Bourne Archive: Muspratt: Tanning
http://boar.org.uk/aaiwxw3MusprattL7Tanning.htm Latest edit 27 Dec 2010
The Bourne Archive
Extracts Concerning Leather, 7: Tanning.
The web pages linked from this introduction are from an article on tanning, under the heading ‘Leather’. The original article is presented here in several web pages, respectively dealing: 1, with leather; 2, with tanning materials; 3, Sources of tannin; 4, crushing mills; 5, varieties of skin; 6, hide preparation; 7, the common tanning process; 8, finishing processes; 9, fancy and speciality leathers.
Vol. 2. pp. 516-521
The Tanning.—The many improvements in reference to the combination of tannin with the gelatinous tissue of the skin, that have been put into practice within the last fifty years, seem almost to exhaust the subject. Hydrostatical pressure, the laws of exosmosis and endosmosis, exhaustion, or the vacuum principle, combined with mechanical action, have been successively tried, and the result is, that the time required of the operation, which formerly extended over two years, and sometimes longer, is now diminished in some cases as many weeks. Unfortunately, it appears that, as the period occupied in the tanning is diminished, so also is the quality of the leather deteriorated, for although by the modern accelerated process, a large amount of tanning agent is rapidly taken up by the skin, its union with the gelatinous matter is far from being complete, and the pressure or exhaustion applied to bring about the rapid combination has the bad effect of leaving the leather porous, and without compactness or body. Hence, when a good article is required, such as shall combine density with closeness of grain and thorough homogeneous tanning, that which has been manufactured in a manner somewhat resembling the old system is still preferred.
This system consisted in spreading a layer of waste tan in the bottom of the pit, another of fresh tan to the depth of an inch; upon the latter the hides was spread out evenly, and over it another stratum of tan. This alternate arrangement of tan and hides was continued until the pit was filled, when another layer of tan, six inches thick, was placed upon the whole, and well trodden to compress the mass; the final covering was termed the hat. After a period of four months, when the bark was considered to be exhausted of its tannin, the contents of the pit were removed, the partially-tanned hides stretched, and again replaced with a supply of fresh tan in the same order as before, left for another interval of several months, then taken out, again stretched, and so on till they were deemed sufficiently tanned; the bloom being ultimately given with a concentrated extract of the bark. No moisture was allowed to enter the pit, except what was retained and absorbed hygroscopically by the bark; and as the tanning agent can come in contact with the skin only in a state of solution, it is evident that in the absence of sufficient liquid to bring it to this state, the method must have been slow and tedious.
Subsequently, the plan was adopted of filling the pit with tepid water, the hides and bark being as above stated, and new materials substituted for those exhausted every three of four months, till the tanning was nearly completed, the finish being given with a strong solution of tan liquor. Even by this course, which was a great improvement on the former, the hides took a period of eighteen months and longer to convert them into good leather. Another innovation followed; it consisted in filling the pit with tan-liquor or infusion of the bark, instead of water, the hides and bark being disposed as formerly. This method is still much followed by Irish tanners, but is now deemed obsolete by most of the leather manufacturers of this and other countries. Leather can, however, be made by this method, unsurpassed by any other in quality.
It is rarely that ground tan is now used, the time of tanning being reduced, and leather of average excellence obtained by employing a solution or extract, termed ooze, instead. In the slow process of working, very little difference exists between the course followed by the various tanners; and where a difference is found, it is more in the manner of making the ooze, than in the succeeding manipulations. The adoption of solutions of tan has been gaining ground regularly from the time of Seguin 1, who first recommended it, and there is no doubt that it is much more effectual, as evidenced by the abridgement of the tanning period, than even the most finely powdered bark. There are several methods of preparing the ooze; cold water being employed by some; hot by others; while many extract the strength of the material by boiling; and latterly, the spent or weak liquor from the pits has been resorted to for this purpose. When cold water is preferred, the better way is to have a gallery of vats, in which the ground tan is exposed to the solvent action of the water. That which receives the water direct from the reservoir contains the almost exhausted tan, and, consequently, as the liquid passes from one vat to another, it meets with a richer material, and takes up more and more tannin, until, as it passes off from the last filled with the fresh bark, it possesses a high degree of concentration.
