66 BULLETIN 150 0, 17. S. DEPARTZMEXT OE AGRICULTURE 
temperature of the glue within them can be controlled and therefore "are not 
always accurate. However, a number of more refined instruments based on flow 
through capillaries have been devised and give satisfactory results. 
The National Association of Glue Manufacturers has adopted an instrument 
of the pipette type (17) which perhaps will become the standard viscosity 
instrument for glue. Differing in principle from the pipette type are the 
friction-type viscometers, in which a suspended disk, cylinder, or other object 
is revolved in the glue or the glue container may be revolved about the sus- 
pended object. The resistance of the solution to the passing of the disk or 
cylinder is indicated directly upon an attached scale. 
Some animal glues can not be correctly evaluated by a viscosity test ; hence, 
there is the need for the jelly test. For example, the raw material as well 
as substances which may be added by the manufacturer affect the viscosity 
without always having a corresponding effect upon the jelly strength. Viscosity 
and jelly tests together, however, will usually determine the grade of the glue 
with sufficient accuracy for all woodworking purposes. 
Miscellaneous Tests 
Aside from the jelly strength and the viscosity which form the principal 
tests upon which animal glue is graded, there are a number of miscellaneous 
tests which may also be used in judging the quality of a glue. 
MELTING AND CONGEALING POINTS 
The temperature at which an animal glue will melt or congeal is usually 
termed the melting or jellying point. Investigators (44) have shown that they 
are not identical, but it is known that either or both have a general relation 
to the grade of the glue. High-grade animal glues have a relatively higher 
melting or jellying point than low-grade glues. It has been suggested by 
Bogue (6) that the melting point is controlled by both the jelly strength and 
viscosity and that no other one test so correctly parallels the actual adhesive 
strength of a glue as the melting point. However, Alexander (1) considers 
that as a general rule the melting point varies simply as the jelly strength. 
Perhaps the greatest drawback to a more extensive use of the melting-point 
test is the difficulty encountered in measuring the melting point accurately. A 
gradual change takes place as a soaked glue is warmed or cooled, and it is 
therefore difficult to determine the exact temperature at which it changes from 
a solid to a liquid state. The melting or jellying point of a solution used for 
gluing is of further importance, however, in enabling the consumer to use it 
properly and to get the best results with any given grade. It generally deter- 
mines the necessity for warming the wood, the assembly time, and other 
conditions in the gluing operation. 
ODOR AND KEEPING QUALITY 
The odor of a glue gives some indication of its source and condition. Glue 
which has an offensive odor is not considered of the highest quality. A bad 
odor may be due to the fact that partly decomposed material was used in its 
manufacture or that the glue itself is decaying. For high-grade joint work 
it is usually specified that the glue be sweet. The odor of different glues varies 
considerably, and it is difficult to express the different kinds. It is not usually 
difficult, however, to determine whether the odor indicates decomposed or sweet 
glue. 
The keeping quality of a glue may be tested by allowing the jelly which is left 
from the jelly-strength test to stand in the laboratory at room temperature 
for a number of clays. The odor and condition of the glue are noted at 
intervals. Glues with good keeping qualities will stand several days without 
developing an offensive odor, or showing any appearance of decomposition. A 
more vigorous test frequently adopted is to keep the solution in a closed con- 
tainer in an incubator at about 98° F. The glue should remain sweet at least 
48 hours under these conditions. 
For woodworking a small amount of grease in glue is not a serious objection. 
"When glue is to be used in a machine where foaming is liable to occur, a small 
amount of grease is even desirable, because it tends to prevent foaming. The 
presence of grease can be determined by chemical means (25), but it is ordi- 
