Glues And Gelatine - Introductory
( Originally Published 1906 )
1. Nature and Properties of Glue. 2. Sources of Glue. 3. Principles of Manufacture. 4. Commercial Forms.
1. Nature and Properties of Glue. — Through laxity of usage, the term "glue," from "gluere to draw together" has been extended to include all solutions of substances exhibiting binding or adhesion. Its legitimate application comprehends only those viscous adhesive bodies resulting from the treatment of animal bones or tissues, as well as the bladders and certain membranes of fish. These products possess the property of absorbing cold water in quantity, which softens them and causes them to swell, when, by the aid of heat, they are rendered soluble in the absorbed water, from which solution they gelatinize or form a jelly, after standing and cooling for a short time.
Of itself, this property of gelatination is not sufficient to distinguish true glues, animal or fish, from the mass of vegetable agglutinants, the solutions of which are often erroneously classified as glue. All starches gelatinize upon treatment with boiling water, the soluble varieties first forming a clear solution from which they subsequently jell, much in the same way as glues. Solutions of Irish moss, agar-agar, "Japanese gelatine" and gelose gelatinize upon cooling, and gum tragacanth will absorb large quantities of water in the cold, forming a gelatinous paste similar to the jelly of a weak glue.
It is in the behavior of the jellies, as well as in the deportment of true glue towards certain chemical reagents, that we find a means of distinguishing the true from the false. Thus, if the glue jelly be dried, we procure practically the original hard glue, which can again be soaked up, melted or dissolved, and from this solution it will gelatinize a second time. This operation may be repeated a number of times. Each treatment, how-ever, so impairs the power of gelatination, that a point is reached where the glue goes into solution but refuses to jell. On the other hand, the products which result from drying out the solutions or jellies of starches and vegetable gums, are dissimilar to the originals; and, while they permit of softening by soaking in cold water, no amount of heating will bring them into permanent solution a second time.
The chief constituent of all glue, that which, in the main, imparts to it the property of gelatination, is GELATIN, or, as it is called by some, GELATIN. The latter term is somewhat objectionable, as it is synonymous with GLIADIN, the essential principle of wheat and other starches. While it is true that the analysis of purest GELATIN corresponds markedly with that of GLIADIN or GLUTIN, this does not justify the assumption that the nitrogen, carbon, hydrogen, oxygen and sulphur, which both contain in corresponding amount, are combined in exactly the same way in each.
According as to the nature of the materials selected for the .preparation of the glue, there are associated with GELATIN such substances as CHONDRIN, which is derived from chondrinogen, the characteristic cartilage of young bones and tissues; and KERATIN, from the horns of animals. These substances are more viscous and adhesive than GELATIN, while displaying less gelatinizing power. MUCIN, a slimy substance present in the intercellular structure of cartilages, is frequently found in glue. It has some adhesive, but little or no gelatinizing properties, and its presence must be regarded as the evidence of faulty manufacture.
As the ratio of the GELATIN to the other substances present increases, so does a glue advance in the scale of purity; and those hard, glassy products, ranging in shade from dark through pale yellow to colorless, are the gelatines of commerce. They are refined glues and are prepared from stock known to be rich in those elements yielding chiefly GELATIN and but little else, i.e., the epidermis and carefully selected bones.. These undergo special processes with a view to eliminating the other constituents of glue.
It is difficult to define the exact point of transition from glue to gelatine. The two classes of substance possess in common the properties of water absorption and subsequent gelatination. The strength of the jelly is not a determining factor, many glues being far superior in this respect to gelatines. Commercially, the distinction is based largely upon the appearance of the product. Gelatines are usually transparent or at least markedly translucent. Their solutions and jellies are very clear. The viscosity of gelatine solutions — that is, their rate of efflux from a vessel of known capacity as compared with that of the same volume of water, is considerably lower than the viscosity of a glue of corresponding strength. This may be ascribed to the fact that the CHONDRIN associated with GELATIN, in glue, imparts to it the added degree of viscosity. The very processes which serve to eliminate the binding and viscous elements of glue stock, thereby producing gelatine, debar the latter from use as glue; and, conversely, the presence of these elements in glue classify it as such. An intermediate product, known as gelatine-glue, displays some of the properties of each and often serves as a gelatine, and, again; finds frequent application where glue is required.
