TESTING AND GRADING GLUE
There is as yet no uniformity of opinion among glue manufacturers and
glue users as to how glue should be tested and graded. In a general way
the manufacturer knows what kind of glue a certain stock will produce;
but on account of variations that are sure to occur, it is necessary to
subject each glue to certain tests, according to standards more or less
definitely established.
The grades in general use are those originally employed by Peter
Cooper, and are as follows:
A EXTRA
No. 1
1 x
1¼, 1⅜, 1½, 1⅝, 1¾, 1⅞
The only way to determine which is the best glue to use, is by trying
out various grades in actual practice. The best glues for ordinary
uses are well understood (see pages 79 to 85 for suggestions on this
subject). Between two or more glues of any one type, actual experiment
is the only safe guide. Most glue men will give you good advice; but
above all, keep accurate records of results of the different grades
used. When you have found the right glue, keep to it.
THE TESTS
Glue is graded on physical characteristics rather than on chemical
composition. Various chemical tests have been proposed, but they are
unimportant so far as practical working value is concerned.
The most important physical tests are those for viscosity, and jelly
strength.
VISCOSITY OR FLUIDITY TEST
The test for viscosity, or fluidity, is based on the idea that
the greater the tenacity of the glue, the greater will be its
cohesiveness, and the less will be its flowing power. In other words,
the higher will be its viscosity.
In testing for viscosity, water is used as a standard. A solution is
made of the glue, and the rate of flow of the solution at a certain
temperature is compared with the rate of flow of water under the same
conditions.
Several devices are on the market under the name of “viscosimeter” for
measuring the viscosity of glue.
While they vary in detail they are in principle a pipette from which
the glue-solution flows at a given temperature. The time required for
the glue to run out of the pipette as compared with the time consumed
in the same operation by the same quantity of water gives the relative
viscosity of the glue.
The viscosity test is not entirely accurate in itself, but taken in
connection with the jelly test it forms a very satisfactory basis for
grading.
THE JELLY TEST
The jelly test is based on the comparative resistance power of the
various glue-jellies. Several mechanical devices for determining
jelly-strength have been perfected. One of these consists of a brass
vessel which rests upon the glue-jelly, and into which shot is poured;
the weight of the cup and the contained shot upon having penetrated to
a certain depth in the glue-jelly, gives a figure which expresses the
comparative strength of the jelly with the standard.
APPARATUS FOR MAKING JELLY TEST
[Illustration: Apparatus for Testing Jelly-Strength of Glue.]
Another apparatus that has been found accurate and practical is the
device illustrated on the next page. It consists of a pressure tube
(A), over the mouth of which is stretched a thin rubber diaphragm (B).
The tube connects to a rubber bulb (M) and to a pressure gauge, or
manometer (E) with a scale (F). The pressure tube is filled with water
to the point (C). The manometer tubes also contain water. A three-way
stop-cock (D) connects the tubes either to the bulb (M), or to the air,
depending on the position. Below the pressure tube is a brass table (G),
on which is placed the glass containing the glue-jelly to be tested.
When the glass is in position, the table is raised by means of a
threaded wheel until the glue surface forces the water resting on
the flexible diaphragm up to the fixed mark L. Then the stop-cock is
turned to connect the pressure tube and gauge to the rubber bulb. By
pressing the bulb the water is forced down in the pressure tube and so
expands the diaphragm into the jelly, the liquid in the gauge rising
simultaneously. Pressure on the bulb is continued until the water
reaches the mark N.
Thereupon the stop-cock is again turned, the water is held at the point
N, and the pressure is indicated by the height of the liquid in the
gauge. The degree of pressure is the measure of the consistency of the
glue-jelly.
The initial contact between the jelly and the rubber diaphragm is
always the same, all jellies therefore having the same initial
pressure. When the diaphragm is forced down into the jelly, the
pressure required depends entirely upon the resistance that the jelly
offers. The slightest difference in the consistencies of the various
jellies will alter the pressure required, the differences being
accurately recorded by the gauge upon the scale.
Advantages of this apparatus are that the relative value of the jelly
compared with the standard is expressed in concrete figures; the method
of operation is simple; and the instrument is so sensitive that it
will record a change in reading between two samples of glue in which a
difference of 1/100th ounce of dry glue is used. Repeated tests may be
made on the same jelly, as the surface is not broken. With a little
practice a single glue may be tested in twenty seconds, or less.
THE FINGER TEST
One of the most satisfactory methods of determining jelly-strength--and
the one perhaps in most general use, is the finger test.
In this test the various glue-jellies are arranged before the tester,
who presses each with the tip of the finger, comparing it with the
standard as to resistance power. While this may seem to expose the
final decision too greatly to the personal equation, as represented
by the personality of the glue-tester, it is nevertheless true that
an expert develops the most extraordinary precision, arriving at
conclusions that are corroborated by the results of other tests
and by the results in actual practice. The work of the glue-tester
is analogous to that of the coffee-taster and the tea-taster, or
experts in other lines, who through a highly developed and keenly
discriminating sense of taste, or touch, or smell, determine with
extreme nicety the physical characteristics of the substances that they
are accustomed to test.
SOME SIMPLE, PRACTICAL TESTS FOR GLUE USERS
To perform the tests described requires a degree of experience and an
equipment beyond the reach of the ordinary buyer and user of glue.
Certain tests may however be made that are of great value in
determining important facts about the glue it is intended to buy and
use.
These tests could not be used as a basis for grading glue
scientifically, but they are exceedingly valuable in determining its
purity and its adaptability to the work in hand.
