THE GLUING OF WOOD 9 
A properly mixed vegetable glue is translucent, colorless or of a 
brownish shade, viscous, and usually tacky. Caustic soda is usually 
added to vegetable glues by the user to make them stringy and less 
viscous. The caustic soda also lengthens the working life of the glue, 
but it discolors certain woods and may therefore be objectionable. 
BLOOD-ALBUMIN GLUES 
The use of blood-albumin glues is not extensive in the United 
States but is more common in European and Asiatic countries. 
Blood albumin, a slaughterhouse by-product, has the property of 
coagulating and setting firmly when heated to a temperature of about 
160° F., after which it shows marked resistance to the softening 
effect of water. This characteristic makes it a desirable glue for use in 
the manufacture of highly water-resistant plywood. The chief draw- 
backs to the use of most blood glues in woodworking are the necessity 
for hot pressing, which requires expensive machinery and consider- 
able handling; their usually low dry strength; and the fact that 
they can not be marketed in a dry-mixed form ready for the addition 
of water. However, recent tests have shown that a highly water- 
resistant and moderately strong blood glue can be prepared which 
does not require hot pressing. (See Appendix.) It has not yet been 
possible to secure as uniformly good results by cold pressing as by hot 
pressing the glue. 
The blood of cattle is generally used for glue making. The 
albumin is separated from the other substances of the blood and then 
dried at a temperature below its coagulating point. It may vary in 
color from dark red to very light red or even be almost colorless, 
according to the, manufacturing process. The dark albumin ijs 
lower in price and is generally used in glue making. 
Various formulas used in mixing blood glues are patented 9 and 
others are held as trade secrets. Alkalies such as hydrated lime or 
caustic soda are used in formulas to increase the adhesiveness of 
the glue. If a sufficient quantity of alkali is added, partial coagu- 
lation of the albumin occurs, and a jellylike mass results. By a 
proper control of alkalinity, temperature, and stirring, a large pro- 
portion of water can be added without breaking down the jelly 
form of the glue. 10 Blood-albumin glues with the highest propor- 
tions of water, however, are often lacking in strength. 
GLUE TESTS 
Whether or not tests of glue are necessary must be determined 
by the user, taking into account the importance or volume of work 
to be done. Sampling and testing of large consignments to deter- 
mine whether the purchased glue conforms with specifications are 
advisable. 
9 Two formulas, developed at the Forest Products Laboratory and patented by it for the 
benefit of the public are covered under United States patents Nos. 1,329,599 and 1,459,541. 
(See appendix.) Heinning, S. B. glue and manufacturing samei. <u. s. patent no. 
1,329,599.) U. S. Patent Office, Off. Gaz. 271 : 48. 1920. Lindauer, A. C. blood-albumin 
glue. (U. s. patent no. 1,459,541.) U. S. Patent Office, 'Off. Gaz. 311 : 669. 1923. 
10 Allen, C. B. waterproof cement. (U. s. patent no. i.,23i_,468.) U. S. Patent Office. 
Off. Gaz. 239 : 1160. 1917. 
