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Testing and grading


HERE 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:
1 1/4, 1 3/8, 1 1/2, 1 5/8, 1 3/4, 1 7/8

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.

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.

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 is based on the comparative resistance power of the varityus 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 testing jelly

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 1-40-th 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.

One of the most satisfactory methods of determining jelly-strength - and the one perhaps in most general use, is the finger test.
In this tcst 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.

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.

In the first place you should .carefully sample your glue with a view to testing.
Take several samples frfom 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.

If you should notice white bubbles, in the shape ar 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.

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.

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.

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.

This is a simple test that affords an important indication of the quality of glue. Take a small piece bgtween 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.

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.

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 over-heated on which a scum rises has an excessive amount of grease. It shows that the glue has not been properly skimmed in manufacture.

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.

Deterioration is always accompanied by a sour odor. Avoid using any glue that does not smell clean and sweet.

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.