The same plan may be followed when hot water is used, in which case branch steam-pipes are laid to each vat for the purpose of heating the contents. By this means the extract is obtained of any degree of concentration, according to the quantity of water employed, and the time of digestion allowed. Sumach, divi-divi, and valonia 2 may be exhausted in the same way; but kino, and the several varieties of catechu, should be macerated by repeated agitation in the water. Divi-divi extracts should be made as much as possible in close vessels out of contact with air; and the tanning, also, when this substance is used, ought to be performed with as little exposure as possible, with a view of preventing the coloration of the leather.
Latterly, hydrostatic pressure has been resorted to for abstracting the virtues from barks and similar substances. A well-bound vessel is taken, and a perforated false-bottom inserted as short distance above the real one, and from the space between the two, and outlet pipe draws off the contents. It must be fitted with a strong and water-tight cover, supplied with two trap-doors, through which the bark 3 may be inserted and removed, but capable of being perfectly secured. When filled with bark, water is introduced through a pipe, which may be twenty or thirty feet high. By keeping the latter always filled, considerable pressure is exerted upon the contents of the tub, and by this means the tannin is more readily dissolved. The liquor is drawn off from time to time through a pipe, according as it filters through the mass.
Whatever the means adopted for making the ooze, the next operation is the immersion of the skins in it, in order to convert them into leather. This is done by filling a number of pits sunk in the floor of the tan-yard, and introducing into them the swelled or partly-raised skins 4, beginning with those pits which contain a weak infusion, and after proper intervals, passing the skins forward into those which contain stronger solutions. In the construction of these pits, the practice is to excavate the earth to the depth of nine or ten feet, and to cover the bottom with a bed of retentive clay. The pipes for conveying the ooze, waste, tan liquor, et cetera, are then laid, and the intermediate spaces filled in with well-compressed clay. Instead of boards, bricks are sometimes used, and the inner surface is well secured with hydraulic cement. A composition of asphalte 5, made of the usual components, only substituting clay for chalk, would answer the purpose admirably. Such pits as contain the weak infusion or the tanning agents are called handlers, and those employed for finishing the process, and, therefore, having a stronger liquor, are known as layers or bloomers. In the weak handlers, the hides are taken out twice a day, placed double one over another in a heap, and again introduced into the same or the next pit. After their removal to stronger pits, the handling, as the operation is called, is performed only once a day, and after they arrive at the layer-pits, they are taken out only once in the week, and finally once a month. The object of these handlings is to equalize and quicken the combination of the tannin with the skin; for during the time that the hides lie upon one another on the ledge over the pot, their weight forces the absorbed liquid out of their pores, so that they are more free to imbibe the juice on their re-introduction to it than before. Some are of opinion, however, that during the one or two hours they remain exposed, they contract some injury, and that the quality of the leather is much improved by transferring them at once into other pits; but, as will appear further on, a directly opposite view is often held. It is a common practice with many to introduce some powdered tan into the last pits or bloomers, between the hides, more especially when it is desired to produce a superior leather in point of quality and color. On the other hand, several manufacturers continue to use the powdered tan with the ooze throughout. To avoid the repeated handling of the hides, and the alleged injury from exposure, some tanners submit the hides to the tan-liquor in a vertical instead of a horizontal position. This method has the advantage of exposing every part of the skin to contact with the juice; but if care be not taken to alter the position at proper intervals, the lower part is liable to be better tanned than the upper, owing to the bottom of the pit containing the richer ooze. This, however, is very easily obviated by reversing the hides. Patents on this principle were taken by Mr. Keasley in 1845, and by Mr. Barrett in 1850.
