50 BULLETIN 1500, U. S. DEPARTMENT OF AGRICULTURE 
Staining may be reduced by drying the wood to a low moisture 
content before it is glued and then drying the glued stock as soon as 
possible after the glue is applied as described on pages 16 and 29. To 
further aid in the prevention of veneer surface discoloration a thick 
veneer and a thick glue mixture should be used. Surface stains may 
be removed by bleaching. 31 
GLUING RECOMMENDATIONS FOR DIFFERENT SPECIES AND GLUES 
The results shown in Figures 12, 13, 14, and 16 indicate that many 
woods glue similarly under the same gluing conditions and that there 
is a gradual transation from those requiring the least to those re- 
quiring the most care in gluing. Consequently, there is no sharp line 
or division point by which the wood species can be separated into 
distinct classes or groups. It is also apparent that conditions which 
may give satisfactory joints on some species may be entirely unsat- 
isfactory for others. 
The gluing schedules, presented in Table 7, and their application 
to the 40 species of wood listed in Table 8, are intended as a guide 
to good gluing practice. Gluing schedules Al, A2, and A3 apply 
to animal glue ; Cl and C2 to casein glue ; and VI and V2 to vege- 
table glue. The Al, Cl, and VI schedules are less exacting than the 
A2, C2, and V2, respectively, and the A2 schedule is less restricted, 
in turn, than the A3. Species for which one of the less exacting 
schedules is recommended may also be glued under a more restricted 
schedule. For example, if ash, both heartwoocl and sap wood (Table 
8), is glued with animal glue, the gluing schedule A3 should be used. 
Any of the four sets of conditions listed under A3 (Table 7) may 
be used successfully. Ash can not be glued under less exacting- 
Schedules A2 or Al without some sacrifice in joint strength. Like- 
wise, with casein and vegetables glues the more restricted Schedules 
C2 and V2 are recommended for ash. Yellow poplar is glued more 
easily than ash, and Schedules A2, Cl, and VI are recommended, 
respectively, for animal, casein, and vegetable glues. Any of the 
gluing conditions listed for yellow poplar as well as those given 
under the most restricted schedules of A3, C2, and V2 may be safely 
used. Gluing Schedule Al is not recommended for yellow poplar. 
In a few cases, for example, in the Schedules A3, C2, and V2 listed 
for the heart wood of beech, the most restricted conditions do not 
give entirely satisfactory results, and an improvement in the gluing 
of such woods is possible by treating it previous to gluing (p. 48). 
01 Sponging the stained surface with an oxalic-acid solution, prepared by dissolving 1 
ounce of oxalic-acid crystals in about 12 ounces of water, will ordinarily remove glue dis- 
coloration. A more effective way is to moisten the wood first with a sodium-sulphite solu- 
tion (1 ounce sodium sulphite to 12 ounces water) followed by the application of oxalic 
acid. The acid must be thoroughly removed from the wood afterwards or it may affect 
the finish. The oxalic acid is poisonous and should be handled with care. 
