CHAPTER IV.

MANUFACTURE OF SKIN GLUE.

The thorough preparation of the raw materials will materially facilitate all succeeding operations, which may be classified as follows:

1. Boiling the glue.

2. Clarifying the glue-liquor.

3. Forming or moulding the glue.

4. Drying the glue.

However, before entering into the description of these operations, it will be necessary to refer to an intermediate product, which has been previously mentioned under the name of crude glue, and is prepared, for instance, by tanners and manufacturers of parchment, but also forms in some localities a special branch of industry.

This crude glue is actually not glue, but a glue-yielding substance in such a state of preparation that it can be directly used for the first operation, namely, boiling. It consists of waste of skins and leather of all kinds, completely cleansed, dried and limed, and in the case of leather treated with agents for the extraction of the substances used in the tanning. As will be readily understood, the operations required for the preparation of this stock are virtually the same as those described in the previous chapter for raw materials and need not further be here referred to.

The bulk of such stock is prepared by tawers and manufacturers of parchment, though a considerable quantity of it is also derived from waste in the manufacture of gloves. The product from the latter source is also found in commerce under the French names _Colle franche_ or _Brochette_. However, if such stock is used, it is best to again immerse it in lime water, after which it should be thoroughly washed.

The manufacture of glue from hide and leather waste differs materially in many respects from that of bone glue, it being the more simple process, as no other preliminary operations than the preparation of the glue-stock are required. The first operation is

1. COOKING OR BOILING.

For this operation any kind of boiler may be used, but the materials should be supported on a perforated grid a little distance above the bottom, so as to save them from risk of scorching. In the centre of the grid stands a conical pipe 2 to 3¼ feet long, perforated like the grid and communicating with the space between the grid and the bottom of the boiler. The height of the boiler can be increased 1 to 1½ feet by placing an annular piece upon the rim which is bent upwards for its reception.

The size of the boiler depends on the quantity of raw material to be worked at one time. It is best to choose boilers holding from 110 to 440 lbs. of glue-stock, and to place two, four or more of such boilers in one hearth.

The manner of using such a boiler is very simple. Straw is placed upon the false bottom in such a manner as to cover its entire surface, and extend up the sides of the boiler at least as far as it is touched by the flame. The object of the straw is to serve as a filter, and protect the materials from injury by the flame. But for the production of entirely pure gelatine or glue, straw cannot be used, as, by boiling, it yields a yellow coloring matter, which passes into the glue. Barley straw gives a less intense coloring matter than rye straw.

In case straw cannot be used, the material is placed in a large bag, previously thoroughly boiled, and suspended in the boiler so as not to touch the sides. By this means scorching is prevented even if the fire touches the bottom as well as the sides of the boiler.

The boiler having been heaped with material so high as to overflow the brim and fill the annular piece placed upon it, is filled with water as far as touched by the fire. The fire may now be started. The hearth in which the boiler is placed should, of course, be so constructed that the gases are uniformly distributed and the water quickly brought to the boiling-point. When the water commences to boil, bubbles of steam ascend from the space beneath the grid and, passing through the perforations of the conical pipe, penetrate the glue-stock. Thus the first formation of glue takes place, and the stock begins to settle down gradually as it goes into solution. The stock heaped up in the annular piece also sinks down gradually, and being partly heated by the hot vapors and thus prepared for solution, is finally submerged in the boiling solution and becomes soon entirely dissolved.

Waste of hide and horn piths are completely dissolved in five to seven hours. No more water should be used than is absolutely required for cooking the entire quantity of stock, because too much water renders the solution too thin and gives a jelly of little consistency and difficult to dry. Concentrating the glue solution by continued boiling is bad practice, as it is detrimental to the resulting product by reason of the glutin undergoing a gradual transformation.

It is best to start with a slow fire to give the stock time to soften and thus prepare it for solution. When somewhat softened, the mass is brought to boiling and the latter kept up, gently and uniformly, until solution is complete. Solution is promoted by careful stirring, but care should be had not to disarrange the straw upon the grid and on the sides of the boiler as this would interfere with proper filtration of the glue solution.

The duration of cooking depends on the nature of the raw materials. Scraps of skin from young animals, antlers, sheep trotters, etc., dissolve in three to four hours, while waste from ox and horse hides, or bones from old animals, require six to eight hours.

The progress of the operation is readily ascertained by pouring a small sample of the gelatinous fluid in half an eggshell, and setting it aside for a few minutes to cool. If a clear and consistent jelly be obtained, boiling has been carried on to a sufficient extent, and the liquid is drawn off. Any undissolved glue-stock remaining upon the straw filter can be boiled by itself, and the resulting gelatinous liquor utilized in the next boiling.

It is evident that quick and uniform solution of the materials, which enhances the quality of the glue, is promoted by comminuting the glue-stock either by grinding, stamping, or mechanical means.

