How Glue Should Be Used
( Originally Published 1906 )
IT would seem superfluous, after discussing in detail the nature and properties of glue, as well as emphasizing the necessity of exercising the greatest care in its selection, to remark that its success in work will depend in a large measure upon intelligent preparation and application. The normal and logical inference is that this principle is self-evident to him who uses glue constantly; yet a careful study of the situation reveals the fact that a number of consumers are totally indifferent to first principles in the preparation of glue for work, and that many, of seeming intelligence, entrust the melting and distribution to those who are ignorant of its very nature and seemingly regard it as less worthy of consideration than the sweepings from the factory floor. The average dealer in glue is the recipient of so many and varied complaints regarding the product he supplies, the majority without justification, that it becomes necessary, in fairness to him, to call attention to the fact that he is often as little to blame for the difficulties experienced with glue as he is for an unexpected fall of rain. The commonest complaint in regard to glue is that it has a bad odor. Harrowing tales are heard by the dealer regarding the number of employees who were compelled to suspend work owing to the unbearable stench arising from the glue which he has supplied. Quiet investigation reveals the fact that the glue melting-pot has not been cleaned, say for six months, and that there is nothing "off" in the odor of the glue. The author has seen this trouble arise only too frequently in wall-paper factories, where the carelessness of the men in respect to cleaning the melting-pots is proverbial. It is no exaggeration to state that 80 per cent of the difficulty experienced with glue arises from carelessness or ignorance upon the part of the user, and that the remaining 20 per cent may be legitimately ascribed to the glue itself.
The adage, "Cleanliness is next to godliness," has a beautiful and significant application to the preparation and treatment of glue, not only it regard to melting it, but also to its distribution and other factors. Storage under adverse conditions will ruin glue long before it is used. In the matter of assimilating foreign odors, some glues are extremely sensitive and hence should not be stored with odoriferous chemicals, crude carbolic acid in particular. It is the practice of some consumers to store glue in damp and even wet places, and subsequently complain that it contains undue moisture or is moldy. Others, again, unhead the barrels pre-maturely, exposing the contents to dust and dirt which, when the glue is brought into solution, accumulates at the bottom of the melting-pots, thus contaminating subsequent melts, or, if the solution is heavy, remains suspended therein and soils the Work. While the consumer frequently has cause for legitimate complaint, it would redound to his credit if h3 would but apply the rules of common sense to the treatment of the glue.
Soaking or Softening Glue. — Considerable difficulty in manipulation arises from faulty practice in this respect. Precautions must be observed that will insure the uniform softening of the glue prior to melting. To this end, if in the form of flake, sheet, cake strip or ribbon, it should first be broken into small pieces with a hammer. It is essential that all the glue be immersed in the water. If pieces are permitted to stick out beyond the level of the water, the natural result is that such pieces will be only partially softened. When the melting proceeds, these will slip to the bottom of the kettle, and, in order to melt them, protracted heating at an excessive temperature is required. This means the radical impairment of the strength and adhesiveness of the glue.
In the case of extremely thin-cut, high-testing glues it is often impossible to have all immersed uniformly, unless the melting-pot be capacious. Such glues are very light and hence the required weight frequently represents considerable bulk. The needful quantity of water may fail to cover one half or even two thirds of the glue in the pot, with the result that the uncovered portion is not softened. This difficulty may be avoided at the expense of minimum time and trouble. Thin-cut, high-test glues absorb water rapidly, and hence if the requisite quantity of water is first placed in the melting kettle and the glue added little by little, each portion will soften rapidly and may then be shoved down toward the bottom of the kettle to make room for the addition of the remaining portions. In this way the glue will be uniformly softened.
Even though the pieces are well covered with water it is necessary that they be stirred in the latter, when it is added, to insure uniform wetting. If this precaution is neglected, pieces will adhere to each other and the center of such masses will be quite hard even though the exterior is well soaked. These partially soaked masses require protracted heating at the expense of the whole solution, in order hat they shall melt.
