THE GLUING OF WOOD 31 
CONDITIONING GLUED THICK STOCK 
When the moisture increase in the wood due to a glued joint is 
small, such as in thick laminations (fig. 17, A, B, C, E, I, and J) 
dried to a suitable moisture content before gluing, only conditioning 
to a uniform moisture content is necessary. 
Sunken joints are common defects in the manufacture of thick 
edge-glued lumber and are caused by surfacing the stock too soon 
after gluing. The wood right at the joint absorbs more water from 
the glue than the rest of the wood and therefore swells more. If the 
piece is surfaced before this excess moisture is distributed, more wood 
is removed along the joints than at intermediate points. Then, dur- 
ing subsequent drying and conditioning, greater shrinkage occurs at 
the joints than elsewhere, and permanent depressions are formed. 
Such depressions along the glue line may show very conspicuously 
in the finished panel when viewed under a side light. To avoid 
sunken joints in edge-glued lumber 1 inch thick it should be piled on 
stickers and dried for a period of two clays in a kiln at 100° F., 
or five to seven days at 70° F. 
DRYING PLYWOOD 
It is necessary to dry a part of the moisture from plywood and 
veneer panels. (Fig. 3.) For example, assuming a moisture content 
of 3 per cent in the veneer and an increase of 14 per cent from the 
glue, the panels when removed from the clamps or press would con- 
tain about 17 per cent of moisture. Such percentages are common in 
many types of plywood immediately after gluing. For use in cabi- 
nets, in furniture, or in the interior of buildings more than one -half 
of this moisture should be removed before the panels are ready to be 
put into the finished article. For use outdoors or in unheated build- 
ings, plywood containing about 12 per cent moisture will generally 
prove satisfactory. Where veneer is glued over a lumber core, the 
increase in moisture content of the whole panel at the time of remov- 
ing the panels from the press is not so large as with thin plywood. 
However, in thick core panels the moisture from the glue is largely 
confined to the outside of the core and to the veneer. Therefore, the 
excess moisture of these parts is as great as in thin plywood and must 
be dried out or allowed to equalize through the core. 
If the thick cores (l 1 ^ inches) are dried to a low moisture content 
before gluing, the water added in gluing the veneer onto the core 
may not bring the whole panel above 7 per cent moisture content. 
Under such a circumstance, the panels are sometimes stacked solid 
in large piles and allowed to condition. This requires a long condi- 
tioning period, and the absorption of moisture by the core after the 
crossbands have been glued to it subjects the whole panel to severe 
stresses. 
A better and rapidly increasing practice for conditioning thick- 
core panels which contain excessive moisture is to place the panels on 
stickers and allow them to dry in panel kilns or in factory work- 
rooms This practice allows the excess moisture to be dried from 
the panel faces where it is largely concentrated ; and does not necessi- 
tate drying the thick-core stock to an extremely low moisture content 
before gluing. Panel kilns permit more rapid drying than in fac- 
