The Gluing of Wood 



637 



stroyed by prolonged wetting, with 

 customary care in use the service value 

 of a good glue joint is reliable, a fact 

 that unfortunate experiences in home 

 gluing, the result of faulty surfaces, 

 preparation, and methods, should not 

 obscure. 



Animal glue, most of it made by 

 cooking hides, fleshings, tendons, or 

 bones of cattle in water, was long the 

 world's principal wood adhesive and 

 is still in common use. 



The development of new glues be- 

 gan with the need of modern industry 

 for adhesives that were cheaper or 

 that would provide greater water re- 

 sistance and thereby longer durability 

 under the more severe service condi- 

 tions of new and potential uses for its 

 increasingly diversified products. 



The first of the new glues, intro- 

 duced about 1905, were vegetable or 

 starch glues, a large part of which were 

 derived from tropical cassava root, 

 which easily yielded its large starch 

 granules for the purpose. Despite their 

 low resistance to moisture, the cheap- 

 ness and prolonged workability of these 

 glues in the cold state early recom- 

 mended them for quantity manufac- 

 ture of plywood and veneer products. 

 The present production volume of veg- 

 etable glues equals or exceeds that of 

 animal glues. 



Present-day emphasis upon water 

 resistance in glues began during the 

 First World War, when this property 

 became important in aircraft construc- 

 tion. At the end of that war, casein 

 glue, of ancient but vague history, had 

 won for that purpose a recognition 

 which it still commands in such man- 

 ufactures as doors, plywood, furniture, 

 pianos, and trucks. 



Casein, the dried and ground curd 

 of milk, is relatively inexpensive. After 

 it is dissolved in water, water resist- 

 ance is commonly imparted by adding 

 slaked lime to form with it a jelly that 

 will set permanently and not redissolve 

 upon wetting. Other chemicals, usu- 

 ally sodium salts, are added to provide 

 satisfactory working properties. The 

 mixture, applied cold, sets to a hard 



and cementlike solid, whose bonding 

 strength approaches that of animal 

 glue. It is commonly marketed as a dry 

 mix that contains all essential ingredi- 

 ents except water. 



Blood-albumin glue, which is made 

 from blood from the packing house, 

 was a forerunner of the resins in its 

 requirement of a hot press to obtain 

 proper setting and bonding of its joint. 

 In its dry state it ranks somewhat below 

 casein in adhesive strength, but it has 

 better moisture resistance. It has been 

 largely replaced by synthetic-resin ad- 

 hesives. 



Amid all the magic claimed for the 

 soybean, the recent development of a 

 practical glue from it is not surprising. 

 Soybean glue is cheap. It can be ap- 

 plied cold. It has inherent water re- 

 sistance comparable to casein glue, al- 

 though somewhat lower in strength. 

 Made from the meal residue of soy- 

 bean-oil extraction, the glue has a 

 rather mushy texture, but sets to a firm 

 bond in the cold press. It has won a 

 commanding place in the great Doug- 

 las-fir plywood industry and in other 

 fields like wooden-box construction, 

 because its inexpensive water-resistant 

 joints permit mass production never 

 before attained. Because of its rela- 

 tively high alkalinity, which may 

 cause staining, the glue is not adapted 

 to fine veneers. 



Synthetic resins, the newest adhe- 

 sives, impart to the glue joint the high- 

 est water resistance yet attained. In 

 contrast to the earlier glues that at best 

 could withstand only a moderate 

 amount of dampness, a first-class syn- 

 thetic-resin glue appears to withstand 

 direct and repeated wetting almost in- 

 definitely. In tests at the Forest Prod- 

 ucts Laboratory for more than a dec- 

 ade, certain synthetic-resin glues did 

 not appear to soften or hydrolyze even 

 on continued soaking of bonded wood 

 specimens. These glues are not at- 

 tacked by molds or decay fungi and 

 maintain their hold as long as there 

 is any wood left to test. Thus the bond 

 that cannot be destroyed without de- 

 stroying the wood appears to have been 



