pilchard, 

 gizzard shad, mullet, menhaden. Pacific Coast salmons, Alaska herring, and the 

 whitefish and cisco fisheries of the Great Lakes. 



Scales accumulate in the bottoms of the boats used to transport the herring 

 from the traps or weirs to the canneries and fish salting plants. To eflFect an easy 

 separation of the scales from the fish special boats with false bottoms, called 

 "scale scows", have been devised. The false bottoms permit the scales to find their 

 way to the space between the two bottoms, and the fish are held by the upper 

 or false bottom. A hogshead of herring will yield 27 to 34 pounds of scales. 



Processes of Making Pearl Essence. First the lustrous material from the scales 

 is scrubbed oflF in a large agitator, constructed like the old-fashioned ice cream 

 freezer. Only a httle water is used. Then the lustrous sediment is separated from 

 the wash water. This is usually done in a basket centrifuge. When the partially 

 dried sediment is scraped out of the basket, it is ready for purification. 



Aqueous Suspensions. Aqueous suspensions of the lustrous crystals may be 

 prepared by repeatedly treating the crude product with a dilute ammonia solu- 

 tion. After each treatment the crude essence is allowed to stand in a cool place 

 until the crystals settle; then the ammonia solution is decanted and the treatment 

 repeated several times. The ammonia gradually dissolves the epidermis and leaves 

 the crystals sufiiciently clean for use. Pearl essence prepared in this way can be 

 preserved indefinitely if sufficient ammonia is present. 