The usual practice in wetting ground glues is to place the glue in the kettle and then pour on the water, stirring the while. No matter how carefully this is conducted, particles of the glue will rise to the surface of the water and float there, and, coming in contact with the sides of the kettle, stick there, giving up any absorbed water with great rapidity and so do not dissolve. This very objectionable feature may be avoided as follows: Put the glue in the kettle and add about half of the requisite quantity of water stirring the glue with a stick, beginning the stirring before the water is added and continuing until the glue has absorbed as much water as it will. Now add the balance of water, stir-ring the while. Through the absorption of water from the first portion added, the glue becomes much heavier and when the balance is added there is no possibility of particles floating at the surface.
Melting the Glue. Two forms of melting-pot or kettle are commonly known, the one jacketed, and the other in which steam is introduced direct or "live." The former is the better. Direct steam is injurious to solutions of glue, the drastic action of the steam rapidly weakening them. If a solution of A Extra be boiled continuously with direct steam for a period of twenty four hours, the gelatinizing power of the glue will be destroyed. The gelatinizing power of weaker glues is destroyed in correspondingly shorter time. If a solution of glue is boiled with direct steam for one hour, it will lose a grade in strength, and a half-grade if kept at 200° for one hour in a jacketed kettle. It is to be borne in mind throughout the melting of the glue, that no more heat is to be applied than is absolutely necessary. If direct steam is employed, it is folly to boil the water until all is dissolved; besides, it is not necessary to do so. Let the steam pass through the water until the temperature is 180° F. If no thermometer is at hand to measure the degree of heat, this may be approximately gaged by the cessation of the characteristic rattling noise produced by steam when passed, under pressure, into cold water, and is supplanted by a subdued rumbling. When this degree of heat is reached, cut off all steam and stir the contents of the kettle until all the glue is uniformly dissolved. If the glue has been properly softened, it will dissolve at a low temperature.
Under no circumstances should hot or even warm water be poured upon dry glue. The heat, so far from facilitating the softening, actually retards it, if not preventing it altogether. We have a parallel phenomenon in the treatment of iron with nitric acid, the action of which is first violent and finally ceases altogether, owing to the fact that a slight film of oxide of iron is deposited upon the surface of the metal under-going solution. This is repeatedly dissolved and re-deposited by the acid. In order to promote the solution of the metal, it becomes necessary to pour off the acid and wash the metal well with pure water.
When the acid is returned, it again attacks the iron violently. The action of the hot water on the glue is to dissolve a little, which, when once in solution, seems to retard, if not altogether prevent, the further absorption of water by the glue. This may easily be verified by preparing a 2 per cent solution of glue and placing in this a piece of glue to be softened. It will be found that a much greater time is required than if the piece had been immersed in pure water.
Where steam is not available for melting the glue, and the direct heat of gas or oil has to be used, the melting-pot must be jacketed, as otherwise the solution will burn and char. The outer jacket is to be filled with water, which, when brought to the boil, will impart sufficient heat to the water of the inner vessel to melt the glue. Once the water in the outer jacket is at the boil, the heat should be withdrawn.
The facility with which glue melts will depend upon its proper softening. If this detail is attended to, there should be no difficulty in obtaining a uniform melt at the minimum temperature. In reply to the oft-emphasized dictum that the glue solution should not be kept continuously at a high temperature lest the adhesiveness and strength of the glue be impaired, it is justly con-tended that, since the solution must be hot in order to be effective, especially in joining wood, it is imperative that the melt must be kept at a certain degree of heat in order that time shall not be lost by having to stop work to reheat the cooled or jelled solution. Since this continuous heat is maintained to save time in work, it is inevitable that a skin form on the melted glue, which, brushed aside, clings to the pot and dries there, usually beyond the level of the water added for the next melt. How continued high temperature, and consequent formation of skin may be avoided, will be seen further on.
Cleaning the Melting-Pot. — Not only is it essential that the melting-pot be kept scrupulously clean in order that there shall be no gradual contamination of the glue, but much waste of material is avoided through this observance. Dirt enters the melting-pot through the glue itself, the introduction of dirty brushes, or the exposure of the pot to dust, etc. If glue is melted in a dirty pot, the skin forming on the surface of the melt gradually accumulates at the sides of the kettle and slowly decomposes. This may or may not fall into subsequent melts, thus contaminating them. The only way to make sure that it will not do so is to clean the pot.
