Selection Of Glue For Various Industries
( Originally Published 1906 )
Preliminary Considerations. — While no definite rule can be enunciated relative to the selection of a glue for a specific usage, attention must be paid to certain fundamental considerations. Experience is the safest and surest guide in glue selection; but experience, with glue, is to be acquired only at great pains and expense. Hence the necessity of applying the dicta of common sense to the selection of an article that will answer the requirement of the consumer. It is manifest that stronger glues are required for some purposes than for others, and that common bone glues will not do the work of high-grade hide. It is to be regretted that the average consumer, in selecting glue, is influenced altogether by the question of price. While strenuous competition in all branches of manufacture renders it imperative that raw material be purchased as cheaply as possible, where glues are concerned too little attention is paid to the quality of the material.
Through a lamentable ignorance of its properties and, more particularly, through ignorance of proper methods of application, many a good glue is condemned and frequently an inferior article, at even higher price, substituted. Throught his same misconception of the nature and properties of glue, an article is oft chosen that is unfit for the particular requirement. The average consumer proceeds upon the basis that he does not wish to pay more than, let us say, eight cents per pound for the glue, regardless whether the proper glue for his work can be purchased for the money. Utterly failing to consider that a glue at twelve cents is often far cheaper in actual work than one at eight, he tries every make of glue that is offered him at the latter cost and eventually clings to that furnishing the best results. His supply of this cut off, he is at sea regarding further selection, and it is highly probable that he is never successful in obtaining the glue best fitted for his work.
Glues that exhibit palpable defects of stock or manipulation are unfit for any purpose. True, they can be used; but the results will never be entirely satisfactory. As has been previously emphasized, the consumer's only path to safety, no matter for what purpose the glue is intended, lies in the test. It is only through continued testing that he can learn to distinguish glues and arrive at that state of enlightenment that will enable him to select the glue best adapted to his requirement, independent of any and all suggestions on the part of the dealer or manufacturer. Further, in this way alone will he be able to select the proper glue and at the same time protect himself in the matter of price.
Glues for Joiner-Work. — Prior to entering into the discussion of the relative merits of different makes of glue for joining wood, certain axiomatic principles may be enunciated in reference to the method of applying the glue. These once understood, it is not a matter of great difficulty to obtain a good glue for the work, at a satisfactory price.
1. Glue exerts a far greater hold on surfaces of wood that have been cut across the grain than on those that have been split, or cut with the grain.
2. When two surfaces of split wood are laid together, the hold of the glue is the same whether the fibers are laid parallel or crosswise to one another.
3. The value of a wood joint is dependent upon the union of the glue with the fiber of the wood. For glue to be properly effective, it must penetrate the pores of the wood; and the greater this penetration, the more substantial the joint.
4. All other factors being equal, glues that dry slowly are in-variably stronger in the joint than those that dry rapidly.
5. Except in the case of veneering, both surfaces of the wood should be properly glued before junction.
6. Do not use-thick solutions of glue for joint-work. They congeal too quickly, and hence fail to penetrate the pores of the wood, yielding, as a result, a weak joint. In every case, the glue must be worked well into the wood with a brush, much in the same manner as a coat of paint is applied.
7. If glue is applied to hot wood, all the water of the glue solution will be absorbed by the wood, leaving a thin inadhesive coating of glue at the surface of the joint, which, if made in this fashion, will hold only a limited time.
In the fact that thin solutions of glue produce far stronger joints than thick, we find some basis for the contention that foreign glues are superior to domestic for this class of work. Given two solutions, of equal concentration, one of foreign glue and the other of domestic, both glues testing alike, the foreign glue will make the stronger joint for the reason that the viscosity of its solution, owing to clarification, is comparatively lower than that of the domestic glue, and hence penetrates the wood with greater ease. Too much of the domestic glue will lie at the surface of the joint, and hence, despite the fact that it is as strong as the foreign, will not make so strong a joint.
On the other hand, if the comparison be made between a foreign and a domestic glue of equal price, the verdict will be in favor of the domestic, inasmuch as a far stronger domestic product can be purchased for a given price. If, now, the domestic glue be brought into solution of viscosity equal to that of the solution of foreign glue, it will be found that the jelly strength of this dilution is practically equal to that of the undiluted solution of foreign glue. As a practical example, let us assume that the glues under discussion are a foreign and a domestic, each costing ten cents per pound. The test of the foreign product is apt to be no higher than 1 3/4 whereas that of a domestic glue purchasable for this price is likely to be 1 3/8. Assuming, further, that in the preparation of the working solution, one pound of the foreign glue is dissolved in two pounds of water,. since the domestic glue tests three grades stronger, to make a solution of this equal in viscosity and jelly strength to that of the foreign glue, it is possible to dissolve the domestic product, one pound in three and one half of water. Such solution will equal in penetrating power and binding strength the solution of foreign glue; and it is readily seen that it costs fully one cent less per pound solution. This must not be construed as applying equally to all domestic glues. It is not to be expected that glue made from junk bone will answer as well as hide glue for binding wood. On the other hand, a well made acid-treated bone glue, if properly applied, will make a very strong joint.