In their deportment towards certain chemical re-agents, both GELATIN and CHONDRIN display marked characteristics. These reactions, an account of which will be found under the chapter on Analysis, appeal chiefly to the chemist, in that they furnish some clue to the constitution of these substances. They are of little interest to the general consumer, as they do not enter into the commercial testing of either glue or gelatine. Tests based upon them are found to be inaccurate and, save for alkalinity or acidity, the chemical properties of glue are not factors in rational systems of test.
2. Sources of Glue. — The hides, horns and bones of cattle ; the skins of the sheep, goat and pig, as well as the bones of some of these; the "sounds" or air-bladders and other membranes of fish, constitute the chief stock from which glue is derived. Rabbit skins yield a very strong glue, of unpleasant odor, which was formerly marketed in the form of a jelly, or size. Parchment yields considerable glue of high quality. Excepting, perhaps, clippings from old gloves, leather is not avail-able as glue stock, as it is difficult to separate the glue from the tanning materials in combination with it.
Of the above-mentioned hides and skins, only those portions rejected by the tanner as unfit are employed as glue stock. The better portions are converted into the respective leathers, and only the splits and pieces, the ears and head portions, and the muscular tissue contiguous to the skin, known as "fleshings," come to the glue-maker. The epidermis and carefully selected bones comprise stock for gelatine which, in addition, is frequently derived from old buttons and similar horn products.
3. Principles of Manufacture. — In the selection of the stock for glue making, the manufacturer is called upon to exercise considerable ingenuity; and, while the subsequent stages of preparation are no less important, the greatest care in cooking or treating will not over-come the more radical defects of the stock. Glue is not merely the solution of the materials present in the stock, these being greatly modified by the action of boiling water. The exact nature of the change is but little understood, but is assumed to be that of hydrolysis — i.e., the addition, chemically, of a certain number of molecules of water to the glue-yielding elements.
Almost any stock, upon treatment with boiling water, will yield some glue, but very little. To facilitate the extraction and, more particularly, to insure the maxi-mum yield, the stock undergoes treatment preliminary to boiling, varying in character and duration with the nature of the materials selected.
In the manufacture of glue from hides or skins, the first step is LIMING, or treating the stock with weak solutions of lime. Its object is, first, to dehair the hide; secondly, to cause it to swell or "plump," bringing the pores to a condition which insures not only the rapid extraction of the glue, but also the extraction of the maximum amount; thirdly, liming is intended to rid the hide of substances which, if present in the finished glue, impair its value.
The operation is conducted in shallow, rectangular vats of wood. The stock is immersed in the lime solution and stirred from time to time, at which intervals it is examined and the conditions of the pores noted. Some varieties of hide stock require longer liming than others. By far the greater quantity of hide stock has been limed to some extent before reaching the glue-maker. The tanner subjects the skins to the action of lime, because, like the glue-maker, he desires to rid them of all deleterious material anti to "plump" them, bringing the pores to a condition where they will properly respond to the action of the various tannins. The tanner's liming process completed, he then rejects those portions unfit for leather, but rich in glue-yielding elements, and hence these come to the glue-maker partially limed. Where tannery and glue factory are in close proximity to each other, the stock is delivered in the wet state. In other cases, it is dried and then shipped.
The glue-maker is under necessity of subjecting the stock to further treatment with lime in order to prepare it properly. Heavy hide pieces, notably those from steers, which are rich in glue-yielding material, frequently undergo six weeks' liming, while sheep stock receives little or no further treatment; this, of course, assuming that it has been delivered from the tannery, as otherwise it receives considerable liming. Goat stock is subjected to the process for a period ranging from twenty-four hours to ten days. - In this there is no hard and fast rule, the individual preference of the glue-maker being a factor of considerable importance.
The liming of glue-stock is an operation of great delicacy. In the endeavor to rid the stock of non-gluey material, and to bring the pores to the proper condition, the operation may easily be carried too far and much glue-yielding substance be lost through solution in the alkaline waters. Over-liming creates defects in the stock, which cannot be overcome in the subsequent stages of manufacture, no matter how care-fully conducted. The result is that the finished pro-duct is inferior and unfit for the majority of usages.