The succeeding clarification of the glue is much facilitated by removing while boiling the scum, consisting of fat, coagulated albumen, lime-soap, accidental admixtures, and other impurities. Before drawing off the gelatinous liquor it is advisable to withdraw the fire and allow the contents of the boiler to rest for fifteen minutes.

The residue remaining upon the straw filter consists of hair, lime-soap, undissolved particles of hide and bones, lime, etc., and is utilized, after repeated boiling, as fertilizer or for the manufacture of gas.

The mode of glue boiling above described is the oldest and at present is only in use in small establishments. Fig. 11 represents a convenient apparatus for the purpose. It consists of three boilers upon as many different levels. The lower boiler, _b_, serves for the settling and clarification of the glue. It communicates with the second boiler, _a_, which contains the material to be acted on, by means of a pipe provided with a stopcock, and is sufficiently heated by a small fire to keep the glue liquid without allowing it to reach ebullition. The upper boiler, _c_, which is heated by the waste heat of the chimney, serves as an economical reservoir for hot water. The end of the discharge-pipe of the settling boiler is provided with a filter of woven wire. As the sides and bottom of the second boiler are lined with straw, which acts as a preliminary filter, the glue runs off quite clear from the settling boiler.

When this mode of manufacture is adopted, two boilings can be made per day, under favorable circumstances, so that, if the boiler has a capacity of 220 lbs. of stock, which will yield from 110 to 132 lbs. of dry glue, the daily fabrication will be about 220 lbs. of finished product.

In larger plants, the above described mode of extracting the glue-stock with water has been superseded by the use of steam in a cylindrical wrought-iron boiler, twice as high as wide, and capable of withstanding a pressure of three atmospheres. The boiler is furnished with a perforated false bottom underneath which terminates a steam pipe. It is filled from above with previously softened glue-stock and the charging hole hermetically closed. Steam is then gradually admitted and exerts at once a dissolving influence upon the stock. A portion of the steam condenses and forms with the dissolved glue-stock a concentrated jelly which collects between the true and false bottoms.

For the escape of air a cock is provided which is closed as soon as steam commences to escape from it.

The advantages of this process are obvious. A larger quantity of glue-stock can be extracted than in the boiler previously described, and there is no danger of injury by scorching and consequent damage to the color of the glue. More highly concentrated solutions are obtained in a shorter time, and the spoiling of the glue solution by too long continued cooking is prevented by drawing off the solution as quickly as formed. The escaping hot vapors may be utilized for drying the glue, softening the raw material, etc., the entire quantity of heat being thus utilized. A further great advantage of this method is that there is less annoyance from badly-smelling vapors than when boiling is done over an open fire. A number of such boilers can be arranged in one room and supplied from a common steam boiler.

Fig. 12 represents a boiler for extracting glue-stock with the use of steam. It is provided with a lid, _D_, which is removed for charging the boiler. The aperture, _E_, in front, serves for the removal of the residue. Above the true bottom there is another false bottom, perforated and movable, which can be covered with straw for preliminary filtration. The steam reaches the glue-stock through a pipe which passes through the actual and false bottoms, and is perforated above the latter. The resulting jelly collects between the true and false bottoms, where it is less exposed to the action of hot steam. The escaping steam passes through the pipe, _F_, which is provided with a stock-cock. The pressure in the boiler is indicated by the manometer, _K_. After throwing the materials into the boiler they can be covered with warm water, or, after the lid is closed, warm water is introduced from a reservoir through a special pipe and distributed over the material through a rose.

The boiler stands upon a frame sufficiently high to allow of conveniently placing a vessel under the pipe _G_, through which the jelly is discharged. The vessel, when full, is conveyed to the settling vat, or the arrangement may be such that the jelly is directly run into the settling vat.

In many large plants open jacketed pans heated by steam are still used for treating the material. Fig. 13 shows an arrangement with two of such pans; of course one, or a larger number may be used, according to requirement. In the illustration the pan _I_ on the left is shown in front view, and the pan _II_ on the right, in section. _K_{1}_ is the actual pan enclosed by the jacket _K_. Steam circulates in the space between pan and jacket, whereby the stock in the pan is heated. _K_{1}_, in addition, is furnished with a steam coil _S_, which may, however, be omitted.

The steam enters through the pipe _D_, the space between pan and jacket, passes into the coil _S_, and escapes at _b_. The water formed by the condensation of steam in the space between pan and jacket, as well as that which runs off at _b_ from the coil _S_, is carried away by the pipe _A_.

The pipe _L_ serves for conveying hot water to the pans, and the pipe _F_ for the discharge of the finished glue liquor. The stirrer _R_, is furnished with two paddles, and is set in motion by a transmission on the ceiling of the room. It serves for keeping the stock in the pans constantly agitated, solution being thus very much promoted.

The mode of working with this apparatus is very simple. Water being admitted into the pan through _L_, the glue-stock is introduced and the mass brought to boiling by admitting steam. The finished glue-liquor is from time to time drawn off through the pipe _F_ into the settling vessel.