Since the sine qua non of a good glue for joint-work is that its solution shall properly penetrate the wood, it is important that the cabinet-maker test the viscosity as well as the strength of the glues submitted, before making a selection. While it makes but little difference, for this class of work, whether a glue is alkaline or acid, those exhibiting either of these conditions to excess should be rejected. A glue that has been pre-pared from over-limed stock (the evidence of which defect is extreme alkalinity), is manifestly unfit for joint-work. Such a glue is constantly weakening, even in the original dry state, and needs but to be brought into solution to promote rapid decomposition. The weakening and decomposition will proceed gradually after the glue has dried in the joint, and the joint in this way gradually weakens. Extreme alkalinity is not always evidence of over-liming; but if this be attended by abnormal viscosity, together with a tendency to foam, the glue-stock has suffered over-liming. A strongly acid glue is likely to have an effect on the wood itself, the fiber slowly yielding to the solvent action of the acid.
Of equal importance to the considerations of acidity and alkalinity of cabinet glues is the question whether the glue has an abnormally high viscosity owing to solution of soaps, alum, etc. Alum-clarified or alum-thickened glues are always to be regarded with suspicion, for, despite the clearness of their solutions, the viscosity is very high in proportion to the jelly strength. It is obvious that, in order to prepare a solution of such glue that will properly penetrate the wood, the solution becomes so dilute as to be radically deficient in glue strength, consisting largely of the salts that are imparting factitious viscosity to the solution. Similarly, hide glues, with solutions of high viscosity owing to the presence of mucin or soaps, are to be rejected for the same reason. In this connection, glues prepared from rabbit skins may be cited as typical of very strong glues that are unfit for joint work. Despite the fact that these seldom test lower than A Extra, the viscosity of their solution is abnormally high, owing to peculiar methods of treatment. The dilution necessary to insure proper penetration of the wood renders the glue very weak. These glues may always be recognized by their peculiar rabbity odor, coupled with great clearness of both dry piece and solution, the latter being very thick owing to presence of alum. Some straight sheep glues, clarified by means of alum, are equally unfit for this class of work.
That domestic glues, of good strength, answer equally as well as foreign for joint-work is evidenced by the fact that many cabinetmakers use the former exclusively. No glue consumer is more conservative and particular than the manufacturer of pianos, who frequently uses several grades of glue, both ground and flake. Some piano-makers will employ none but imported glues; others, again, none but colored and opaque domestic hide glues. No definite data as to the relative superiority of foreign or domestic glues can be obtained from a review of the opinions expressed by individual manufacturers of cabinet work. In the selection of glue for almost every purpose, custom is the controlling factor. Through long and continued use, the consumer becomes accustomed to some particular brand of glue. He has learned its limitations as well as its merits, and hesitates to make a change, even though such change might result in a material saving of money, or the procurai of a superior article for the same price he is accustomed to pay, which is an equal saving. While there is considerable justification in the contention of the intelligent cabinet-maker, that one cannot say positively how a glue is going to work until actually tried and the test of time applied, much unnecessary expense may be avoided by a rational preliminary test. It is not sufficient that jelly-strength alone be roughly assayed by noting the "water-taking" properties of the glue. As has been pointed out, the viscosity of the glue solution is of equal importance. By properly testing the glues submitted for his inspection, the consumer will at once be able to distinguish them in a practical way; and, having rejected those manifestly unfit, he may then subject the balance to the test of actual work, confident at least that no work will be harmed through their use, even though they do not supply results of maximum efficiency.
In the manufacture of carriages, imported glues are used almost exclusively, the Star & Anchor being a favorite grade. Long habit in use is responsible for this, as there can be no question but that some domestic grades will work equally well. We must account for this so-called conservatism in the use of glue, in the absence, heretofore, of a reliable system of preliminary test. Having obtained satisfactory results from some one particular grade, it is but natural that the consumer should hesitate to change lest much valuable finished material be damaged. This "conservatism," however, is responsible for much useless expenditure.
