4 BULLETIN" 150 0, IT. S. DEPARTMENT OF AGEICULTUEE - 
The purposes for which glues are used in woodworking may be 
grouped broadly as the gluing of veneer and the gluing of joints in 
thick stock. Blood-albumin glues are used chiefly on veneer, whereas 
animal, vegetable, and casein glues are used both on veneer and on 
thick stock. Liquid glues are used mainly for joints of small area 
which are subjected to low stress, such as those in repair work and 
in small jobs of hand gluing. The other glues are used mainly in 
larger-scale production. 
CONSIDERATIONS APPLICABLE TO ALL USES 
It is necessaiy in all cases that the glue be capable of producing 
joints strong enough and durable enough for the use to which the 
wood is to be put. Glues should remain in usable condition long 
enough (working life) for use by ordinary gluing methods without 
undue waste or inconvenience. A glue which requires little attention 
to temperature in mixing and application is more convenient, to 
use than one requiring careful temperature control. Cost, spreading 
capacity, ease of mixing and applying, and tendency to foam are 
other points that should be considered in selecting a glue for quan- 
tity production of practically any type of wood joint, 
Both the original cost and the spreading capacity of the glue affect 
the final cost of the joint. Spreading capacity is chiefly dependent (ex- 
cept in liquid glue) upon the proportion of water that can be added 
without reducing the strength of the joint. Cost is of greatest im- 
portance in the manufacture of cheap products and in veneer gluing 
and other large-scale operations. 
Ease of application, by which is meant the simplicity and ease 
with which a satisfactory coating of the glue may be spread upon 
the wood is always important, but less so on flat surfaces, where 
standard mechanical spreaders are the rule, than on irregular joints, 
where special spreaders are used or where spreading is done by hand. 
The degree to which glue tends to foam on account of the air 
whipped into it in mixing and on mechanical spreaders is also impor- 
tant. Foamy glue may contain air bubbles or may have a froth or 
lather on its surface. It produces inferior joints in all classes of work 
because the air bubbles prevent complete contact of glue and wood. 
PROPERTIES IMPORTANT IN SPECIAL CASES 
Considerations that may affect the 3uitability of a glue for par- 
ticular uses are the rate of setting in the joint, the water resistance 
of the set glue, the tendency to stain wood, and the dulling effect 
on tools. Each may become the deciding factor in the selection of 
a glue for a particular use. For example, in edge-gluing lumber 
the desired speed of production usually requires a quick-setting 
glue. Water resistance of the joint is a first consideration in exte- 
rior doors, aircraft, and many other constructions. Thin face 
veneers require a glue that does not stain. Glues which have a 
marked dulling effect on tools are undesirable for certain products. 
The dulling effect causes frequent changing of planer or simper 
knives, with consequent increase in expense and decrease in pro- 
duction speed. 