It is generally preferred not to concentrate the glue-liquor in the pans to such a degree as required to obtain a jelly, which after cooling, can be immediately moulded, experience having shown that less concentrated liquors can be more readily and better clarified, and yield a lighter and more transparent glue.

Mr. Thomas Lambert gives the following process of cooking: The skins are taken to the glue-boiler, which is an open vessel, 8 feet in diameter at the top and 7 feet deep, and provided with a perforated false bottom, through the center of which passes a two-inch pipe, one end dipping below a layer of water at the bottom, the other projecting about half the height of the boiler, this part being covered with a perforated hood to spray the liquor through the mass. The skins are placed on the false bottom and the added water at the bottom of the boiler is brought to the boil by means of a steam pipe. The steam not being able to escape quickly through the dense mass of glue-stock above, exerts a pressure on the water, forces it through the pipe, to be sprayed through the mass, and ultimately works its way to the bottom of the boiler to be forced up again. This continual circulation of the hot liquor rapidly dissolves the gelatinous matter, and when a strength of 18 per cent. dry glue is reached, the first run is made to the evaporating pan, the liquor passing through a filter of fine shavings, to remove any suspended matter. Fresh water is added to the boiler, and the boiling renewed. Three extractions are usually made, the last being used for size.

In order to avoid annoyance to the neighborhood from foul odors, Terne’s glue-boiler shown in Fig. 14 may be recommended. The lead-lined iron boiler _A_, with manholes _B_ and _C_, on top and side, is furnished with a false perforated bottom upon which the glue-stock is placed. Underneath the false bottom lies the coil _E_ with valve-box _e_. The boiler is filled through the upper manhole with glue-stock and water admitted, steam being at the same time introduced in the coil and in order to quickly heat the water, direct steam is also admitted to the boiler through the pipe _F_ and cock _G_. When the water is boiling the cocks _G_ and _F_ are closed, the coil furnishing now sufficient heat. During boiling some steam is allowed to escape through the partly-opened cock _L_, all badly-smelling gases being thereby carried to the fire-box of a boiler where they are burned. When boiling is finished, the glue liquor remains for a short time in the boiler to allow the melted fat to separate on the surface, the cocks _K_{1}_ and _K_{5}_ and serving for drawing off the fat. The insoluble residues of the glue-stock remain upon the false bottom and are taken out through the manhole _C_.

2. CLARIFYING THE GLUE-LIQUOR.

The clearness of glue, _i. e._, its freedom from undissolved substances, is by no means a criterion of its value as an agglutinant, since pulverulent inorganic substances (white lead) are frequently intentionally introduced into some varieties, for instance into Russian glue, without injury to their adhesive power. But as a turbid appearance may also be an indication of unsoundness and decomposition, the manufacturer endeavors by all means to obtain a clear product.

A strict distinction should be made between clearness and color. Very dark-colored glue may be very clear, and a very pale variety the reverse, yet both possess excellent qualities. Both properties, clearness and light color, cannot be obtained by the same process.

Clearness will be first referred to. If the glue-stock has been properly prepared by rendering adhering particles of blood and fat innoxious by liming and subsequent careful washing, the separation of the few remaining impurities, which may have passed through the straw filter, is readily effected by allowing the liquor to stand, care being had to keep it liquid as long as possible to give the grease time to rise and the flocculent and fibrous impurities to settle. This is best effected in a wooden vat surrounded by a wooden or sheet-iron jacket, the intermediate space between jacket and vat being filled with a non-conductor of heat, or, if required, it may be heated by the introduction of steam. The grease is skimmed off as it rises, and when the solid particles have settled the liquor is drawn off through a pipe placed a short distance above the bottom of the vat.

The size of the clarifying vat depends on the size of the boiler. It is, however, best to have two vats for each boiler, in order to keep the first liquor, which is always clearer and more concentrated, separate from the last run. To be able to draw the upper layers of purer liquor into cooling boxes by themselves, the vats are provided with faucets at different heights.

To prevent putrefaction of the liquor which readily sets in during settling at a higher temperature, the vats should be kept scrupulously clean, and from time to time rinsed with clean, hot water. It is also advisable to line them with sheet-iron.

Should the above-described mechanical separation not prove sufficient, recourse must be had to other means. Alum and sulphate of alumina have long been used for clarifying, 1 lb. of either of them, pulverized, added to every 300 gallons of liquor, being as a rule sufficient. Either of these chemicals removes the albuminous and extractive constituents of the solution, and converts the dissolved free lime into sulphate of lime, which settles readily, and prevents putrefaction of the glue solution while drying under unfavorable circumstances. The quantity of alum mentioned above does not impair the quality of the glue.