Glues for Wall Paper. — In the manufacture of wall papers, glue is employed for applying the clay, paris-white, etc., with which the papers are "grounded," as well as the sizing medium for the ground colors. In some instances the top colors are applied with glue, although it is commoner practice to use dextrine or "color-size" for this purpose. In no other industry has the search for an available substitute for glue been so rigorously prosecuted as in wall paper manufacture. Keen competition, resulting in the production of quantities of papers, selling for a few cents per roll, some printed in as many as twelve colors, with mica and bronze effects, has rendered imperative the purchase of raw material, of which glue is an important part, at rock-bottom cost. Casein is the only available glue substitute for the heavier work, and has been successfully employed in times of glue scarcity and consequent prohibitive price. Sizes made by partially saponifying rosin with caustic alkali, and combining the emulsion with some starch such as tapioca or sago, have been tried without success. Their failure is due to the same causes that preclude the proper working of a number of glues, i.e., they do not work uniformly with all pulp colors, causing a number to "liver." Thus, a sizing-compound that may work with reds, greens, and blues may fail utterly with yellows and blacks.
In the selection of the proper glue for sizing the colors, much depends upon the colors themselves. Even the best of these are not always perfect, and the trouble arising from imperfectly or improperly sized colors is too frequently ascribed to imperfections of the glue. The same applies to difficulty arising from lack of union between the clay and glue, in the application of grounds. Difficulties of manipulation experienced at one factory will not obtain at another, for the reason that some manufacturers turn out only a very cheap line of papers, while others produce none but high-grade. It is at the works of the former that the greatest and most frequent difficulty is experienced, inasmuch as only the cheapest raw material is purchased, and cheap pulp-colors, for example, are extremely troublesome.
The importance of the relation between glue, on the one hand, and the colors, on the other, can best be understood from a limited exposition of the methods of manufacture of the latter. In order that the colors shall be at all serviceable in the manufacture of wall paper, they must be in the insoluble or lake form. Otherwise, unless they were sized to such an extent that the mixture would consist of much glue and little color, the latter would "bleed" or spread through the paper and over its surface. Again, it is not sufficient that these colors be lakes, for, were they supplied in the dry state, it would be necessary to incorporate them mechanically with water, before they could be mixed with the glue solution; and this would necessitate the installation of mixing machinery at the wall paper plant, in order that the mixture should be uniform and not gritty. Accordingly, the aniline or coal-tar color, and, in some instances, natural dye-stuffs, are precipitated from aqueous solution upon an insoluble base such as finely divided barytes. The precipitated mass is settled and, after the supernatant fluid is withdrawn, is washed several times to free it from excess of precipitant or un-precipitated color. The mass is then conducted to the filter press and much of the moisture pressed out, or else this is gotten rid of by means of a centrifugal hydro-extractor. The resultant color is in the form of a heavy paste or pulp, and in this state is readily and uniformly sized with glue solutions. Exposed to the air, the pulp colors dry out and may be triturated to an insoluble dry powder, which is difficult of uniform incorporation with water.
Pulp-colors may roughly be divided into two classes — those that are made by first suspending the dry mineral base in the solution of color prior to precipitating the latter, and those that are prepared by precipitating the base simultaneously with the color. The former are poorer in quality and are frequently deficient in color strength. The latter cost more to prepare, but are productive of far better results. As an example of each class, the following formulae of preparation may be cited. Lemon yellow, a color much used in printing wall papers, either direct or as a reducing agent for other colors, may be made by precipitating a solution of bichromate of potassium with one of lead acetate (sugar of lead). If a quantity of barytes or other suitable mineral base be suspended in the bichromate solution prior to the addition of the lead acetate, the chromate of lead will be precipitated upon the barytes and a pulp-color, of a sort, will result. In order that the whiteness of the base shall be fully hidden as well as that the resultant yellow shall be adequately strong, very concentrated solutions both of bichromate and acetate are required. The resultant precipitate will not stand much washing, as the color is not absolutely fixed upon the base.
Contrast with the above the pulp colors prepared from Ponceaux and Bordeaux reds, nigrosines, eosines, etc., in the following way. Four solutions are prepared, one of calcined soda ash (carbonate of sodium), one of barium chloride, one of magnesium chloride, and one of caustic soda. The color to be precipitated is separately dissolved in water, the amount of color used being subject to the shade desired; and to this solution of color is added a proportion of the above in the order in which they are named. The carbonate of soda does not affect the color, but the barium chloride precipitates it and at the same time reacts with the sodium carbonate to form insoluble carbonate of barium. Similarly, the caustic soda and magnesium chloride react to form insoluble hydroxide of magnesia. The insoluble basic precipitates, being formed simultaneously with that of the color itself, are more finely divided than if corresponding quantities of barium carbonate and magnesium hydrate had first been mechanically suspended in the color solution, and the color precipitated upon them by means of barium chloride. Again, the amended procedure insures the intimate incorporation of precipitated color and bases, which it is otherwise practically impossible to obtain.