Liming finished, it is the practice of some manufacturers to kill excess of lime by means of sulphuric acid, converting it into insoluble calcium sulphate. Other, again, wash out as much of the lime as possible, permitting the balance to remain. As a result, the finished glue is more or less alkaline in reaction. While moderate alkalinity is of no material disadvantage, interfering in no way with tensile strength, it is prefer-able to have the glue slightly acid, inasmuch as the bacteria which effect the decomposition of a glue jelly, thrive in an alkaline medium, whereas in an acid one, they have a much harder struggle for existence. Hence the solution and jelly of an acid glue keep better and longer, independent of artificial preservatives, than those of an alkaline glue.
In the course of liming, some of the grease inherent in the stock is converted into insoluble lime soap, the subsequent treatment with acid destroying this; and, while the mineral salts wash out, the separated fatty acids remain in the stock. They have a disagreeable odor and must be removed, else the finished product is too greasy. Their removal is usually effected in the course of boiling, when they rise to the surface of the liquor and are skimmed off.
After liming, the stock is washed. This must be done carefully, lest some glue-yielding material be lost. Washing frees the stock from excess of lime, where treatment with acid has not been resorted to for the purpose. It further serves to remove from the stock substances which have only been softened and not dissolved by the lime. In washing, the stock is subjected to the action of a stream of water, while a roller passes over it, squeezing it out thoroughly. The water employed for washing, as well as for boiling, is the subject of special consideration. It must be free from all salts or chemicals which may interfere with the production of a good glue, or tend to precipitate the glue from solution.
Where bones constitute the glue-stock, the simplest treatment consists in comminuting them in a suitable mill and then steaming the mass to rid it of the bulk of grease. In many instances the grease is extracted by means of some hydrocarbon solvent, special machinery, designed for the recovery of both solvent and grease, being employed. Where selected bones comprise the stock, they are treated with weak alkali to cleanse them and are then leached with acid for a long period. The object of this treatment is manifest, and to understand its operation it is but necessary to recall the experiment of childhood, in which a chicken bone was softened by means of dilute acid, tied into a knot, and once more hardened by immersion in lime-water. Bones consist of the phosphates of calcium and magnesia, supported by a framework of cartilage. The acid dissolves out the mineral salts, leaving the skeleton of cartilage, which is the glue-yielding substance. This, after careful washing, is subjected to boiling. As with hide stock, this washing must be thorough. If the bones have not been previously degreased, some of the fat is converted into acid soaps. The crude treatment of common or "junk" bones, as outlined, results in the ordinary bone glues of commerce; while the product of leached stock is known as "acid-treated" bone glue. Where the stock has been carefully selected and manipulated, bone gelatine is produced.
It is the practice of many manufacturers, prior to boiling, to blend different hide stocks after they have been limed and washed. Herein lies one of the great secrets of making good glue. Sheep stock and goat, of themselves, yield exceptionally strong glues, though rather greasy. Combined with fine ox fleshings, hide pieces etc., the resultant glue is vastly improved in strength and in quality, all other manufacturing factors being equal. As a result, the bulk of hide glue in the market is not "straight" — that is to say, it is not made from any one exclusive hide stock, but from mixtures of two or more. Straight sheep glue is quite common, but straight goat is rarely, if ever, met with.
Many of the cheaper grades of glue consist of mixtures of hide and bone. In preparing these, the respective stocks are not mixed beforehand, but are extracted separately, the resultant liquors combined in different proportions, and the process carried to completion. Similarly, liquors extracted from different hide stocks may be combined before jellying, although it is by far commoner practice to mix the initial dry stocks, as previously pointed out.
