534 MARINE PRODUCTS OF COMMERCE 



mostly the former. In Venezuela and Brazil the Siluridae are the chief source of 

 isinglass. 



Isinglass is merely carefully washed and dried fish sounds made into special 

 forms by mechanical means. The fish sound, also called the air bladder or swim 

 bladder, is a hollow compressible sac, containing air, situated in the abdominal 

 cavity just below the vertebral column. Its principal function is mechanical; since 

 it is compressible, it serves to regulate the specific gravity of the fish and enables 

 it to rise and sink or to maintain its level in the water. The size of this organ 

 varies greatly in difi^erent species; in sturgeon, hake, catfish, and carp it is large and 

 well developed. In some fish the sound is securely fastened to the backbone, the 

 intestines, and the abdominal wall, while in others it is nearly loose in the 

 abdominal cavity. The swim bladder consists of several tunics, the outer layer of 

 which is thick and fibrous; the collagen contained in this layer is the source of 

 isinglass. The inner layer of tunics is thin, contains crystalline substances, such as 

 guanin, and often has a silvery luster. 



No complete analyses of fish sounds have been published; but the composition 

 of the fresh sound probably does not differ greatly from that of the prepared isin- 

 glass as washing and cleaning removes only a small amount of membranous mate- 

 rial and dirt. The analyses of the prepared isinglass are presented in Table 116. 



Methods of Manufacture 



Russian Methods. If fresh sturgeon sounds are used, they are split open and 

 carefully washed either in cold or warm water to remove the blood, membranes, 

 and any adhering extraneous matter. If the sounds have been previously pre- 

 served by drying, they are soaked in water for several days, with frequent changes 

 of water to soften them and permit the removal of membranous matter. 



The cleaned sounds are treated in various ways depending upon the form of 

 isinglass desii'ed. Common forms of Russian isinglass are long-staple, short-staple, 

 leaf book, and cake isinglass. 



In preparing leaf isinglass in the U.S.S.R. the thoroughly washed sounds are 

 cut longitudinally into sheets which are laid out, inner side up, to dry in the 

 sun. When partially dry the inner layer, which consists of pure isinglass, is re- 

 moved from the coarser external layer. These sheets (the inner layer of the 

 sounds) are flattened with pressure and then dried. When dry they are tied into 

 packages, weighing about 20 oz. each. The external layers of the sounds are used 

 chiefly for glue-making. 



The "staple" form of isinglass is produced by rolling each bladder and folding 

 it around a few pegs set in the form of a horseshoe. Book isinglass is prepared 

 by folding the sounds and then covering them with a damp cloth. 



American Methods. The best quality of American isinglass is prepared from 

 the sounds of hake caught in deep waters ofi^ the coast of Nova Scotia. One ton 

 of these fish yields from 300 to 500 large swim bladders, weighing a total of 40 to 

 50 pounds. Hake sounds from shallow waters are smaller and of a lower grade. 

 The sounds are removed from the fish when the hake are dressed. This operation 

 is usually carried out on board the fishing vessel soon after the fish are caught. 

 If the fishing grounds are far from shore, the sounds are immediately salted in 

 barrels to prevent their putrefaction. 



Upon arrival at the isinglass factory the sounds are slit open and thoroughly 