This digression from the subject in hand to that of pulp-color manufacture is deemed necessary in order that the working of different glues with different colors may be fully understood. Since so many ingredients enter into the composition of a pulp-color, and the cheaper are poorly fabricated, often containing an excess of precipitant; from what has been said in previous chapters as to the effect of divers chemical reagents upon glue, it is of the greatest importance that the colors be free of all material tending to react with the glue used for sizing them. Conversely, it is of equal importance that the glue be free of substances that may react with any excess of precipitant in the colors. Far greater latitude is to be exercised in the selection of the glue for sizing, than of the colors to be sized. A given shade of color may be obtainable only through the use of certain chemicals, and since this shade may be essential to the requirement, it becomes necessary to adapt the glue to the color and not the color to the glue. For example, it is difficult to prepare Prussian blue lakes that are not greenish, and their manufacture involves the use of a number of chemicals without which the desired shade could not be achieved. De-spite the fact that such colors are known to be excessively over-loaded, the wall-paper manufacturer has no alternative save to use this color and find a glue that will work properly with it.
To return to the colors themselves. Since these must cost as little as possible, the manufacturer of them is under necessity of selecting, for the base, such materials as are-cheap and heavy. A definite quantity of color is required to produce a given shade, and the color must be of good quality. Hence there is no opportunity of reducing the cost at this end. Therefore, if the color maker is to produce an article of standard shade at low cost, and still make a profit, he must make his colors as heavy as possible.
The difference between the sum of the constituent weights and total product, in all of the above, is water. These may all be somewhat cheapened by the substitution of calcium chloride of good strength for the chloride of barium.
It is now seen that the constitution of the average pulp-color is far more complex than its appearance would indicate, and is anything but the simple result of precipitating the color upon a single insoluble base. It is obvious that where so many widely divergent chemicals enter into the constitution of a product, and where, particularly, their proper interreaction is dependent upon certain mechanical or physical factors, it is highly probable that they will contain an excess of one or more materials that will either affect the glue or be affected by some substance which it contains. In the case of Bremen blue, for example, if there be any excess of tannic acid, it is not difficult to forecast what will occur when this is brought into conjunction with the glue solution. The insoluble, stringy precipitate of tannate of gelatin, even though small in amount, will be sufficient to "liver" the sized mass. In the case of the maroon, the presence of soap may or may not prove detrimental. This will depend upon what kind of soap is used.
Since the colors, as such, must be accepted and used, the glues for wall paper must be chosen with a view to their possible effect upon the colors, rather than of the possible effect of the colors upon the glue. The glue may be neutral or it may be acid or alkaline. How will either of these latter conditions affect the colors in use? The careful examination of the glue for foam is necessary, not only because of the mechanical difficulties that may arise from this cause, but also to determine if the cause of the foam is some glue substance that will affect the colors. Grease, in excess, unfits a glue for wall-paper work, but not for the usually accepted, reason. The ash of the glue revealing chemical salts, the query follows, are these present in sufficient quantity to react to disadvantage with the colors? Each of these contingencies is worthy of individual consideration.
Acidity and Alkalinity. — Were the glue employed for sizing gold or aluminium bronzes, the question of acidity or alkalinity would be entitled to greater consideration than it is. The moderate amount of alkali or acid present in glue is not apt to affect the colors so far as changing the shade is concerned. Once precipitated, colors are not so susceptible to the action of acid and alkali as before precipitation. The chemist has only to view either of these conditions from the standpoint of their possible reaction with substances present in the colors; and, save within the limits previously defined, viz., acidity or alkalinity in excess due to defects of manipulation, these may be disregarded.
Grease. — A popular superstition exists among wall-paper manufacturers, to the effect that greasy glues are to be avoided because of the likelihood of their making grease spots in the paper. That this belief is without foundation is evidenced by the fact that for many years the Baeder, Adamson B4, a markedly greasy glue, was the favorite brand for wall-paper use. It is common practice, even at the best plants, whenever trouble arises through the foaming of glue and clay, or glue and colors, to "kill" the foam by the addition of lard oil, or grease and soda emulsions, to the mass. If the addition of grease itself, in this way, does not result in the formation of grease spots, it is to be assumed that the grease in the average glue never will. An excess of grease in a wallpaper glue is to be avoided chiefly on account of its possible action with the colors. While, on the one hand, grease does much to prevent foam, and very frequently has a brightening effect upon the printed colors, on the other there is always the possibility that there are substances in the colors which will convert the grease into insoluble metallic soaps which will certainly interfere with the uniform flow of the sized colors from the printing machine.
In applying the test for grease in the wall-paper factory, it is advisable to use the ungrounded hanging stock itself for the purpose, and to incorporate the glue solution to be tested with a standard pulp-color. In this way, a glue that might exhibit considerable grease if painted out with a plain aniline color, will in all probability exhibit none at all when a pulp-color is used, as the porous base of the color will absorb it. If, on such test, no grease permeates either color or paper, the glue is not too greasy for use; and it may be added that a glue that fails to qualify under this test is rarely to be found. Unless it is known that the colors contain excesses of material that will be affected by the grease, or affect it, it is wiser to choose a greasy glue in preference to a grease-free, as the grease will promote uniformity in the flow of the color from the roller of the printing machine. Many patent sizings are characterized by the addition of soap to the glue and base, as the former promotes the "slip" of the size. Grease itself will act in much the same way, and is so rapidly absorbed by the porous bases of the colors that it cannot interfere radically with the drying of the printed colors.