In the preliminary treatment, all of the grease in the stock is not converted into soap by lime or acid. Indeed, only a small portion is so modified, and the bulk must be removed by other means. This is usually done in the course of boiling, when the grease and separated fat rise to the surface of the liquor and are skimmed off as closely as possible. Contrary to popular belief, the glue-maker is at especial pains to recover most of the grease. Apart from the fact that its presence, in excess, is hurtful to the finished product, barring it from certain uses, the manufacturer has often to rely upon its recovery and sale to reduce the cost of the glue. Experienced consumers often remark that their favorite grade is at times greasier than at others. We account for this in the fact that at times the price of grease is so low as not to warrant the additional labor-cost attendant upon its recovery. In times of scarcity, however, the manufacturer, by recovery and sale of the grease, realizes an increased margin of profit.
Boiling or Extracting the Stock. — The prepared and washed stock is now subjected to boiling or extraction; and if the initial treatment is of importance in pro-curing a good product, this is no less so. Here, again, the individuality of the operator asserts itself and, while certain fundamental principles are universally observed, it is safe to assert that no two glue-makers proceed in exactly the same way throughout, each having his little secrets and tricks of the trade.
The stock may either be dried and stored for a time, when it must again be softened prior to extracting, by immersion over night in pure cold water, or else it is at once placed in the boiling vats. These vary in construction from simple wooden vats fitted with a steam coil and vent for discharging the liquors, to complex iron kettles equipped with apparatus for temperature control and agitation, and provided with both jacketed and direct steam. The stock to be extracted is covered with water and the cooking begun. At this juncture, two alternative methods present themselves. The operator may thoroughly extract the stock in one operation and complete the glue from the resulting liquor, or, as is often done, he may produce his glue liquor in "runs that is, he begins the cooking using very little water and at the minimum temperature necessary for the extraction of glue; and, maintaining this for a short time, he withdraws the liquor which constitutes the first "run." More water is now added, and the cooking continued at an increased temperature and for a longer time, and so the second run is produced. According to the nature of the stock and the preference of the operator, as many as five runs may be obtained, the last at the maximum temperature, practically that of boiling water, in order to insure complete extraction. Prolonged heating is injurious to solutions of glue, gradually destroying the gelatinizing power, and, for this reason, where glue is produced in runs, the first is the strongest as it contains the bulk of choice material, extracted at a low temperature. Each succeeding run is weaker, the last having but little strength, owing to the conditions under which it is produced.
In the course of boiling or extraction, the preservation of the liquor and thus, ultimately, of the finished glue, receives attention. For antiseptic and preservative purposes a number of substances may be employed with varying degrees of success. Chief among these is carbolic acid, though this, as a rule, is barred because of its characteristic. and permeating odor. Formaldehyde is a splendid preservative, but its use, also, is attended by disagreeable features. It tends to precipitate a constituent element of the glue and, while its operation may not be manifest throughout the processes of jellying and finishing, at a later day the consumer, in melting the finished glue preserved by its use, is confronted with the rather startling spectacle of the glue solution separating. Formaldehyde is extensively employed as a temporary preservative of very dilute bone liquors which are to be concentrated by evaporation, during which most of the formaldehyde passes off as vapor, the final preservative being then added to the concentrated liquors. Salicylic and hydrofluoric acids are most useful as glue preservatives, the latter, although difficult to handle because of its corrosive properties, being a powerful germicide. Sulphate of zinc has some preservative properties and is often used in conjunction with other substances.
While still in the form of liquor, the glue may be subjected to the action of bleaching agents, where a very pale finished product is required. The bleaching is usually effected by means of sulphurous acid which is conducted in the gaseous state through the hot glue liquor. From time to time a sample is withdrawn, and the process is terminated when the desired shade has been achieved. The color may be improved by the addition of such substances as zinc white (oxide of zinc), chalk or finely divided barytes or talc. One or more of these is added to produce a perfectly white or "opaque" glue. Again, only a limited amount of these substances may be added, when the resultant glue is described as "colored."
As has been stated, the temperature at which the glue is extracted requires careful regulation. Bone and other liquors are often produced so dilute that they will not jell sufficiently to permit of proper cutting, and must be concentrated by evaporation. Under normal conditions the degree of heat required to effect proper concentration would gradually destroy the gelatinizing power of the glue. To overcome this, the operation is carried out in a vacuum evaporator.