Albumen is sometimes used for the better qualities of glue, and generally for gelatine, but a cheaper substitute is fresh blood, which contains albumen and fibrin. Dry albumen is dissolved in cold water, or white of egg is used direct, if procurable. Before adding either of these substances, the liquor is cooled to 130° F., and the clarifier well stirred in; then the temperature is raised to about 200° F., when coagulation occurs, and the precipitate entangles the impurities and falls to the bottom, requiring, however, from twelve to twenty-four hours to clear. It is said that glues clarified with albumen have a characteristic soapy smell and show a tendency to foam.

The precipitation of the lime might be better effected by oxalic acid, and the organic substances removed as scum by adding to the boiling mass some astringent matter, such as a decoction of oak bark or hops; but the purification has, in either case, to be done at the expense of glutin.

A glue liquor, which does not clarify by these means, is not sound, and is derived either from spoiled raw materials, or such as have not been thoroughly prepared, or has been injured in boiling.

A far more difficult matter than the removal of mechanical admixtures is to free the liquor from the coloring substances from which it derives its color, and to discolor it without injury to the characteristic qualities of the glue.

The use of animal charcoal for such large quantities of somewhat thickly-fluid solutions, which are liable to spoil at the high temperature at which they would have to be filtered, is very difficult, and the result not favorable, except the solutions could be successfully deprived of their tendency to putrefy. The use of carbolic acid is also in this case the only means of removing the great tendency of the liquor to putrefy, and hence, if the liquor is to be discolored by treatment with animal charcoal, it can only be done without danger to the glue, by mixing it with carbolic acid.

The object is more easily effected by bleaching the raw materials previous to boiling them to glue.

This is accomplished by placing the glue-stock, thoroughly limed and while still moist, in a bath of chloride of lime, not too strong, as otherwise the solution of the materials becomes difficult. A bath of the proper concentration is made by dissolving about 9 ozs. of chloride of lime in sufficient water to cover 110 lbs. of glue-stock. After one hour add sufficient hydrochloric acid to obtain an acid reaction, which is recognized by litmus-paper dipped in the bath turning red.

Although the glue-stock is not bleached entirely through by this process, the thin portions and outsides of the thick material acquire a lighter color, and the first run of glue solution will have a light color and can then be treated further without much difficulty.

Sulphurous acid has been successfully used for the production of colorless glue without the necessity of boiling.

Waste of hides and skins is the only available material for this process. Place the waste in water until putrefaction sets in. When this is the case wash the material in a bag or wicker basket in running water. Then pour 2½ parts of sulphurous acid over 12 parts of wet material, mix the whole thoroughly and let it stand in a hermetically closed vessel for 24 hours. Now draw off the acid, and after washing the material thoroughly repeat the operation. When the vessel containing the mixture of material and sulphurous acid is opened for the second time the foul odor should be entirely superseded by that of sulphurous acid, this being a sure indication of the correct execution of the process. Wash the material, and, after squeezing, throw it into a vat large enough not to be filled by it more than two-thirds full. After filling the vat with water allow the mass to digest at a temperature of 109.4° F. for 24 hours. The result will be a gelatinous solution, which is drawn off and converted into glue. The undissolved residue is transformed into gelatinous solution by pouring water over it and allowing it to stand at a somewhat higher temperature.

For carrying out this process and that of bleaching with chloride of lime it is best to use a vat provided with a stirring apparatus, somewhat like a hollander used by pape-rmakers, as being most suitable for washing, disintegrating and mixing the material.

Glue-liquor may also be successfully bleached with sulphurous acid, and in speaking later on of the manufacture of bone glue, a very practical apparatus for this purpose will be described.

Glue-liquor bleached by sulphurous acid clarifies very readily and is protected from spoiling. The resulting glue remains, however, quite acid, and cannot be used for all purposes, especially not in combination with colors, chemicals, etc., upon which the acid has a destructive effect.

3. FORMING OR MOULDING THE GLUE.

After clarifying, the liquor is run into moulds of deal wood or sheet iron, lightly joined and of a rectangular form, slightly converging towards the bottom so as to allow the more ready detachment of their contents. They are about 3.25 feet long, 10 inches wide at the top, and 7¾ inches at the bottom, and 5 inches deep. When very regular cakes of glue are desired, cross grooves of the required shape are cut in the bottoms. After being well cleansed and ranged upon a level the boxes are filled to the brim through large funnels with strainer cloths affixed to their barrels. It is best to place them upon perfectly clean stone flagging slightly inclined towards a reservoir for the reception of such portions of their contents as may run over. The apartment in which the work is performed should be clean and airy, a dry cellar being the best for the purpose. In place of a large number of boxes, a shallow vessel lined with sheet-iron and capable of holding the entire quantity of liquor is sometimes used, from which the solid jelly is cut out in cubic masses, which are further divided.