Foam in Wall-Paper Glues. — The same considerations limiting the acid, alkali, or grease in wall-paper glues apply to the presence of foam. When trouble arises from this source, it is well to investigate the colors, rather than the glue, unless the latter, on preliminary test, was found to be foamy of itself, in which case it should never have been selected for the work. The chemist at the wallpaper factory will do well to observe the following, in testing glue, apart from deter-mining viscosity and jelly strength. If the glue reacts strongly alkaline with litmus and by this is meant, if it turns the litmus a very deep blue the glue should at once be tested for foam, and if found to be foamy should be rejected without further examination. If the glue be only slightly or moderately akaline, the foam test may be relegated to its proper place in the general test scheme. Extreme alkalinity points to over-liming, which means that the glue will foam badly in solution and also that it contains mucin and soaps which will not affiliate with the colors or clay. In certain quarters there exists a prejudice in favor of alkaline glues, based upon the assumption that the alkali brightens the colors. The contention has its origin in the fact that preparations like "Tenacetine," which are powerfully alkaline solutions of starches, certainly brighten the printed colors to some extent. Such preparations are advocated for use in conjunction with the glue solution for sizing the colors, it being claimed that they increase the tenacity of the mass so sized. What has been said in previous pages in regard to glue substitutes applies equally to these preparations.
Viscosity of Wall-Paper Glues. — In the selection of wall-paper glues, certain mechanical considerations claim attention. Since the sized colors are applied from a roller, or, in the case of hand-printed papers, from a block bearing the design, the flow of the glue solution is a factor of considerable importance. The lower the viscosity of its solution, the better is a glue adapted to sizing work. Wall-paper manufacturers usually employ two grades of glue, one comparatively cheap and used with the clay, etc., and the other of better quality, as it is used for sizing the chief colors. The latter is usually from three to four grades stronger than the former, which is, as a rule, a bone glue, but in some instances a hide, like the better quality. The strength of these is of less importance than their facility of flow and surface-covering qualities. True, the stronger these are, independent of abnormal viscosity, the more economical in use, for the same reasons applying to cabinet glues.
In connection with the sizing of pulp-colors, we have again to compare the relative adaptability of foreign and domestic glues to a particular requirement. Foreign glues work exceptionally well in wallpaper work, as the viscosity of their solutions is such that they flow with the utmost readiness. Their serviceability is enhanced by the fact that, owing to clarification, nearly all those substances tending to affect or to be affected by the colors have been removed. Despite this, comparatively little straight foreign glue is used by American wall-paper manufacturers. Some avail themselves of acid-treated bone glues which answer equally as well as the imported. Save such as employ a chemist to examine the raw material purchased, the majority have not the slightest idea what kind of glue they are buying, and the purchasing department bases its verdict upon the opinions of the color mixers. It is obvious that colors will not show to equal advantage, if printed with a cloudy glue, to those printed with clear. Hence it is to the interest of the manufacturer to use clarified glues. These, however, must be carefully selected and examined, for the reasons enumerated above.
In the general testing of wall-paper glues, those with abnormally high viscosities must be rejected, as this peculiarity is due to the presence of substances that will react to disadvantage with the colors. Preferably those glues which, in solution, display viscosities disproportionately low to the jelly strength should be selected. These will give the best results with colors.
The bulk of glue consumed in the wall-paper factory is in the ground form. In no other class of work is thorough and rapid solution of the glue so to be desired. Apart from facility in use, ground glues are of advantage in that two or more grades may be so mixed as to produce a glue fully answering all requirements as to flow and strength.
Attention is directed to the futility of "testing" glues by determining the specific gravity of solutions of equal concentration, the measurement being effected by means of a hydrometer. This practice is altogether too prevalent in our wallpaper factories. The same factors that unduly increase the viscosity, such as soaps, mucin, alum, etc., will increase the specific gravity, and hence this, per se, is as little indication of glue strength as is viscosity. Intelligently applied, the specific gravity test may be substituted for the determination of viscosity. Its intelligent application, however, presupposes a thorough acquaintance with the viscosity test; and, as the latter is but little more complicated, its use is to be recommended.