Jelling the Glue Liquor. The glue liquor, from one run or several, is conducted from the vat into oblong pans or molds with slightly sloping sides and about one foot deep. These are then placed in the cooling room, where the liquor gradually sets to tenacious jelly. These jellies, in the form of oblong blocks, are next conveyed to the cutting' room where they are sliced. The simplest device for cutting consists of two uprights permanently fastened upon a long board or runway, to the sides of which are attached boards equal in height to that of the jelly block to be cut. These prevent the jelly from slipping or swerving. Between the uprights stretch fine wires, the space between which may be increased or diminished at will. The space between the two upper and two lower wires is smaller than that between the others, in order that the top and bottom slices shall be thinner than the rest. The block of jelly is placed in the runway and is firmly pressed against the wires, which traverse it longitudinally, cutting it into sections which average a half inch in thickness. In many plants, more elaborate cutting apparatus, operated by power, is employed; but these complex types are but amplifications of the principles embodied in the simplest.
Drying the Cuttings. The sections or cuttings are now placed upon nets, made either of galvanized iron wire or fish twine, to dry, in a room set apart for this purpose. This room is heated artificially and fans convey the heated air over the nets. In many instances, the drying is effected by simply exposing the cuttings to natural air currents. In the operation of drying it is possible to undo, through carelessness, much, if not all, of the good accomplished in the previous stages of manufacture. Too much heat, in drying, is as fatal to a good product as in boiling and cooking. On the other hand, if dried too slowly, the cuttings are apt to rot. Some manufacturers rid the cuttings of the bulk of moisture by subjecting them first to artificial drying, and then finish them by exposure to the air. This is thought to "season" the glue. Natural drying in very cold weather is productive of frozen glue, much of which comes into the market at intervals.
The grease which fails to rise to the surface of the liquor in the course of boiling, and so fails of removal by skimming, presents itself in the glue jelly and is to be found chiefly at the top and bottom of the jelly block. For this reason the top and bottom cuttings are not, as a rule, spread upon the nets to dry, but are set aside and remelted with the next boiling of glue. There are times, however, when they are dried with the rest, and come into the market under the name of Greasy Tops and Bottoms. In the majority of instances where the manufacturer goes to the trouble of drying these greasy sections, he grinds them to be mixed with other ground glues.
4. Commercial Forms. As taken from the nets when dry, glue is in the form of oblong sheets of varying thickness. The majority of foreign glues come to this market in this form, or as square cakes, the latter very thick. Few, if any, domestic glues are marketed in cake form. Gelatines are for the most part sold in the sheet. Prior to packing in barrels, the sheets of glue are broken into small, irregular pieces known as flakes, and hence this form acquires the designation of FLAKE GLUE. These may be GROUND, or even powderd. Certain glue jellies are cut into long, prismatic strips, which, when dry, are slightly twisted and bear a faint resemblance to noodles, whence the name, NOODLE or STRIP GLUE. Glue jellies are at times so cut as to dry to a strip one inch wide by six long, and from one quarter to one-half inch thick. This form is known as RIBBON GLUE.
Gelatines are met with in the FLAKE, SHEET, STRIP, GROUND, and POWDERED forms. SHRED GELATINE is in the form of fine sections similar to shredded cocoanut.
The bulk of gelatine consumed here is of foreign manufacture, and is in the sheet form.
The form of a glue bears no relation to its tensile strength or working qualities. It is merely a question of individual preference based, perhaps, upon what the consumer has been accustomed to use, that impels one to employ nothing but flake, where another will use only ground or strip. Facility in use is dependent upon the form of the glue. A thin cut flake soaks up faster than a thick cut, a ground glue faster than a thin cut flake, and powdered glue fastest of all. The chief objection to foreign glues is that the majority of cheaper grades are so thick cut as to require long soaking before they can be melted.
Flake glues are cut in varying thicknesses. The stronger varieties are cut very thin, and where this has been done unskilfully the glue is curled up. To prevent this, the glue liquor is made as dilute as is consistent with proper jellying and rather thick sections cut from the jelly. These consist chiefly of water which passes away in the drying, leaving a thin, dry glue; and as the drying is conducted somewhat slowly, the curling of the edges does not occur. It is readily seen that this method is applicable only to a very strong glue liquor, as otherwise the necessary dilution would interfere with the formation of a jelly sufficiently firm to cut.
