THE GLUING OF WOOD 65 
therefore restricted somewhat, especially among glue users, but it can be 
depended upon to supply information of value. 
The modern jelly test consists in preparing a solution of a definite concentra- 
tion, placing a given amount of the solution in a container of a standard size 
and shape, allowing the solution to cool at a definite temperature until it forms 
a jelly, and determining the firmness or elasticity of the jelly. The strength of 
the jelly may be expressed relatively by comparison with a glue of an arbitrary- 
standard or by numerical units. It is necessary to have suitable equipment and 
it is important that the solution be chilled at a given temperature for a definite 
time and tested at the same temperature. 
The oldest method employed for measuring the strength of a glue jelly is the 
finger test. In this test the strengths of two or more jellies are compared 
empirically by pressing the glues with the fingers. If used in grading glues, 
it is necessary to compare the unknown glues with an arbitrary standard or set 
of standards, handling the samples at the same time and in exactly the same 
manner. This method is the simplest means of measuring jelly strength and 
with practice may be applied with surprising accuracy. The personal factor 
and the absence of numerical values are its chief disadvantages. 
Numerous instruments have been devised for measuring the jelly strength of 
a glue (6), many of which give numerical values. With some of these instru- 
ments a measure is made of the pressure which is required to break the surface 
or to sink a plunger a given distance into the jelly or to compress a free column 
of jelly a given distance. In others the distance which a given weight of a 
definite shape' will sink or displace the jelly is observed. In still others the 
torsional force required to break a column of jelly is measured. These instru- 
ments are often difficult or slow to operate, but they afford a numerical basis 
for judging jelly strength and therefore a means of accurate comparison. 
The National Association of Glue Manufacturers has adopted the Bloom 
gelometer of the plunger type, as a standard instrument and has outlined a 
procedure for making the test (17), which is reputed to give results that can 
be duplicated in different laboratories. The method outlined, while accurate 
and doubtless a satisfactory basis for grading glues, can not be regarded as 
generally applicable by the users of glue unless such users are equipped with 
a glue laboratory and use glue in considerable quantities. It promises, how- 
ever, to become a generally accepted standard for properly equipped laboratories. 
The jelly strength alone is not always an accurate measure of the value of 
a glue, for although it usually has a definite relation to viscosity (the other 
main property upon which grade is based) there are exceptions. The jelly 
strength of some glues may be higher in proportion to the viscosity than that 
of other glues. Likewise, certain substances when added to the glue are known 
to affect its jelly strength (6). The jelly test is valuable, however, when used 
in connection with the viscosity and other tests. 
Viscosity Test 
The viscosity of a glue is usually determined by allowing a specified amount 
of glue solution at a given concentration and temperature (usually 140° F.) to 
flow through an orifice or by measuring the resistance offered by the glue to an 
object moving through it. As with the jelly test, it is necessary to adopt a 
standard procedure to secure results of value (17). The important points in 
making the viscosity test are briefly summarized as follows : 
(1) A definite, constant concentration of glue solution, calculated on the 
moisture-free basis. 
(2) Approximately constant temperature and time used in melting sample. 
(3) A constant temperature of solution at the time of test (140 ±0.1° F. is 
recommended ) . 
(4) Accurate measure of time of flow or of the resistance offered by the 
solution to a moving body. 
The measurement of the viscosity of glue should preferably be made on an 
instrument capable of expressing the result in poises, which is the absolute unit 
of viscosity. Various types of instruments are capable of doing this if they 
are correctly calibrated. 
For viscosity tests of glue, the capillary-tube type of viscometer is the most 
commonly used ; the simplest form of which is a pipette or a straight glass tube 
constricted at one end. These viscometers are not always arranged so that the 
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