To recapitulate, good wall-paper glue should be of adequate tensile strength, 1 5/8 for the grounds, and at least 1 3/8 for the colors. The glue to be used with the clay and in the coarser work should preferably be an acid-treated one; provided this is not too acid in re-action. Some clays are in a measure disintegrated by excesses of acid or alkali in the glue. This may or may not cause foaming of the mixture, but is sure to make it lumpy. The viscosity of the glue destined for the work must be normal or sub-normal. Not more than traces of mucin, soaps, and foreign salts must be present. Glues that are slightly colored work to advantage with the grounds. They do not work so well with delicate colors where clearness is desired.
Glues for Surface-Coated Papers. — Much, if not all, of the foregoing is applicable to the selection of glues for surface-coated or glazed papers. It does not follow, however, that any glue adapted to wall-paper work will answer this particular requirement. Exceptions, due to a difference in the result to be obtained, must be considered. Thus, the glue employed in this work must, as a rule, be clearer than that used for wall paper, for not only is a high gloss in connection with the color required, but more glue is used in sizing the colors. Hence any cloudiness of the glue is apt to result in dulness of the coated paper. Again, the glue must be fairly strong, corresponding to the better grade employed by the wall-paper manufacturer. It suffices the latter if the sized color, when dry, yields but a little when rubbed with a sheet of clean paper. With glazed papers, however, the color must be bound with sufficient firmness so as not to rub off at all. As a rule, a better quality of pulp-colors are employed in the manufacture of glazed papers than are in the printing of wall papers. Despite this, the same precautions governing the selection of glue, as to the possibility of its affecting the colors, are to be observed.
Since great clearness of the colored surface is a requisite, foreign glues are extensively used in the preparation of glazed papers. Domestic acid-treated bone glues answer well and are frequently used. A water-proof coating is made by sizing the colors with a solution of shellac in wood-alcohol, crude acetone, or other solvent. Such surface, while water-proof, has not the clearness of the glue-sized surface. Some prepare a sizing medium by dissolving shellac in a strong, aqueous solution of borax. So long as the alkali is present, this cannot be water-proof and has not the clearness of good glue. A far better result is achieved by adding a proportion of potassium or ammonium bichromate to the glue solution, and exposing the coated papers to the influence of sunlight for a time, which renders the coating practically water-proof. This practice is at-tended with some risk, however, as every glue will not admit of treatment with a bichromate without suffering damage. Again, the colors themselves may not be permanent and the subsequent exposure to sunlight may cause them to fade. The cheaper varieties of surface-coated papers are prepared by coating them with a mixture of unprecipitated aniline color, sized with casein. The use of casein in this connection is subject to the same limitations restricting it as a sizing medium for wall-paper colors.
Glues for Paper Boxes. — While many glue substitutes will answer for paper-box making, a cheap grade of glue is usually employed for the heavier work, and substitutes, such as Boston gum and Liquid Glue, for the lighter. The average paper-box maker seldom pays more than six cents per pound for glue, and at this price only a weak product may be purchased. This usually tests No. 2 and rarely exceeds 1 3/4 in strength, averaging 1 7/8. Ground glues are always used, and these, as a rule, very dirty. The standard grade of paper-box glue is the Swift Box. This is a medium brown, low-grade hide, averaging, in test, 1 7/8. Very similar to this is the Nelson Morris No. 92. It must not be inferred that these are the only glues fit for paper-box making. They are merely cited as types.
Almost any glue, weak or strong, that is properly made and sweet in odor will answer for this class of work. Apart from the considerations that an evil-smelling glue is to be avoided and that the boards used for colored boxes are, as a rule, so cheap and poorly colored that alkali or acid in excess is likely to spot them by discharging the color, no particular examination of the glue is required, save to determine whether or not the glue has adequate binding strength. Fully 99 per cent of the trouble experienced by paper-box makers is due to their own carelessness. Owing to a deplorable ignorance of the elemental properties of glue, it is constantly exposed to influences and conditions that cannot fail to impair its efficiency. The melting-pot is exposed to the dust of the work-room and it is a common spectacle to see. strips of board, sticks, and other refuse floating in the glue solution. Frequently muddy water is used to dissolve the glue. Intelligent application is essential to the successful working of glue for any purpose; and the fact that the paper-box manufacturer need not exercise the care in the selection of his glue that is demanded of the cabinet-maker, for example, does not warrant the lax methods of treating the glue in vogue at the majority of paper-box factories.
