12 BULLETIN" 150 0, U. S. DEPARTMENT OF AGRIQULTTJBJES 
unused should be discarded or kept in a refrigerator to be used sub- 
sequently only where a lower grade will suffice. 
The temperature of the glue is difficult to control at the work- 
bench without some automatic temperature regulator. Electrical glue 
pots that furnish a moderate, steady supply of heat or that are 
equipped with a thermostatic control are now in use. It is best to use 
pots which keep the glue within the recommended temperature range 
from 140° to 150° F. 
Many users are firm in the belief that animal glue must be boiled 
before it is ready for use, although the contrary has repeatedly been 
proven by tests. This erroneous opinion is apparently founded on 
the fact that when a glue is 1 " cooked," water is evaporated and the 
viscosity of the solution is increased. A thick consistency makes up 
to some extent for deterioration in quality, and as a result good joints 
are sometimes made with glue that has been boiled. If, however, 
water were added to make up for that lost by evaporation, the lower- 
ing in grade owing to the boiling would quickly become apparent. 
The solution would become thinner and thinner as boiling progressed, 
and decidedly inferior joints would result from the use of such a 
glue. When the loss of water by evaporation is not replaced there is 
a decrease in the volume of the glue mixture and hence a costly 
decrease in the covering capacity or spread of the glue. 
DETERIORATION OF ANIMAL GLUE CAUSED BY MICROORGANISMS 
Decomposition by bacteria is a well-known cause of serious trouble 
with animal glue. The bacterial action is usually accompanied by 
a characteristic offensive odor and quickly deteriorates the glue. 
Decomposition takes place rapidly at high room temperatures, and 
" sweet " glue in contact with infected glue soon becomes " sour." 
The utmost cleanliness should therefore be the rule in handling and 
using animal glue. 
PREPARING VEGETABLE GLUES 
Vegetable (starch) glue is prepared by three different methods. 
In all three the dry glue is first mixed with cold water and stirred 
until of a uniform consistency. On account of the high viscosity 
of the mixture, mechanical stirring is a practical necessity. One of 
the several types of mixers used for mechanical stirring is shown in 
Plate 1, B. The proportions of water and dry glue used in all three 
methods vary according to manufacturers' directions from 1% parts 
of water to I part by weight of dry glue, to 4^4 parts of water to 1 
part by weight of dry glue, depending upon the particular kind of 
glue and the type of work for which it is to be used. The two chief 
causes of this wide variation in proportions of water and glue are 
differences in the starches used and in the methods of processing. B} r 
the first method of preparation, caustic soda equal to about 3 per 
cent by weight of the dry glue is dissolved in a small amount of cold 
water and then slowly stirred into the water and glue mixture. The 
resulting mixture is then heated to the temperature recommended 
by the glue manufacturer. While heating, the mixture should be 
constantly stirred. After the proper heating and stirring, the mix- 
ture changes into a viscous and translucent mass. The temperature 
must be carefully controlled at all times, as excessive heat will 
