FISH GLUE AND ISINGLASS 527 



oils and preservatives are used for its preservation. Only the first two runs of the 

 best quality of fish-skin glue stock are taken. Special care is used to prevent the 

 admixtvure of stock or glue containing any considerable amount of grease as grease 

 and oil cause spots on the photoengraved halftone plates. Moreover, a uniform 

 admixture of the essential oil in the glue must be insured; improper mixing causes 

 some of the bottles of glue to be of poor quality because of an excess of oil. 

 Photoengraving glue also may be clarified either by a special settling process or 

 filtration before it is bottled. Even with the greatest care in manufacture each 

 batch of glue must be tested very carefully to determine whether or not it is 

 suitable for use in delicate work. 



Waste Glue. Waste glue, as previously explained, is prepared from salted 

 ground-fish trimmings and bones and from haddock, hake, and pollock skins. This 

 stock is washed and converted into glue in practically the same manner as that 

 used in the preparation of fish-skin glue. The type of washer already described 

 is used. Cooking is accomplished in the galvanized iron-jacketed cookers, although 

 direct-steam cookers are occasionally used. More acid is used for the conversion; 

 the amount depends somewhat upon the proportion of the bones in the stock— 

 the more bones the more acid required; but the usual amount is about 1 gallon 

 of glacial acetic acid per ton of glue stock. Two and sometimes three cookings 

 are made; the first is usually of about 10 hours' duration, less time being required 

 for subsequent cookings. Much higher percentages of preservatives are usually 

 added to the stock before cooking. The filtration and evaporation of glue hquor 

 are effected in the same manner as previously described for skin-glue liquor. The 

 total amount of preservatives in the finished glue is often in excess of 1 per cent, 

 although this varies with the nature of the preservatives. Phenol, cresol, beta- 

 naphthol, boric acid, and cresylic acid are commonly used. Larger amounts of 

 essential oils are usually added to waste glues than to fish-skin glues. The essential 

 oils in common use include cassia, wintergreen, sassafras, artificial sassafras, and 

 peppermint. 



Waste glues are often sold expressly for wood-joining work; therefore, small 

 amounts of zinc oxide are often added to partially opaque the glue. If transparent 

 liquid glues are used for joining work, the dried joint is dark and very noticeable. 



Fish-Head Glue. The methods used to convert fish heads into glue are not 

 as well known as those for fish skin and waste stock, although, needless to say, 

 they are much the same. Washing is done in the same way, although larger washers 

 are commonly used, as more fish-head stock must be handled to obtain the same 

 quantity of glue. Two types of cookers are in general use: one resembles that used 

 for the conversion of skin and waste stock, except that it is much larger— about 

 12 feet long, 8 feet wide, and 5 feet deep; the other is a large, pear-shaped steel 

 boiler. Sodium bisulfite or sulfurous acid is often added to the stock just before 

 cooking is commenced; these agents effect considerable bleaching. Because of the 

 relatively large proportion of bone in fish heads much larger percentages of acid 

 are required. Two gallons of acetic acid per ton of stock is the usual proportion. 

 If much less acid is used, the residue is difficult to press and dry. 



Fish-head glue liquors are evaporated according to the procedure described 

 for skin and waste glue liquors. The same antiseptics are used to preserve the 

 finished glue; larger amounts of essential oils are usually added to fish-head glues 

 to mask their disagreeable odor. 



