THE GLUING OF WOOD 67 
narily not necessary to be so exact for a glue for woodworking. A common 
method of testing for free grease in glue is to mix a little water-soluble dye — 
for example, methyl violet — with the glue solution and then paint the mixed 
solution upon a piece of unsized white paper. Grease which is not highly dis- 
persed will show as spots, giving the paper a mottled or spotted appearance. 
This test permits only a rough comparison between glues and gives no accurate 
numerical measure of the grease content. 
Glue which foams excessively is objectionable because air bubbles are liable 
to get into the joint and thus reduce the area in which the glue is in contact 
with both faces. Foamy glue is especially undesirable for use in glue spreaders, 
as the glue is agitated much more than when it is used by hand, and the 
danger of incorporating air bubbles is greater. A test for the amount of foam 
in a glue is made by beating the glue solution in a container of specified shape 
and size for a given time with an egg beater or other form of agitator ; then 
noting the height to which the foam rises and the quickness with which it 
subsides. The foam should be measured after standing for a few minutes. 
The solution used in the viscosity test may be used also for foam determination. 
ACIDITY AND ALKALINITY 
If it is either strongly acid or strongly alkaline, animal glue is likely to de- 
teriorate rapidly. A small amount of acid or alkali is not of itself particularly 
objectionable, although it has been shown that even slight acidity or alkalinity 
affects to some extent the viscosity, jelly strength, tendency to foam, and other 
characteristics (6'). The test for acidity and alkalinity of glue is easily and 
quickly made by dipping strips of a chemical indicator, such as litmus paper, 
in the glue solution and noting any color change. If litmus paper turns red 
it indicates acid. If it turns blue, alkali is indicated. More sensitive indi- 
cators may be selected, or electrometric means may be used with greater 
accuracy. 
ASH AND FOREIGN SUBSTANCES 
Tests may also be made to determine the ash content and the presence of 
foreign substances such as salts, which are at times added to give the glues 
special properties. Such information, while indicating to some extent the 
origin, treatment, and history of the glue, can not be regarded as an adequate 
basis for judging its strength. The amount 37 and character of the ash depend 
upon the source of the glue, its treatment in manufacture, and the presence of 
inorganic materials which are sometimes added by the glue manufacturer. 
The ash content will show the presence of zinc oxide, lead and barium sulphate, 
calcium carbonate, or other inorganic salts added 3S usually to make the glue 
line less conspicuous in the joint. Such materials are normally not considered 
adulterants, and their presence in small quantities does not, of itself, indicate a 
poor or inferior glue. 
MOISTURE CONTENT 
Animal glue is a hygroscopic substance, and its moisture content varies with 
the humidity of the atmosphere surrounding it. The moisture content com- 
monly ranges from 8 to 20 per cent of the dried weight. 39 Because of this 
37 Fernbach (19) gives the ash content of glue as varying from 2 to 8 per cent; Rideal 
Ul) finds the usual range from 1.5 to 3 per cent; Alexander (1) 3 to 4 per cent; Linder 
and Frost (32) from about 1.5 to 5.5 per cent; and Bogue (6) 1 to 5 per cent where 
no inorganic material has been added directly to the glue. 
35 Bogue (6) states that where inorganic materials have been added to .eive color to the 
glue the ash content may rise to 10 or 15 per cent. Fernbach (19) and Rideal (W state 
that the ash of a bone glue fuses, is neutral, and contains phosphates and chlorides, and 
that the ash of hide glues does not fuse, is alkaline, and is generally free from phosphates 
and chlorides. However, Bogue points out that phosphoric acid is now often added to 
neutralize the lime used with the hide stock and that the fusing of the ash and the pres- 
ence of phosphates is no longer a reliable means of distinguishing between the two types 
of glue. 
39 Fernbach (19) gives 8 to 16 per cent; Rideal (J,l) 12 to 18 per cent; Bogue (6) 9 to 
18 per cent: Teesdale (-'/9) 8 to 16 per cent; and Linder and Frost (32) 10 to 18 per cent. 
Bateman and Town U) show that the moisture content varies at a temperature of 80° F., 
and with relative humidities of 30 per cent to 90 per cent from about 13 to 35 per cent, 
respectively, and found approximately the same percentage of moisture for two grades of 
glue. Bogue on the other hand reports a variation of moisture content of about 5 per 
cent from the lowest to the highest grades of animal glue. 
