FISH GLUE AND ISINGLASS 535 



washed and the black outer membrane is scraped off. The sounds are then 

 dried in the open air, usually on large trays which may be quickly taken indoors 

 in case of rain. 



The final step in the manufacture of isinglass is carried out only during cool 

 weather on account of the difficulty of preventing softening and putrefaction 

 during the warmer months. The process is very simple; it merely involves the 

 union of the sounds into a long ribbon. This is accomplished by soaking the fresh- 

 ened, scraped, and air-dried sounds in water for 4 to 6 hours, or until they become 

 soft and pliable. The softened sounds are usually run into a cutting machine pro- 

 vided with a roller and a set of knives which chop the sounds into small pieces. 

 These are mixed and macerated between a set of iron rollers, from which the 

 material passes to so-called sheeting rollers— hollow iron rollers, through which 

 cold water flows, provided with a scraper which removes all adhering isinglass. 

 Thus the isinglass is converted into sheets Vs to }i inch thick, 6 to 8 inches wide, 

 and of any length desired. These long sheets are finally passed through ribbon 

 rollers until the ribbons produced are ^4 inch thick, the width being the same 

 as that of the sheets. The ribbons must be quickly dried to prevent putrefaction. 

 This is accomplished in a few hours by suspension in moderately warm, light 

 rooms. The ribbons are then rolled on wooden spools into coils weighing less than 

 a pound each. 



Composition and Properties of Isinglass 



Isinglass does not dissolve in cold water, but merely swells slowly and absorbs 

 some water, though not nearly the amount that would be taken up by a similar 

 weight of gelatin. Isinglass has been popularly considered to be pure gelatin, but 

 it is really collagen and does not become gelatin until after it has been heated 

 with water. Under these conditions it hydrolyzes to form gelatin, which dissolves 

 and upon cooling forms a tough gel closely resembling those formed by cooling 

 animal-gelatin solutions. If this gel is dried, it is then found to possess all the 

 properties of hide or bone gelatin; it will be clear and almost colorless and will 

 make a glue of great adhesiveness. Isinglass always has a distinct fishy odor. It is 

 insoluble in alcohol. 



Isinglass should contain but little ash and a low percentage of matter insoluble 

 in hot water. The best grades of Russian isinglass have less than 0.50 per cent 

 of ash and not more than 3.0 per cent of matter insoluble in hot water. 



Since isinglass is very valuable, attempts are often made to adulterate it with 

 gelatin, blood fibrin, agar-agar, and several other substances. Gelatin is sometimes 

 rolled between layers of isinglass. This adulteration may be easily detected by 

 microscopic examination of the jelly formed by soaking the material in water: 

 the isinglass retains its fibrous structure whereas the gelatin does not. 



Bogue (1922) carried out detailed analyses of Russian isinglass. His results 

 are given in Table 116 (p. 536). 



Uses 



The most important use of isinglass is in the clarification of wine. White wines 

 are sometimes clarified by treatment with isinglass. The fining process is simple. 

 The isinglass is first swollen in water and then in wine until it is nearly transparent. 

 It is thoroughly beaten with more wine, after which a little tartaric acid is added. 



