THE GLUING OF WOOD 29 
hours. The exact time of pressing is partially determined by con- 
venience of operation; for example, glued panels of one day's run 
are left under pressure overnight and removed the succeeding day 
to free the equipment for a new run. The minimum time for most 
vegetable-glue panels to remain under pressure is usually about seven 
hours. However, with casein, animal, and the quicker-setting vege- 
table glues, the veneered stock sometimes is removed after two to 
three hours with satisfactory results. 
Pressing Time Affects Staining 
The alkali present in some glues discolors certain woods. (Table 
6.) Removing glued veneer from the press as soon as possible and 
drying it on stickers reduces the staining, because alkali does not 
readily penetrate dry veneer. 
USE OF HEAT IN PRESSING 
The presses used for blood albumin glue are equipped with steam- 
heated platens. 19 The exact time of pressing depends upon the tem- 
perature, upon the thickness of the glued stock, and to some extent 
upon the kind of wood used. The temperature of the platens ranges 
from 212° to 300° F., and the corresponding pressure period usually 
ranges from three minutes to one-half hour. Temperatures near 
300° set the glue quickly and give strong water-resistant joints, but 
they may cause steam blisters to form in the joints and thus ruin 
a part of the joint. Steam blisters are sometimes prevented by sub- 
jecting the panels to the heat necessary to set the glue and then by 
running cold water through the platens while the panels remain 
under pressure. This practice may, however, result in warping the 
platens of the press. 
DRYING AND CONDITIONING GLUED JOINTS 
So much moisture is added to the wood adjacent to the glue line 
by the large amount of water contained in glue mixtures that it must 
be taken care of. Table 2 shows the calculated percentages of mois- 
ture added to wood in gluing certain types of construction with 
different glue mixtures and spreads. The table is based on the 
assumption that the wood absorbs all of the water added by the glue. 
This is not strictly correct, since some of the glue squeezes out of the 
joints and some water evaporates during the pressure period. The 
calculated percentages, however, are reasonably close to the results 
obtained in actual gluing. 
In order to obtain strong joints and to prevent warping, checking, 
sunken joints, and other defects in the finished article, it is essential 
that the wood after gluing be brought to the moisture content most 
suitable for the subsequent use of the article and that the moisture be 
evenly distributed throughout. 
19 The hot-press process of plywood manufacture is not extensively used in the United 
States, but in European countries it is the prevailing method and is used not only with 
blood-albumin glues but often with casein glues as well. 