SAMPLING
In the first place you should carefully sample your glue with a view to
testing.
Take several samples from various parts of the barrel. Flake glue is
often made up of different varieties, and a single sample may not be
at all representative. Ground glue, from its very nature, is easily
adulterated. It should be examined in a good light, for evidence of
foreign substances. Examine flake glue carefully also for uniformity
of odor and general appearance. If glue has been adulterated while
in original form it is practically impossible to determine the
adulteration by external appearance. Subsequent adulteration may be
detected.
BUBBLES
If you should notice white bubbles, in the shape of round blots, on the
surface of the glue, you have found evidence of decay. If there is any
doubt in your mind you can complete the evidence by moistening the
glue. If it gives off a sour odor you have an additional indication of
putrefaction. Such glue should be avoided.
Bubbles may appear within the glue--not on the surface--without
being an indication of putrefaction. As a matter of fact, bubbles
are practically always found in certain high-grade glues, though
practically never in low-grade bone or hide glues. They are supposed
to be due to the air which gets in when the glue is poured into the
moulds. When glue is originally dried on nets in very cool and dry
weather, such bubbles are frequently found. Always beware of glue
showing surface bubbles.
SURFACE INDICATIONS
Besides being free from bubbles or blots a good glue is smooth, though
not necessarily glossy. Sometimes the very best glue will be of a dull
color, and many inferior glues even have a very shiny surface. The
surface should be uniform in color and in appearance.
COLOR INDICATIONS
These are not important, as a rule. The color of any particular lot of
glue should be nearly uniform; otherwise it is subject to the suspicion
of adulteration.
Bone glues are usually darker than hide glues, but some bone glues
go through an artificial clarifying process which gives them the
appearance of high-grade glues but really detracts from their strength.
Very frequently oxide of zinc is added to glue, the effect being to
make it set quickly, as well as to give it a light color. Some glues
contain so much oxide of zinc that they are milk-white. Zinc oxide is
not harmful except when added in very large quantities.
ALKALINE OR ACID QUALITY
The best glues are neutral as to acid and alkali. Glues with an excess
of acid should be avoided, especially when used with oak or chestnut
or other woods with strong acid qualities, as the acids in the glue
may unite with those in the wood in such way as to have a destructive
effect upon the glue. In such cases the glue will granulate after a
time and the work will pull apart.
When a wood is being used that is strong in acid it is advisable to
use a glue containing enough oxide of zinc to neutralize the acid in
the wood. In making sizing for paper a glue containing either acid or
alkali in excess should be avoided. It is also held by some authorities
that acid in glue tends to bring about decay.
To test for alkali or acid, dissolve a small quantity of glue in water
and dip a piece of litmus paper into the solution. Acid will turn the
paper violet or red. Alkali will turn it blue. Litmus paper may be
procured at any drug store.
BREAKING QUALITY
This is a simple test that affords an important indication of the
quality of glue. Take a small piece between the thumb and forefinger
of each hand and bend it. A very thin piece of good glue will bend
without breaking. When it does break, if the edges are splintery, great
tensile strength is indicated. A clean fracture, on the other hand,
indicates a brittle, low-grade glue, which has been subjected to heat
so long as to destroy the tissue; or else it has been made from bone
stock. High-grade glues never show glassy fractures, but bone glues do.
In making this test, the air conditions of the room should be taken
into account. If the glue has been kept in a dry room it will naturally
break much more readily than if it has been in a moist atmosphere. This
is especially important to bear in mind if comparative tests are being
made.
FOAM
A simple test for foam is to beat a solution of glue with an ordinary
eggbeater. Glue which shows foam, or in which foam does not quickly
subside, probably contains impurities. Foam is especially frequent in
alum-dried glues and in cheap bone glues.
Some authorities believe that foam is caused by overheating, due to
scalding by contact with steam jacket, or by steam coming into direct
contact with the glue, or by heating for too long a time, or it may be
due to the fact that all the grease has not been eradicated.
Glue that foams at ordinary temperature should be avoided for good work.
GREASE
A moderate amount of grease may be a good thing when using with clay or
with colors, but a large proportion of grease should be avoided when
making glazed or coated papers, or in general use.
A glue at ordinary temperature which is not overheated on which a scum
rises has an excessive amount of grease. It shows that the glue has not
been properly skimmed in manufacture.
KEEPING PROPERTIES
The keeping property of glue may be determined by letting the
glue-jelly stand for several days exposed to the air, and noting
any deterioration. It is customary to let the jelly stand at room
temperature--but if the glue is to be kept under any special conditions
the test should be made as nearly under these conditions as possible.
ODOR
Deterioration is always accompanied by a sour odor. Avoid using any
glue that does not smell clean and sweet.
LABORATORY TESTS
Up-to-date practice in all the larger concerns using glue demands a
laboratory for making tests. An expert is put in charge and the glue
analyzed chemically as well as for its physical properties. This method
cuts the guess-work down to a minimum. Manufacturers whose output would
not permit the employment of an expert all the year round can have glue
analyzed in laboratories maintained for such purposes. It means often a
great saving of money in the end to learn the exact properties of the
glue you propose to use, or that you may be actually using.
We may also repeat what we have already said about securing competent
advice from manufacturers. The glue user who takes a responsible
manufacturer or glue house into his confidence will secure valuable
counsel.
It is to the glue-maker’s and glue salesman’s interest to have you
secure good results. They have a large experience to draw on, and when
checked up by the results from actual practice in your glue-room their
advice in regard to the selection of glues is usually worth heeding.