This arrangement can only be recommended for establishments where but one variety of glue is produced, and the different layers in the clarifying vat are not separated according to their clearness. Before running the liquor into the boxes the latter should be moistened with water, or, if made of wood, coated with oil, stearine, or paraffin to prevent the liquor from penetrating the wood and the solidifying glue from adhering to the sides.

After the solidification of the glue, which generally takes place in twelve to eighteen hours, the boxes are inverted upon a table with a smooth top of wood or stone previously wetted, so as to prevent the adherence of the gelatinous cake to its surface. To detach it from the sides of the boxes the moistened blade of a large knife is generally used.

Cutting the cubes of glue into commercial cakes or sheets is readily accomplished by observing the following instructions:—

The shape of the cakes depends principally on custom. The consumer is used to a certain variety of glue, and if it is not offered to him in the customary shape, he might refuse it and take his custom elsewhere. The quality of the glue is the next point to be considered. If very dark, it is advisable to cut the glue into thin cakes, and if turbid, into thick ones, in order to make this defect the less apparent. Thicker cakes can also be cut if the conditions for drying them are favorable, and thinner ones if the reverse is the case.

The mass is first divided by a steel or brass wire stretched over a frame, like a bow saw, into horizontal layers. The size of these layers is regulated by guides which are placed at distances corresponding with the desired thickness of the cake of glue. Instead of one wire, as many as the cakes of glue to be cut, can be stretched over the frame, which is best made of iron and provided with conical pins by means of which the wires can be tightened, in the same manner as piano strings, when they have become slack by use.

The width and thickness of the cakes of glue are regulated by the distance of the wires from each other, and the length by the width of the box. The cakes thus formed are dexterously lifted from the block with the moist blade of a large knife and placed upon nets.

Instead of using wooden or sheet-iron cooling-boxes, it is recommended to pour a layer of liquor of the desired thickness of the glue cakes upon large polished stone slabs, and when congealed, cut it into sheets, which are placed upon the nets to dry. The advantages of this method are obvious. The liquor cools more quickly by being exposed in a thin layer upon a large surface, which reduces the danger of spoiling, and a strong evaporation of water and consequent concentration take place. Besides, the cakes show the smooth surface of the polished stone, and become in a short time so hard, that when placed upon the nets, the twine will make no impression upon them.

Liquors which in gelatinizing do not become very solid, are not run into forming boxes, but upon glass or zinc plates, and thus spread out in a thin layer, acquire sufficient solidity to be removed cake by cake after being cut. The plates upon which the glue is run are placed in frames and laid upon a table furnished with a rim about 1 inch deep. To accelerate gelatinization of the liquor, the table is flooded with water before placing the plates upon it.

Where cooling-boxes are used, the jelly when completely congealed is placed upon a table with a stone plate, by inverting the boxes, and then cut into cakes. Figs. 15 and 16 represent the tools for cutting the jelly into cakes. The block of glue is laid upon the surface _A_, Fig. 15, and the frame, _B_, is gently drawn along in the grooves, _a_. In the upright portion of the frame are fixed wires at such a distance from each other as required for the thickness of the cakes to be cut.

When the block of glue has been cut in this direction, it is divided by cuts perpendicular to the former, into cakes of a size in which the finished product is to be brought into commerce. The apparatus shown in Fig. 16 serves for this purpose. The vertical bars, _a_, furnished with the wire, _b b_, serve as guides. The sheets thus formed are lifted from the block with the moist blade of a large knife, and laid upon nets.

The machine shown in Figs. 17 and 18 is the invention of Mr. J. Schneible, and it is for slicing and spreading glue-jelly preparatory to drying, and it consists in the combination of a reciprocating cutter with the jelly-box and a traveling belt-carrying frame for receiving the slices as cut by the knife.

Fig. 17 is a partly sectional side view of the machine, and Fig. 18 is a cross-section of the same.

_A A_ are side bars of the supporting frame, fitted at the ends with cross-shafts, _a´_, carrying pulleys, _a a_, around which are endless belts, _b b_. _c c_ are slide-ways upon the bars, _A_, and _d d_ are slides carrying a cross-plate, _e_, and also a plate, _f_, to which plate _e_ is attached a knife or cutter, _g_, the cutting edge of which is at the edge of the plate, _f_, and about the same thickness as the slices to be cut. The cross-shaft, _h_, is fitted in boxes on bars, _A_, and near one end thereof it is provided with cranks at its ends, which connect by rods, _i_, to the slides, _d_.

From the opposite ends of the slides, rods, _k_, pass to loose arms, _l_, on the shaft at the opposite end of the machine, and the arms, _l_, carry pawls, _l´_, that engage ratchet-wheels, _m_, fixed on the shaft, so that the shaft, _h_, being revolved, the slides, with plates, _e f_, are reciprocated, and at the backward movement of the cutter the pawls engage the ratchet-wheels, and belts, _b_, are moved a distance equal to the movement of the knife.