Glues for Sizing Work. — Glue, as a general size, is employed largely by the house painter, either for the treatment of walls before painting or calsomining, or as the binding medium for the calsomine itself. Fairly cheap opaque glues are in great demand for this class of work, custom having decreed their use. These glues, known among the painting fraternity as "shell" glues, since, by a stretch of imagination, they may be said to resemble certain varieties of laminaria, are frequently made from pig stock, colored by the addition of very cheap material. Several grades of these are produced, the better testing as high as 11, while the poorer average 1 7/8 to 1 3/4. That a pale-drying, uncolored glue answers the purpose as well, if not better, is indisputable, the inferior coloring material of these opaque glues often working to disadvantage. Of late years, prepared sizes have come into the market, consisting of mixtures of chalk and glue, clay and glue, whiting and glue, or chalk, clay, whiting, and glue, sometimes with the addition of alum and soap. To prepare these for use, it is but necessary to add boiling water, in the required proportion. The painter frequently desires to prepare these sizes for himself; and in this connection it must be pointed out that the selection of the glue for the size is largely dependent upon the nature of the materials to be sized. To work properly in so strange a combination as the above, it is essential that the glue be comparatively pure, that is, containing no residual salts that are apt to react with the other ingredients of the formula. The selection of glues for these combinations presupposes their careful analysis. Since the flow of the glue is of importance, those with low viscosity in solution are to be given preference. At the same time, the glue must be fairly strong, in order that a minimum may suffice to bind the basic material of the size.
Glue is frequently used as size in the manufacture of cotton batting, and it is customary to prepare a sizing solution such that, when applied to the cotton as it passes into the drying machine, it is absolutely dry as the roll emerges from the machine. Comparatively few glues fulfil this requirement, despite the fact that the temperature of the drying room averages 200° F. The absorbent nature of the material sized is a source of unending trouble, too thin a solution of the glue being totally absorbed, with the result that the material emerges from the drying machine stiff as board. On the other hand, too heavy a solution of the size lies all at the surface, and does not dry with sufficient rapidity, despite the high temperature of the apparatus. To obviate this difficulty, it is customary to treat a very dilute solution of a fairly strong, clear glue with sufficient alum to thicken it. Herein lies a source of danger. Many glues are precipitated with alum, particularly if the mixture of glue and alum remain warm in the kettle for some hours. This precipitate cannot again be brought into solution without the aid of acetic acid, and, while the supernatant fluid has weak sizing properties, it has absolutely no binding strength. Even though the glue is used up before the precipitate has a chance to form in the kettle, the extreme temperature of operation is apt to cause its separation before it dries, and the work becomes "streaky," clear in spots and cloudy or dense in others. It is essential, therefore, if it is intended to thicken the glue with alum, that a preliminary test of the glue be made in the following way: One pound of the glue is dissolved in fifteen of water. To the hot solution is added about 10 per cent of alum, and the mixture is kept hot (without actual boiling) in the kettle over night. If any precipitate forms under these conditions, the glue is not fit to be used as a size in conjunction with alum. If none forms, the glue will work excellently under these conditions.
This applies equally to the selection of glues for use in conjunction with alum and clay, alum and chalk, etc., to which soap is to be added to promote the slip of the size in. application. The glue must be carefully tested in the presence of all the ingredients with which it is to be combined, individually as well as collectively, and any adverse result noted. The necessity of great care in the selection of glues for the manufacture of prepared calsomines, particularly the colored varieties, cannot be over-emphasized.
Glue in Textile Manufacture. — In the manufacture of silk, wool, and other fabrics, glue plays an important part, either in conjunction with the dyeing processes or as a finish. It finds considerable application to the manufacture of carpets, as well as special wall-coverings such as burlaps and tapestries. An exception to the rule that glue strength is of the first importance is to be noted in this connection. Since, in all these cases, the glue is used as a size in very dilute solution; and since it is brought in contact with sensitive colors, purity is by far a more important consideration than glue strength. Analysis of the glue destined for use in textile manufacture is essential in order that it may be established beyond peradventure of doubt that no deleterious substance is present in the glue. For example, the presence of mineral acids and normal sulphites in a glue renders its use hazardous in the manufacture of woolens, since any notable proportion of these impurities will reproduce light patches on dyed wool. Black woolens owe their color to logwood (haematoxylin plus haematin) mordanted by the oxidizing influence of a chromium salt. The logwood itself contains consider-able glucose, since this is employed to impart the necessary gravity to logwood extracts. It is difficult to combine glucose and solutions of glue. True, the glue solution is applied after the logwood, with its constituent glucose, has dried ; but it is not to be denied that the water of the glue-size has a solvent action on the glucose of the fiber, the glucose not being affected by the chromium salt. Hence, while the reaction between the finish and the glucose of the fiber may be so subtle as not to be at once manifest, it will nevertheless proceed, and at a later day, long after the shipment of the goods, they may be ruined by spotting, etc.
