THE GLUING OF WOOD 7 
however, many manufacturers are directing their efforts toward 
making additional casein glues that shall be cheap, easy to handle, 
and otherwise advantageous, even at the expense of water resistance. 
Among the disadvantages of casein glues for certain uses are 
their dulling effect on tools, their tendency to discolor certain woods, 
their relative short working life, and their high viscosity. All 
strong glues dull the knives of woodworking machines to some 
extent in machining a joint after gluing. Many highly water-re- 
sistant casein glues are especially bad in this respect. The dulling 
effect of a glue line on knives is also affected by the thickness of 
the glue film in the joint (9). The tendency ,to stain has been 
reduced in the case of some special casein glues, but experience with 
such glues indicates that the properties of nonstaining and high 
water resistance have not yet been combined in the same casein glue. 
Means of increasing the working life of casein glues is discussed later 
on this page. The high viscosity of casein glues makes them incon- 
venient to spread by hand unless a special brush is used or the glue is 
mixed thinner than is customary. 
Casein, the principal ingredient of casein glue, is the prepared 
curd of milk. When obtained as the product of natural souring it 
is known as self-soured or lactic-acid casein. It may also be pre- 
cipitated from milk by sulphuric and hydrochloric acids. The 
general method of preparation is to remove the acid and other 
impurities from the curd by washing and then to dry and grind it 
fine enough to pass through a No. 20-mesh or finer sieve. Variations 
in details of the process yield caseins of somewhat different prop- 
erties (16). 
Casein of the very highest commercial grade is not essential for 
glue making. It does not follow, however, that a decidely inferior 
casein will yield a high-grade glue. Some of the desirable properties 
of a casein for use in glue making are low fat and acid content, 
freedom from sour odor, larva?, burnt or discolored particles, or 
marked impurities of any kind. The ash content of the casein is of 
importance, since it is a fairly good measure of the amount of water 
which must be used in preparing a glue of suitable consistency (15). 
It is considered safest to use a casein of medium ash content and 
medium water requirement for the highest grade of casein glue. 
Besides casein and water, a third principal ingredient of most casein 
glues is hydrated lime, which reacts with the casein to form a highly 
water-resistant compound. A properly proportioned mixture of these 
three substances will give a strong, water-resisting glue. Such a 
glue, however, will remain in workable condition only a short time. 
To lengthen the working life of the glue a fourth ingredient is 
added. Trisodium phosphate, sodium fluoride, sodium silicate, 
sodium hydroxide, copper sulphate, and copper chloride have been 
used for this purpose in patented formulas. 
Some of the above chemicals have additional desirable properties ; 
for example, the copper chloride also has the property of giving 
the glue a high water resistance. In addition to the above, various 
other chemicals have also been added to casein glues to produce 
improved glues (11). 
Casein glues are classified as " prepared " and " wet-mix." Pre- 
pared glues can be bought in the form of a powder containing all the 
