end of the 8l weeks, however, all specimens had failed, which indicated that 
vegetable-glue joints cannot be depended upon to remain permanently durable 
when exposed to conditions under which the wood may at times exceed about 17 
percent moisture content, which is approximately the equilibrium condition 
for 80 percent relative humidity. Furniture and other products bonded with 
vegetable glues often serve satisfactorily in spite of occasional exposures 
to relative humidities in excess of 80 percent, but in those cases protec- 
tion afforded by the finish may prevent the moisture content of the wood 
from reaching equilibrium values, particularly if the exposure to dampness 
is not prolonged. 
In tests Ncs . 1 and 2, no evidence of significant loss in strength 
could be detected during the 160 weeks that the tests were in progress. 
Test No. 2 approximates the changes in moisture content that can be expected 
in interior woodwork in normal use in heated buildings in the northern 
part of the United Gtates. In this type of service, properly designed and 
well-made joints of vegetable glue should prove permanently durable, and 
experimental evidence, other than these tests, supports that belief. 
Uses of Vegetable Glue 
Vegetable glue is utilized chiefly in plywood and other veneered 
products manufactured for interior use, such as furniture of all kinds, 
pianos, and interior trim. Because of their slow setting the ordinary 
vegetable glues are not extensively used in edge- joint gluing, but it is 
reported that the increased setting rate of some of the newer types has 
allowed vegetable glues to compete successfully in this field. 
The lack of water resistance prevents the use of vegetable glue in 
products that will be subjected to moist conditions. 
Starch Tests 
Although research on starch has been extensive, no simple chemical or 
mechanical test has been devised that will permit the user to test starch 
for quality and for suitability for gluemaking. 
If it is desired, the starches may be distinguished from each other 
by such tests as the temperature of gelatinization, by the form and size of 
the grain, by the appearance under polarized light, by the refractive index, 
by the character of the paste, by the color reactions, and -orobably by X-ray 
determinations of the crystal structure 
Consistency tests at different starch-water ratios may give useful 
information, but the user of the glue must, for the most part, rely upon 
joint tests in determining the quality of starch for gluemaking. 
Report No. R30 