The jelly-box, _n_, is fixed to side bars, _A_, by brackets at its ends, as shown in Fig. 18, and is placed above the cutter and the plate, _e_, so that when the plate, _f_, is drawn out from beneath the box the plate, _e_, takes its place for holding up the block of jelly.

In operation the block of jelly is placed in box _n_, resting on plate _e_. A frame provided with netting—such as is used for drying glue—is placed on belts, _b_, beneath the box, and the shaft, _h_, being rotated by power, the cutter moves forward and cuts a slice from the jelly. The plate, _f_, at the same time moving away, the slice passes upon the frame, and the return movement taking place, plate _f_ is carried beneath the jelly-block, and the belts being at the same time moved, the frame is carried forward in position for receiving the next slice apart from the first one. In this manner, as slice after slice is cut, they are spread on the frame, and the frames, when filled, are carried to the end of the machine for removal. The plate, _f_, is adjustable, so as to vary the thickness of the slices cut.

The box may be divided into cells of any size desired, so that each movement of the knife will cut a slice from the bottom of each cell, and the box extending the full width of the drying frames, all the slices cut at once will be properly spread.

In order to keep the plates, _e f_, moist, so as to prevent the glue-jelly from sticking thereto, there are fitted at the sides of the jelly-box, _n_, open-bottomed boxes, _o_, containing fibrous material soaked with water, which, resting on plates, _e f_, keeps their surfaces moist.

The machine saves the troublesome and expensive work of spreading the jelly by hand, as has been heretofore practiced.

The knife is to be attached to plate, _e_, in any suitable manner, and the surface of plate, _f_, may be corrugated, so as to slide on the jelly more readily.

The cutting apparatus patented by M. Devoulx, of Marseilles, is much used in France. The machine stands upon a board or table, upon which are fastened two uprights, far enough apart to allow of the passage of a truck carrying the glue, which is cut into cakes by blades or wires stretched between the uprights.

Fig. 19 shows the perspective elevation of the machine with its truck. The upper part is filled up for the reception of the glue to be cut up into cakes. The sides are omitted in this figure in order to admit of a better explanation of the separate parts.

Fig. 20 gives the same view, except that the truck, the upper part of which is closed, is between the uprights, and contains the glue to be cut.

Fig. 21 represents the moment the wires have passed through the glue and cut it into cakes. In all the figures, _a_ is the wooden frame upon which the machine rests, _b_ the table-plate fastened to the frame, _c_ and _d_ are the uprights, between which the cutting wires are stretched, and _f_ the truck carrying the glue.

Figs. 22 and 23 show the truck by itself, _g_ representing the bottom, and _h_ the back, which is provided with slight grooves, into which the wires catch to assure the entire cutting through of the block of jelly; _i_ is the upper part of the truck, which opens by means of a hinge, and when closed is fastened with the pin, _k_. This upper part of the truck is fastened to the back part of the truck by means of a screw, which allows it to be set higher or lower, according to the size of the block of jelly to be cut; _m_ is the bar of a rack fastened to the truck, and serves for moving the latter. The driving gear, _n_, the shaft of which carries a crank, _o_, catches into the rack.

Two boards, one on each side of the truck, serve to keep the block of jelly in position, and guide the truck.

With this machine 120,000 to 130,000 cakes can be cut in five or six hours.

3. DRYING THE CAKES OF GLUE.

Drying the cakes is without doubt the most precarious part of the manufacture. The jelly contains a large quantity of water which, to prevent decomposition of the jelly before it is converted into glue, must be evaporated as quickly as possible. In favorable weather, drying may be accomplished either in the open air or in covered sheds.

Drying in the open air is connected with many inconveniences, for if the sun strikes the cakes of jelly when they still contain a large quantity of water, they may become soft so as to run through the meshes of the net, or they may dry so quickly as to prevent them from contracting to their proper size without numerous cracks and fissures. If frost supervenes, numerous cracks may be formed in the cakes from the congelation of their water, or a shower of rain may cause much work and damage. In consideration of all these inconveniences, it is best to conduct the operation in a drying-room.

To insure a constant circulation of air, which is absolutely necessary for the expulsion of the aqueous vapor caused by the evaporation of such a large quantity of water, the drying-room should be at least 10 feet high, even if intended for summer use only, and the windows be provided with Venetian blinds so as to shut out the sun, if necessary, without disturbing the circulation of air.

To dry the cakes in heated rooms in winter is a more difficult matter, as provision has to be made for the removal of the aqueous vapor, and a current of warm dry air has to be kept up at the same time. But such a room is an absolute necessity for the manufacturer on a large scale, who, in order to carry on his business without interruption throughout the entire year, must be independent of the changes of wind and weather.