Much unnecessary waste of glue may be avoided through observance of the following procedure. The contents of the melting-pot exhausted, scraps of dried glue, as well as scraps of partially dried jelly adhering to the sides, should be detached mechanically, as thoroughly as possible, and examined. If clean, they may be replaced at the bottom of the kettle; if dirty, they are to be set aside temporarily. In the first instance, they are covered with the minimum of water necessary to soften them and the sides of the kettle swabbed with a little water in order to soften any glue that has dried and not been detached mechanically. The pot is then gently heated in order to bring the scraps into solution, this solution used in work, and the pot thoroughly washed out with hot water and cooled before soaking a fresh portion of glue. If the scraps have proved dirty, but not sour, they may be re-melted in a separate kettle and the solution kept warm enough to permit the dirt to settle, when the supernatant glue may be used without risk. If sour, they must be thrown away. If the glue pot is properly cleaned, there is no danger of souring, and thus all the glue may be used without waste. It may be contended that much labor may be saved by adding sufficient water for the next melt, and through this means soften all glue adhering to the kettle in conjunction with that added fresh. It will be found, however, that the freshly added glue will absorb the bulk, if not all, of the water, leaving adhering scraps practically unsoftened, which in this way continue to accumulate, interfering with the proper working of the glue.
Distributing the Glue. — The most economical method of handling glue is that adopted at a number of progressive factories where large quantities are consumed. The glue is first softened and melted in a large central kettle, the solution being made very concentrate so that a stiff jelly results. This jelly is then divided into small portions and distributed among the workmen, whose benches are equipped with small melting-pots. The workers have then merely to re-melt the jelly with the addition of water, this re-melting requiring but a minimum temperature. The advantages of such system of distribution are, first, that but a minimum of glue is apt to be damaged in the melting, as this is entrusted to some one who understands what he is about; second, that there is comparatively little or no waste of the glue, the portions of jelly distributed being carefully checked, and it is thus seen that no workman uses more glue than another for a given purpose. Thirdly, the danger of contamination by dirt or partially foul scrap from previous melts is reduced to a minimum, as there is but one large kettle to clean, and the cleaning of the smaller bench-kettles is readily effected. The chief disadvantage of the system arises from the fact that glue, in the form of a jelly or in solution, is far more prone to decomposition than when dry. Unless the system is so regulated that the minimum amount of jelly is prepared and distributed so that it will be rapidly consumed, loss is apt to be occasioned by the souring of the jelly, especially in warm weather. This difficulty may be obviated through the judicious use of any preservative save formaldehyde. The addition of a small amount of pure carbolic acid, for example, will insure against decomposition of the jelly for a day or so, and will not impart sufficient disagreeable odor to affect the work. If the use of preservatives is objected to, the keeping quality of the glue may be enhanced by pouring the solution of glue into small tin boxes to jell. These should be provided with a tightly fitting cover and should always be kept closed save when portions of the jelly are to be withdrawn. Definite quantities of glue jelly may in this way be distributed, as boxes holding one pound, or any other desired quantity, may be selected for the purpose. The atmosphere excluded as far as possible, the glue will keep well. In jelling the glue, the cover of the boxes should be left off until gelatinization sets in.
Davidowsky points out that, while glue is fresh made, every addition of water will, up to a certain point, increase the adhesiveness and elasticity of the glue.
This is true only of glue in the form of jelly, and the fact is largely commendatory of the system of at once converting the glue into a stiff jelly and then distributing this for use. This practice certainly increases the water-taking quality of the glue. Even as glue stock yields more and more glue as water is added up to a certain limit, so glue itself would seem to yield greater strength and adhesiveness to large quantities of water than to small. From this it would seem that glue, as prepared from the stock, is never chemically complete, but requires the addition of water and subsequent treatment with heat in order that the hydrolitic action, to which it owes its very nature, shall be completed. This peculiar behavior of glue jelly would further indicate that gelatinization is attended by partial reversion to the form of the glue-yielding elements of the stock, and that when the jelly is treated with additional water, an increase of actual glue is effected.