THE GLUING OF WOOD 33 
between the wood layers which is unbroken by air bubbles (pi. 9, F) 
or by foreign particles, and sufficient penetration of glue into the cell 
cavities to give adequate surface for adhesion. The thickness of glue 
film may vary considerably without appreciably affecting the strength 
of wood joints (pi. 9, D and E), provided the film is continuous. 21 
Glue may stick wood layers together without penetrating the cell 
cavities 22 in the same way that glass and other nonporous substances 
may be glued. Microscopical studies of glue joints in wood, how- 
ever, show that glue penetrates the cell cavities and other openings. 
Numerous shallow openings such as are made by penetration into the 
fiber cavities immediately adjacent to the glue line in plate 9, D, 
are better than penetration into a few deep but isolated large cells 
(21). 
When glue is brought into contact with wood surfaces it does not 
pass through the cell walls but tends to enter the exposed cell cavities. 
Wherever the grain runs into the wood from the surface, as it nearly 
always does to a slight extent, the glue may penetrate for some dis- 
tance along the cell cavities. (PL 9, A.) As the glue tendrils in 
the larger cell cavities dry and shrink they commonly take the form 
of long, hollow cylinders, leaving a film of glue clinging to the walls 
of the wood cells. (PL 9, I and J.) Deep penetration, which re- 
sults in long tendrils of glue in the cell cavities, is not only unneces- 
sary but may be harmful. When the wood has a dense cellular struc- 
ture and the glue is thick, little or no penetration occurs, but the 
resulting bond is still adequate for some woods. Where no appre- 
ciable penetration occurs, the adhesion of glue to the wood sub- 
stances is confined chiefly to the surface area of the wood and does not 
always give a bond equal in strength to that of the wood. If glue 
is relatively thin it may be pressed out from between the wood layers 
or else forced to penetrate into the open cells, giving what is com- 
monly termed a " starved joint." (PL 9, C, I, and J.) In such cases 
the film of glue is not visible under the microscope and does not make 
a satisfactory bond. Therefore, moderate penetration along the glue 
line is preferable. 
CORRELATION OF PRESSURE AND GLUE CONSISTENCY 
Making strong glue joints depends primarily upon having the 
proper correlation of gluing pressure and glue consistency at the mo- 
ment the pressure is applied. Long study and experiment at the 
Forest Products Laboratory have resulted in the recommendations 
shown graphically in Figure 6. If the glue is thin at the time of 
pressing a light pressure should be used, and if the glue is thick a 
heavy pressure should be used. By controlling the glue consistency 
21 The relation between film thickness and adhesive strength reported by McBain and 
Lee (35) in polished metal surfaces joined with gums, resins, and waxes ; namely, that 
" the thinner the layer of adhesive the stronger the joint," does not have a practical bear- 
ing upon wood joints. Many thousands of tests on wood joints made at the Forest Prod- 
ucts Laboratory failed to disclose any direct relation between glue film thickness and joint 
strength. 
22 The relative importance of specific adhesion between glue and wood substance and of a 
purely mechanical bond in wood joints is a point upon which there is not complete agree- 
ment. McBain and Hopkins (33) have described a wood joint as " purely mechanical " in 
nature and state that specific adhesion is apparently lacking. This conception was later 
modified somewhat (36), but the character of adhesion is still described as chiefly mechani- 
cal. There is considerable evidence, however, that specific adhesion plays an important 
part in wood-glue joints (3, 13). 
31595°— 29 3 