The size of the drying-room should be proportional to the daily production. Constructions are fitted up with the requisite frames for the reception of the glue cakes, and are heated by steam pipes arranged along the walls. In the floor in the immediate neighborhood of the steam pipes are openings, which can be opened and shut at pleasure, for the admission of fresh dry air. The latter on entering the room is heated, and after passing over the frames and absorbing water from the glue cakes, escapes through openings in the ceiling to a space above it from which it is withdrawn by means of ventilators in the roof. A constant change of air must be kept up. The quick drying of the glue is of the utmost importance, as otherwise the jelly putrefies either entirely or partially, and the glue acquires a turbid and mean appearance. Too much heat causes the cakes to bend and crack. The cakes are laid upon widemeshed nets of twine stretched in frames 6½ to 8 feet long and 3¼ feet wide. Fig. 24 represents the form of nets commonly used. The nets are placed upon frames, such as shown in Fig. 25, arranged around the drying-room in the neighborhood of the steam pipes and air flues. As the cakes have to be occasionally turned upside down upon the nets, the latter must be placed at convenient distances, one above the other in the frames.

The use of twine netting has been found to be attended with many disadvantages, the principal ones of which are given by S. Rideal as follows:

1. “Being freely handled in the making, the netting is almost always impregnated with dangerous organisms which penetrate the moist glue cakes, and cause moulding or putrefaction. When this occurs, it is usually attributed to a state of the atmosphere, but if the cakes are examined, the alteration will generally be found to originate along the lines made by the netting. The fault could be cured by sterilizing the net for an hour at 212° to 248° F. in a hot oven, but besides the expense, the fibre is thereby weakened. Moreover, the spores of a few bacteria, such as _Bacillus subtilis_, which is widely distributed and has the power of liquefying gelatine, will bear a heat of 248° F. for over an hour, and still be capable of growing.

2. “However smooth the fibre, the glue will stick in places, leaving small remains, which being hygroscopic, become ’sour,’ and set up the objectionable bacterial changes in the subsequent batches.

3. “Owing to sagging, rotting, scouring, or wearing into holes, the life of cotton or hemp netting is so short that the constant renewal is a considerable item. A whole batch is frequently spoilt by the fault of a net. In some works, heaps of old netting are found, which become very putrid in the rain and sun, and give rise to mysterious bacterial inroads in the factory. In others they are regularly burnt under the boilers.

4. “The considerable overlap or selvedge required for securing the edges of the net involves a waste of the area, and also some difficulty in refixing.”

For this reason metallic netting has been largely adopted. The best material has proved to be a heavily galvanized iron-wire netting having no less than 15 to 25 per cent. of its weight of zinc. It can be strengthened by longitudinal and transverse wires or ribs. It must be examined by the microscope to see that it is perfectly free from holes or cracks, and should last at least two years in constant use.

The temperature of the drying-room requires careful regulation, and should never be allowed to rise above 68° to 77° F., as otherwise the glue would soften and run through the meshes of the net, or adhere so firmly to the twine as to require the nets to be put in hot water for its separation. Dryness of air is of far greater importance in the drying process than a high temperature. To promote this dryness of air and prevent the aqueous vapor from condensing, evaporating, and again condensing upon the cold walls of the room, they are wainscoted. Thus protected by a bad conductor, they acquire a higher temperature, and the aqueous vapor, instead of being precipitated upon them, is carried off by the air-currents.

As the cakes placed in the immediate neighborhood of the steam pipes and near the floor where the dry air enters, dry quickest, the nets containing them are shifted after some time to a higher part of the drying-room and their former places filled with cakes still wet. When the cakes are dry, they are finally desiccated in a room at a higher temperature, which serves to harden and improve them.

In modern times drying-rooms have been almost entirely abandoned and in this country long drying galleries are used, sometimes 250 feet in length and 6 to 8 feet square, with traveling platforms on rails carrying the sheets of glue on stout galvanized netting. Wood is found to be a better material for the galleries than stone or brick.

Figs. 26 to 28 show an apparatus for drying glue which is the invention of W. A. Hoeveller.

Fig. 26 is a plan section, and Fig. 27 a side elevation in section, of this improved drying-alley. Fig. 28 is an end view in section.

The form and arrangement are as follows:—

_A B_ represent the two parts of the alley, separated by the partition _C_, which is shorter than the alley, so as to leave a communicating space at both ends.

At the front of section _A_, is located a blower, _D_, actuated by a steam-engine or other motor, _E_, also located within the walls of the alley. The whole current from blower _D_, is directed through section _A_ of the alley, whence it turns into section _B_, and comes back through it, to be again drawn into and forced out of the blower into section _A_. By this means the contained air of the alley is set in continuous motion through the two sections successively, and as the structure is made as air-tight as practicable in such cases, the air remains unchanged until the doors _F_, or either of them, are opened to discharge the vitiated air and let in the fresh.

In sections _A_ and _B_, is placed the railway _a a_, to admit of the convenient movement of the contents in process of drying, which are generally set on cars or buggies.

