32 
BULLETIN 150 0, IT. S. DEPARTMENT OF AGRICULTURE 
tory workrooms, give a better means for controlling the conditions 
during drying, and save factory space. 
In panel kilns it is very easy to dry satisfactorily 3-ply and 5-ply 
panels in 24 hours. Results of tests at the Forest Products Labora- 
tory in panel kilns show that under normal conditions the moisture 
added in gluing 3-ply panels, three-sixteenths of an inch thick, can 
be dried out satisfactorily in from 8 to 16 hours (7). These tests 
also indicate that the desired essentials in drying can be met by 
maintaining a constant temperature and relative humidity through- 
out the drying. To save time in such kiln operations it is advan- 
tageous to maintain conditions which correspond to a moisture 
content slightly below that to which the panels are to be dried. 
Table 3 shows several combinations of temperatures and relative 
humidities, which will bring the stock to approximately the desired 
moisture content but which will not allow an appreciable amount of 
drying beyond this point. 
Table 3. — Combinations of temperatures and relative humidities suitable for 
drying plywood panels to moisture-content values of 6 to 12 per cent, 
inclusive 
Moisture content desired, per cent 
Relative humidity for use with stated temperature i 
70° F. 
80° F. 
90° F. 
100° F. 
110° F. 
120° F. 
140° F. 
6 
19 
24 
30 
43 
55 
19 
26 
31 
44 
56 
20 
27 
32 
45 
57 
21 
28 
33 
46 
58 
22 
29 
35 
48 
59 
24 
31 
37 
50 
61 
26 
7 
34 
8 . 
41 
10 
53 
12 
65 
i The relative humidities shown for the lower temperatures and moisture- content values are obtainable 
ordinarily only during the. winter season. Where a low moisture content is necessary during warm, humid 
weather, it can be obtained by raising the temperature. 
Drying panels to an excessively low moisture content materially 
increases warping, checking, opening of joints, and other defects. 
Tests show that the amount of warping on 3-ply veneer panels is 
approximately proportional to the percentage of moisture removed 
from the panel in drying (7). 
In a few instances plywood has been dried on mechanical veneer 
driers and on hot-plate presses. These methods, however, have been 
confined to plywood of a high moisture content, which was glued 
with water-resistant glue. Plywood dried in this way is usually 
comparatively thin and not of the highest quality. The high tem- 
peratures and pressures used in these methods have a molding effect 
on the wood and to some extent thus prevent face checking and 
warping. 20 Mechanical driers and hot-plate presses result in a quick 
drying process, but involve the use of more expensive equipment than 
is used in the other methods. 
CONTROL OF GLUING CONDITIONS 
A strong joint is characterized by complete contact of glue and 
wood surfaces over the joint area; a continuous film of good glue 
20 IIood, A. N., Nightingale, J. T., Leland. F. R., and Dike. t. W. making plywood. 
(U. s. patent no. 1,369,743.) U. S. Patent Office, Off. Gaz. 283 : 7o:i. 19-'l. 
