THE GLUING OF WOOD 35 
known, the consistency of the glue used should be such that when a 
firm pressure is applied a small but definite amount of glue is 
squeezed out along the joint. 
CONTROLLING GLUE CONSISTENCY 
After glue has been spread on dry wood it thickens, some kinds 
more rapidly than others. Where coated laminations are laid to- 
gether as soon as spread (closed assembly), the glue thickens much 
more slowly than where such laminations are left exposed for a 
time to the air (open assembly). If the closed-assembly method is 
used on unheated wood, an average spread of glue may be in good, 
condition for 15 to 20 minutes. In the open-assembly method, glue 
quickly dries to the stage where proper adhesion is impossible. 
Where warm wood is used,, dry spots in the glue line may occiur 
within a very short time. Casein glues in particular dry and set 
very quickly on hot wood, and there is consequently the danger of 
poor joints from this cause. Casein, vegetable, or blood-albumin 
glues are at times unintentionally spread on wood which is still 
warm from the driers. Such practice is likely to cause " dried " 
joints (pi. 10, D) with all glues and "starved" joints (pi. 10, B) 
with animal glue. 
Consistency of Animal Glue 
With animal-glue solutions the consistency depends upon the cool- 
ing and the drying effects. For the first few minutes after an ani- 
mal glue has been spread on the wood the cooling effect is much 
more important than the drying effect. Figure 7 shows the vis- 
cosity-temperature relationship for an animal glue mixed for use 
in woodworking. Near 140° F. a decrease of a few degrees makes 
but a slight change in the consistency of the glue solution, whereas 
a similar decrease in temperature in the region of 80° to 90° makes 
a marked change in consistency. In other words, an animal-glue 
solution as it approaches room temperature changes in viscosity 
very rapidly. This temperature- viscosity relationship, although 
general for animal glue, varies slightly with the grade and mixture 
of the glue solution. With, other types of glues applied cold, a 
change of a few degrees in temperature without drying has relatively 
little effect. 23 High-grade animal glues and mixtures of low water 
content thicken to the proper pressing consistency quicker and at 
higher temperatures than low-grade glues of high water content. 
For this reason the grade and mixture of animal glue should be 
chosen to suit a particular gluing operation and to suit the tempera- 
ture to be encountered. 
Animal glues as ordinarily mixed for use are too thin for pressing 
immediately after being spread except for certain woods at low tem- 
peratures. In some gluing operations, therefore, animal glues should 
23 The initial effect of heat in lowering the consistency of glue solutions is rather general 
with glues used in woodworking. Animal, certain jelly forms of blood-albumin, casein, and 
vegetable glues all become thinner with an increase in temperature. Prolonged heating, 
however, has a variable effect. With animal glue it results in a further decrease in vis- 
cosity, with blood glue a solidification occurs if the coagulating temperature or higher tem- 
perature is used, and with water-resistant casein glues the rate of any normal increase 
in the consistency is accelerated, and the working life of the glue is shortened (12). 