In section _A_, in front of blower _D_, is placed a steam or other heating device, _G_, which may be of any form or design adapted to allow the air from blower _D_ to pass through it and to heat such air while passing therethrough. The inventor prefers the radiating coil for such purpose, the steam entering at _b_, and emerging at _c_. At the other end of the alley, which by the double construction is in section _B_, just back of the blower and heating-coil, there is placed a condensing-coil, _H_, of a construction similar to coil _G_, and having inlet _d_, and outlet _e_. Through this condenser there is kept flowing a refrigerating liquid or brine, which renders the condenser very cold. The continuous current of air from the blower passes over the contents of the cars or trays in the alley and takes up moisture in its passage. After such passage the air is charged with moisture and comes in contact with the coils of the condenser _H_, upon which the charge of moisture is condensed, and the air emerges dry again, enters the blower, and is again made the vehicle by which the moisture of the glue or other contents is transported to and deposited on the condenser.

In drying glue by this method do not use the steam-coil at the first stage of drying a charge, as the drying should not be effected too rapidly; but as soon as the product begins to stiffen properly, admit the steam to the coil _G_, and thereafter the operation is continuous, as above described.

By doubling up the alley into two sections, as shown, the inventor is enabled to erect the alley in a more contracted space. In a length of ninety feet he obtains the benefit of a single alley one hundred and eighty feet long. Section _B_ may, if desired, be located on top of section _A_. Doors may be located wherever desired, to facilitate the movement of the trays or cars and the placing of them in and their removal from the alley.

By the above apparatus the drying can be perfected in a very much shorter time than can be done by the old alleys, and operations can be conducted in hot weather without hindrance from the condition of the atmosphere.

In cases where the atmosphere is dry enough to dispense with the heater and the condenser, the inventor can throw the doors _F_ wide open, extend the partition _C_ out to that end of the alley, and then preserve a continuous forced draft of sufficiently dry air in both alleys for the purpose. As there are many days during the year fine enough to give reasonably dry air, operations can be conducted with the blower alone in this way, and thus economize the steam and the refrigerating-brine.

Figs. 29 and 30 show a longitudinal section, with upper and ground floor plans of a modern drying-house, as given by Thomas Lambert. In the ground floor the whole of the liquors are jellied in coolers, and then cut into cakes by the two cutting machines in the centre. Here a hoist, _E_, Fig. 30, is placed which carries the cut cakes on “glasses” to the floor above. This forms the drying-floor, and is partitioned off in three divisions, running nearly the length of the building. The two outer divisions form the tunnels proper, and at the ends are fixed two powerful revolving fans, driven at high speed and drawing the air through the tunnels at a high rate. At the opposite ends of the fans are fixed a series of 6-inch pipes, heated up by waste steam, and the air passing between is warmed up to any desired temperature, of necessity below 78° F. In the center passage a number of girls are employed in transferring the cut cakes to the nets, which are built upon a carriage running on a small railway. The carriage with the complement of filled nets is run to the end of the division, transferred to the lower railway, _C_, by which it is carried either to the right or left hand tunnels as desired; the glue when dried on the nets is run to the other end and by another lower railway brought to the middle division, and ultimately by the hoist raised to a large store-room, seen in section, where the glue is sorted and bagged. At the end of the store-room is placed a grinding machine, and all the off-color and twisted cakes are ground to a powder and sold as powdered glue. The manufacturer by varying the size of the cake, its thickness, and its color, may make any number of grades from the same boiling.

Fleck proposes to accelerate the drying of glue by utilizing the water-absorbing power of some salts, such as Epsom and Glauber’s salts, ammonium sulphate, crystallized acid sodium sulphate, etc., for the purpose of withdrawing water from the glue cakes. For the practical application of this principal a shallow, water-tight, wooden box is required. The bottom of the box is sprinkled with a layer of the water-absorbing salt about half an inch deep, and covered with a moist linen cloth. Upon this is placed the jelly cut into sheets and also covered with a moist cloth, a layer of salt being finally scattered over it. After standing for a few hours, the box is slightly inclined and the salt solution allowed to drain off through a hole in the bottom, the dropping ceasing in from 12 to 18 hours. If now the upper cloth is taken off with its layer of salt, the glue beneath it will be found so far deprived of its moisture that when placed in the sun or exposed to other heat, it will become completely dry in a short time without either melting or spoiling, and in winter may be laid upon drying-floors with the same result. The salt-solution formed may be evaporated to crystallization and the salt thus obtained be again used.

After the treatment with the salt, the jelly contains 70 to 75 per cent. of anhydrous glue, while the content in jelly not thus treated varies between 7 and 28 per cent. according to the concentration of the liquor from which it has been derived. It is claimed that the adhesive power of the glue is not injured by this treatment.

Commercial glue must not only be thoroughly dry, but should also present a good appearance, showing especially lustre. However, after drying, the glue is dull, spotted, dusty and sometimes even mouldy. To give a good lustrous appearance the dry cakes are dipped in warm water and replaced upon the nets to dry.