THE GLXJING OF WOOD 59 
ferences in the shrinkage properties of the adjacent members will 
reappear if the moisture content is changed after having once been 
relieved. 
BUTT-JOINT CONSTRUCTION 
Butt joints are those in which the end grain of two pieces of wood is 
glued together or the end of one piece is glued to the side of another. 
Mitered joints chiefly present end grain and are therefore considered 
butt joints. Several types of butt joints are shown in Figure 2. 
It is practically impossible to make square end-butt joints suffi- 
ciently strong or permanent enough to meet the requirements of ordi- 
nary service. The gluing of end-grain joints is more difficult than 
the gluing of side-grain joints, and the stress placed upon them in 
service is more severe. Tests made in gluing end-grain surfaces have 
shown that such joints are erratic and in strength they rarely exceed 
3,000 to 4,000 pounds per square inch. With the most careful gluing 
possible not more than about 25 per cent of the tensile strength 
parallel with the grain can be obtained. 
In order to obtain good strength in pieces spliced together endwise, 
it is necessary to make a scarf, fingered, serrated, or other sloped 
joint. (Fig. 2, B and C.) To make the joint as strong as the wood, 
it is necessary to have the slope not steeper than the proportion of 1 
across the grain to 8 along the grain, and longer slopes are necessary 
for species which are of high strength or which are not easy to glue. 
The plain scarf joint is perhaps the easiest to glue, and it also involves 
fewer machining difficulties than are involved in the sharply angular 
forms. In tests on plain scarf joints the following slopes were found 
necessary to produce joints as strong in tension 32 along the grain as 
the solid wood of the species tested : 
Species Slope 
Yellow poplar 1 in 8 
Red gum 1 in 8 
Mahogany 1 in 10 
Yellow birch , 1 in 12 
Red oak 1 in 15 
White oak . 1 in 15 
Black walnut 1 in 15 
It is practically impossible to make permanent plain end to side 
grain joints (fig. 2, D) where strength is important because it is not 
only difficult to make a good glued joint on end-grain wood but when 
such a joint is made the stresses to which it is subjected in service are 
unusually severe. Under changing moisture conditions the end- 
grain piece of the joint tends to swell or shrink considerably along 
both dimensions of the joint while the side-grain piece of the joint 
swells or shrinks only in one direction. Joints which are not sub- 
jected to much external stress may serve satisfactorily; for example, 
the joints made by gluing facing strips of veneer on the end edges of a 
cross-banded table top. (Fig. 3, K.) In chair backs and other fur- 
niture parts, however, the stresses placed on end to side grain joints 
by the service to which they are put are very severe, and, when com- 
bined with the internal stresses due to moisture changes, usually re- 
32 In bending- tests on scarf joints in Sitka spruce a slope of 1 in 10 was found necessary 
to make the joint as strong as the wood (20). 