Two other forms of glue are produced, Thimble or Teat, which has its origin in the fact that weak glue jellies frequently drip through the interstices of the net, particularly if the drying room become unduly warm. Another, SERRATED GLUE, is produced by foreing the glue jelly through an apparatus similar to a colander, with the result that the dried particles are no longer than a quarter of an inch, and twisted. This latter form is very rare, and Thimble glue is usually promptly ground.
In the trade, the term "opaque" is used with reference to the presence of coloring matters in the glue, rather than as the antonym of "transparent." If the hard glue be white or almost so, it is called "opaque"; and if only whitish, "colored." When materials are added to the glue to produce color, an inspection of the dry piece may fail to reveal their presence; yet the solution will be more or less whitish. The cheaper white glues, much prized by the house painter, are made from pig stock, to which an inferior quality of coloring material is added. Of this class the Northwestern No. 1, No. 2, and No. 3 are types. High grade glues are rendered perfectly white or opaque by means of zinc white. This adds to the cost of the product. Not all liquors can be converted into opaque dry glues; only such as have an initial light color receive the treatment and are, in addition, bleached prior to the admixture of coloring agents, in order that only the smallest quantity of the latter need be used. The price of opaque glues is always somewhat higher than that of clear glues of corresponding strength. It is assumed by some that the presence of coloring matter in glue is an aid in drying. Practice fails to sustain this theory.
Of the high grade hide glues, many are cut very thin and are almost transparent. Others, again, are thicker in section and less flexible. As has been stated,these conditions have no bearing upon the strength of the glues, save within limits hereafter to be defined. As appearance, however, is a material factor in selling, glues are made as nice looking as possible.
Fish Glues. With the exception of isinglass, which is prepared chiefly from the swimming-bladder of the Russian sturgeon, and which is marketed in various solid forms such as sheet, purse, lump, pipe, honeycomb, etc., fish glues are in the form of heavy solutions or pastes. They are all powerful adhesives having the characteristic odor of fish, combined with that of the materials used to preserve them in the liquid or fluid state. To offset this, they are perfumed with essential oils (sassafras and wintergreen) ; but these disguise the natural odors only to limited extent.
Isinglass is a very hard, transparent and practically colorless substance. It is the most powerful adhesive known and is used chiefly by brewers and wine merchants for clarifying purposes. It is to be remarked that pure gelatine, which is chemically the same thing as isinglass, cannot be used for this purpose. Many substitutes for isinglass are offered for sale, but they possess only to a slight degree the excellent properties of the genuine similarly, liquid fish glues are extensively imitated. Weak animal glues treated with mineral acids or other agents to destroy their gelatinizing power, and per-fumed with various essential oils, are sold as fish glues. They do not and cannot supplant the genuine, as they are deficient in adhesiveness and binding properties. In fact, only very weak animal glues will respond to the necessary treatment, and these, in very cold weather,jell and must be remelted with the aid of vinegar before they can be used. They are properly known as liquid glues; and only such as preserve their fluidity at all times are deserving of the title. The most recent addition to this class is a weak animal glue treated with chloride of calcium which tends to prevent jellying. This is the most spurious of all, as this chemical is markedly hygroscopic and interferes with complete drying.
The "liquid glue" of the paper box manufacturer contains no glue whatever, being a solution of certain grades of corn dextrine. Flour paste is often added to increase the body of the material and, incidentally, the margin of profit.
The various mending cements are prepared from glue, gelatine and, at times, casein. The glue and gelatine are treated with a view to prevent jellying and mixed with other substances which render the dried material more or less waterproof.
Flexible glue, for book-binders' use and for making pads or writing tablets, comes in the form of oblong cakes, about ten pounds in weight. It is a mixture of glue, glycerine and water, the glue being dissolved in the regular way and the glycerine then added to impart flexibility. Printers' rollers are made in much the same way. Flexible glue is made of white glue or the ordinary shades. That intended for use as padding composition is colored to suit the consumer, fuchsine (magenta) being chiefly employed. Flexible glue is frequently worked up into small toys, such as the grotesque heads displayed for sale by street fakirs.