THE GLUING OF WOOD 15 
and checking. These stresses reduce the strength of glued joints 
and should therefore be avoided as far as possible. 
The range of moisture content of dry wood in service commonly 
varies, depending mainly upon the humidity to which the wood is 
exposed, from 4 per cent to 15 per cent or more. The wood in a 
chair may have as low as 4 per cent moisture in a heated building 
in winter and as high as 10 per cent moisture in the same room in 
the summer. The average moisture content of wood in heated build- 
ings is about 7 per cent for the year. Dry wood out of doors has 
an average of about 12 per cent moisture throughout a large part 
of the United States but will average less in areas of low humidity 
and more in areas of high humidity. 
The amount of moisture added to wood in gluing varies from less 
than 1 per cent in lumber of 1 inch thickness to 45 per cent in thin 
plywood where the amount of wood is small in proportion to the 
amount of glue. The thickness of the wood, the number of plies, 
the density of the wood, the glue mixture, and the quantity of glue 
spread all affect the increase in moisture content of the wood when 
glued (p. 30). 
Thin veneer, even if dried entirely free of moisture, will take up 
so much moistuce from the wet glue that its moisture content will 
become too high for the finished article and must be reduced. Mois- 
ture-free veneer is easily split or cracked, so that great care must be 
used in handling it preceding the gluing operation. It also very 
quickly reabsorbs moisture during this handling. Therefore, it is 
not practicable to dry it below 2 or 3 per cent moisture content. 
Furthermore, experience has shown that a moisture content of 5 
per cent or less in veneer at the time of gluing is satisfactory for 
furniture and similar uses. Lumber with a moisture content of 5 
or 6 per cent is satisfactory for gluing into furniture and similar uses. 
Lumber for outdoor purposes, however, should generally contain 
about 11 per cent of moisture before gluing. The moisture added in 
gluing will then bring the total moisture to about 12 per Cent. 
The manufacturer shipping glued articles to various parts of the 
country and making products for various uses can not provide for all 
the moisture variations to be met in service. He must standardize his 
operations, and he can therefore aim at approximate averages only. 
For the best results in gluing it is also necessary to have the joint 
free from casehardening and other internal stresses. Internal 
stresses which are the result of improper drying, are likely to cause 
warping and checking after the wood is glued. Lumber should 
therefore be tested for the presence of such stresses before it is 
removed from the kiln. If found to be casehardened or otherwise 
stressed, it should be treated to relieve the stresses (SO). 
In drying lumber, although the desired average moisture content 
may be reached, there will usually be considerable differences in the 
moisture content of individual boards and even between different 
parts of the same board. As a result, a conditioning period subse- 
quent to kiln drying or air seasoning is usually desirable to bring the 
stock to an approximately equal moisture content. This is best 
accomplished in a storage room in which the temperature and humid- 
ity are controlled so as to maintain the desired moisture content. 