Silk finishing involves the use of more or less glue, which is used in conjunction with dextrines and starches. That a preference should be expressed for very pale glues is but natural, in view of the fact that these dry with a very transparent coating, especially if applied dilute. At the same time, the use of very pale glues for finishing fabrics of any kind is attended by constant risk. The glue owes its very paleness to the fact that the liquors have been bleached with sulphurous acid, traces, and frequently more than traces of which remain in the glue, as well as salts of sulphurous acid. If these are a menace in the finishing of woolen fabrics, their action is all the more inimical to colored silks, in the dyeing of which far more delicate colors are used. A given shade on silk is produced by the combination of the minimum quantities of several different colors; and a glue used for finishing the silk, if it contain any of the above impurities, is apt to discharge one or more of these, producing spots of the fastest color. Those familiar with the methods of the color-chemist in "discharge work " will have no difficulty in comprehending this phenomenon. A pale blue silk, for example, may be produced by dyeing with a fast red in combination with a blue and a violet. Sulphurous acid will discharge the blue and the violet, revealing the red and causing the silk to be covered, apparently, with red spots.
In many instances, silks, particularly black silks, are treated in the dye-house with glue in conjunction with the finishing emulsion of oil, acid, and soda. As a rule, a good quality of gelatine is used, the object being to correct any hairiness or fuzz of the silk produced by extreme temperatures in the dyeing. Dark imported glues answer as well as gelatine for the purpose and are cheaper than gelatine. It is essential that the glue used in this way, or for finishing purposes, be a clarified one. This assured, the color is of little importance, and it is always safer to use a rather dark glue than a light one, for the reasons above mentioned. A dilute solution of dark clarified glue dries to a remarkably light, transparent coating, and may be used for all save pure white silks. In addition to the regular test preliminary to purchase, the dyer should treat dilute solutions of the glue with all the colors he employs, permitting the mixtures to stand for a day or so that any untoward effects may be noted. The finisher, not having definite data as to how each shade of silk has been produced, should subject the glue or gelatine to rigorous analysis and reject those exhibiting any sulphurous acid and more than traces of other mineral acids and salts.
Glue for Matches. The manufacture of matches is attended by considerable difficulty owing to the nature of the ingredients used. As to the identity of the individual ingredients, no secrecy exists; the art lies in their proper combination. The general formula for parlor matches is, glue, phosphorus, chlorate of potash, whiting, plaster of paris, and rosin. The glue serves to work these into a hot paste into which the ends of the match-sticks are dipped.
It is readily seen that in so strange a combination of materials, every glue will not work. It is stated upon authority that, unless the customary proportions of the above ingredients are radically modified, none but imported glues will answer. The Irish Block C. and Block C. L., and the Walsall of English manufacture, are the standards of match glues. Domestic glues have been used with success, but are troublesome in operation owing to the necessity of radically modifying the formula.
Glues intended for use in match making should be subjected to careful analysis as well as tested for strength. The function of the glue is not merely as a binding medium for the mineral ingredients. It is largely depended upon to prevent atmospheric oxidation of the phosphorus. Accordingly, it must be certified that the glue contains nothing likely to' affect the phosphorus, in addition to exhibiting a fairly water-proof coating when dry.
Comparison of Straight and Mixed Ground Glues. — Although ground glues are invariably regarded with suspicion, much of the popular prejudice against their use is unwarranted. To suggest to some consumers that they are using a mixed glue is to seriously affront them. They at once conceive the idea that the glue is worthless, and that an attempt has been made to defraud them. While it is true that the dealer frequently mixes ground glues for the purpose of cheapening them, his motives in so doing are not always ulterior. Not only are ground glues most convenient in use, as they require but the minimum soaking or softening prior to melting, but they offer many advantages over flake, in that they may be made to conform more nearly to a given requirement. The dealer who is conscientious in the endeavor to sell glue on "test," that is, to match the samples he submits with those submitted to him by the prospective purchaser, has often to mix one or more glues in order to obtain a product conforming in strength and viscosity or other factors with those of the samples submitted to him. It is at times possible to mix two or more grades of flake, with the same object in view. Hence the fact that the dealer submits a sample of mixed glue must not be always construed as an attempt at deception.
Recapitulatory Remarks. The uses of glue herein enumerated represent but a small proportion of its possible applications. They are cited as typical of the limits of application, by which is meant the least and the greatest that is expected of glue in actual work; and are chosen with a view of impressing upon the glue-user the fact that it is adverse to his interests to regard glue as so much cheese, to be bought, consumed, and the stock replenished. It lies within the power of every consumer, small or large, to control the quality of a material for which he expends considerable in the course of a year. He has but to apply the rules of common sense to its selection and application in order to obviate 90 per cent of his. "troubles." Price has often to be relegated to second place in selecting glues, although it must always be considered. For many purposes, an impure glue is dear at any price. In other instances, to spend a few cents more per pound for glue is to effect an ultimate saving, paradoxical as this may seem. He who wittingly fails to test glues prior to purchasing them and to insure that delivery is as per sample, on the ground that his time is too valuable to be devoted to such purpose, is assuredly "penny wise and pound foolish." He is a veritable mark for the unscrupulous salesman, who with glib argument, backed by a minimum of scientific fact, has no difficulty in persuading him that a glue that smells to Heaven has absolutely no odor.