THE GLUIXG- OF WOOD 73 
Fineness. — Ground to pass a 20-mesh screen if mixed wet or a 50-mesh screen 
when mixed dry with other materials. 
Moisture. — Not more than 10 per cent. 
Fat. — Not more than 1 per cent. 
Ash. — Not more than 3 per cent for natural sour, 4.5 per cent for acid, or 
8.5 per cent for rennet caseins. 
Nitrogen. — Not less than 14.25 per cent. 
Acidity. — Not more than 10.5 cubic centimeters of N/10 sodium hydroxide 
per gram. 
VEGETABLE PROTEIN AND STARCH 
In general, it is not practicable for the user of glue to test vegetable proteins 
and starch for their quality and fitness for glue making. There is comparatively 
little information about the essential qualities of these materials and the test- 
ing of them for glue-making purposes. Vegetable-protein glues 45 are new in the 
woodworking industry, and, while the class of materials from which they are 
made has been investigated (27) for some time, the available information about 
them is mostly of a general character. Likewise researches on starch have been 
carried on by a number of investigators, 46 but, in spite of the rather large amount 
of investigative work done, there is not yet agreement on such fundamentals as 
the nature of the starch grain (46) and how it changes into glue (2). Such 
information on these materials as is known and applied in glue making is in 
large part a result of private development. 
So far as the present knowledge of the subject goes the tests on starch of 
most significance are a determination of its kind (39), its consistency in solu- 
tion, and its adhesive property or ability to make joints. Different starches 
may be identified by such characteristics as size and form of granules, color 
reactions, appearance under polarized light, and refractive index. A con- 
sistency test at a given starch-water ratio may also give useful information but 
the user of glue must, for the most part, unfortunately rely upon joint tests in 
determining the quality of both starch and vegetable proteins for glue making. 
CALCULATION OF PRESSURE ON JOINTS 
SCREW CLAMPS 
Where it is possible to measure the force exerted on a screw the load applied 
to the glue joint can be roughly calculated from the following formula, which 
uolds on]^ for screws with square or approximately square (acme) threads. 
\irDm—fKJ 
Where — 
TF=load in pounds. 
F=force applied to lever arm in pounds. 
L— length of lever arm in inches. 
Z>ra=the sum of the diameter inside the threads plus the diameter outsid e 
the threads divided by two. 
Rm=Dm divided by two. 
K= pitch of screw (one divided by number of turns per inch) for single 
screw or lead for multiple thread screws, 
/—coefficient of friction (computed to be about 0.20). 
vr = 3.1416. 
43 Soy bean, peanut, or cottonseed meals are generally used as bases for these glues and 
are described in the following patents : Johnson,, O. adhesive, (u. s. patent no. 1460757.) 
U. S. Patent Office, Off. Gaz. 312 : 132. 1923. (Re. 16.422.) U. S. Patent, Office, Off. Gaz. 
350:289. 1926. Osgood, G. H. glue. (U. s. patent nos. 1601506 and 1601507.) TJ. S. 
Patent Office, Off. Gaz. 350 : 861. 1926. 
46 A bibliography on vegetable (starch) glue is given in the following: Brouse-j D. 
vegetable glue. U. S. Forest Products Lab. [Pub.] 30. [Mimeographed.] 
