THE GLUING OF WOOD 13 
caramelize the starch and thus greatly reduce the adhesiveness of the 
glue. For this reason a steamjacketed heater is generally used. In 
the second method of preparation 6 per cent or more of caustic is used, 
and with thorough stirring the caustic itself heats and agglutinates 
the starch to the desired consistency. In the third method of prep- 
aration, the starch and the water are mixed without caustic soda, and 
the conversion of the starch into glue is accomplished by the action of 
external heat. The mixing process for this method ordinarily re- 
quires an hour. The caustic-free glue is not as stringy as the glue made 
by the first two methods and has a shorter working life. It is there- 
fore not considered as satisfactory as the caustic glues for some pur- 
poses, but it is useful where the staining effect of caustic must be 
avoided. 
After being prepared by one of the methods described, the glue is 
allowed to cool. It is then ready for use and may be run off into a 
spreader or storage vat. Vegetable glues do not deteriorate rapidly 
and may be kept for several days without apparent loss in strength. 
However, chiefly on account of evaporation they do change slowly 
in viscosity and workability. After standing several days caustic-free 
glues become appreciably thicker, whereas glues with a high percentage 
of caustic become thinner. 
PREPARING CASEIN GLUES 
Casein glues must not be heated at any stage of mixing. Mixing 
by hand is laborious and often unsatisfactory: A dough mixer of the 
type illustrated in Plate 1, C is one of several machines which may be 
used satisfactority for mixing casein glue. The requirements of a 
satisfactory mixer for casein glues are that it shall agitate the mass 
thoroughly, that the mixing bowl shall be made of a metal that 
does not corrode rapidly from the action of alkali, and that the bowl 
shall be readily detachable for cleaning. 
Mixers, spreaders, and other equipment used with casein glue 
should be thoroughly cleaned at regular intervals to present de- 
terioration of the glue and the inclusion of pieces of dried glue in 
freshly prepared batches. A thorough cleaning at least once every 
working day is therefore desirable. 
A " prepared " casein glue is mixed by simply stirring the prepared 
dry glue into water. Most glues require about 2 parts of water to 1 
part by weight of dry glue. An additive or subtractive variation of 
10 per cent in the amount of water is allowable for different kinds of 
work. The water should be poured into the bowl of the mixer first 
and the glue sprinkled or sifted in slowly while the paddle of the 
mixer is at high speed (100 to 120 revolutions per minute). Care 
should be taken that large lumps do not form. After the dry glue 
has all been added, the stirring is usually continued at half speed 
until a smooth thoroughly dissolved mixture of even consistency is 
produced. This ordinarily occurs in 15 to 30 minutes. With a few 
glues the dry powder is mixed with the water and then allowed to 
stand, without agitation, until the materials are dissolved. 
A " wet-mix " casein glue is prepared by mixing all of the separate 
ingredients at the time of use. The raw casein is first stirred with 
the water. If it is then allowed to stand and absorb water, it will 
dissolve more readily under the influence of an alkali, such as caustic 