The quantity of tan required varies with the weight and quality of the hides. Generally, heavy hides take about double their weight of good oak bark to tan them; but those which are lighter and of less body are perfectly tanned with a less quantity, from 1.6 to 1.8 of their weight. When sumach, valonia, divi-divi, or catechu is the tanning agent, a much less proportion suffices, since these materials, as already stated, are greatly richer in tannin than oak bark. With oak tan, the period required for tanning good sole leather 6 averages ten months; about eight months when valonia is used; and six if terra japonica or catechu 7 be employed.
The method of tanning introduced by M. Ogereau of Paris, is similar to the old system still followed in many parts of Ireland and Scotland, namely the stratification of the pit with alternate layers of tan and hide; only that, in the French method, the pit is furnished with a false or hurdle bottom, through which the liquor in the vat filters, and collects in the space below. Once a day or oftener, as the case may be, the filtered juice is raised by pump to the top of the vat, from which it gradually descends through the materials, to be again successively raised to the surface. By this means the tan juice in the pit is kept in circulation, and brought into close contact with the hides, at the end of a month the contents become exhausted, and fresh materials are introduced, the circulation being kept up as before. It is said that by this method the time occupied in the tanning does not extend beyond four months, and that the leather is of first quality.
Nossiter, in 1844, patented a method by which the pressure of the overlaying hides in the pit, which prevents free contact with the tanning liquor, is counteracted, and the conversion of the hides into leather much accelerated. This method consists in constructing within the tan-pit a frame with ledges, which support other frames on which the skins are placed when introduced into the pit. Even when fixed as closely as possible together, as many skins cannot be introduced as in the old way; but taking into consideration the limited time in which the operations are finished, the patentee states that, in the end, a larger number can be tanned by this method within a given time than by the ordinary arrangement. After all the frames and hides are placed, the pit is filled with ooze, and after it has become exhausted, the spent liquor is pumped out and fresh supplied, and this is repeated till the leather is formed. Another improvement embraced in this patent consists in depriving the hides of the old tan-liquor previous to immersing them in the new, by submitting them in numbers at a time to the action of a screw press, so constructed that the hides do not require to be folded, but are laid flat upon the table or press-board.
The plan adopted by Berenger and Sterlingue for tanning hides in a reasonably short time, without the aid of machinery, is the following:—a row of pits is formed, each furnished with a hollow vertical wooden cylinder, connected with the next pit of the series by a pipe fixed at a depth of six inches from the surface, and communicating with the space between the perforated and true bottom, by means of a punctured pipe. By this arrangement, the liquid in each pit may be made to pass over to the next in the series, and so to the end, without the least disturbance of the contents. Thus, if water from the supply pipe be conveyed into the first vertical cylinder of the series, the liquid flows down through the false bottom into the intervening space, causing the fluid already there to ascend through the body of the vat, and to flow over by the connecting pipe into the cylinder of No. 2 pit, where it displaces an equal volume of liquor in like manner. In working on this principle, a series of eight pits is preferred; No. 1 is filled with tan and hides in the old manner, and as much water as it will retain added. After twenty-one days, No. 2 is filled in the same way; but instead of employing water, as much strong ooze as is required to wet the materials is poured into the first pit, and this drives before it the weak liquor, which ascending by the pipe from the false bottom into the cylinder, flows over by the connecting pipe into, and moistens the contents of, No. 2, with the weak tan liquor of the first pit. After the lapse of another period, a third is charged with tan and skins, and after this a quantity of strong ooze, equal in bulk to what is necessary to moisten the contents of the new materials like the others, is poured into No. 1, when the liquor previously contained in it will flow over from the cylinder to No. 2, the liquid contents of which are in like manner transferred to No. 3. In this way the work progresses till the eight pits are filled. The connecting pipe of No. 1 is then closed, and the ooze in it pumped out, and the remaining materials removed, when the leather will be found thoroughly tanned. A fresh supply of hides and tan are then put into this pit, which now becomes the last or eighth of the series, that which before was No. 2 becoming No. 1, and receiving the strong ooze. By this means the hides are tanned without once removing them from the pits till they are finished, and likewise with a great saving of labor. If the strong infusion be added at intervals of fifteen days, the time required does not extend beyond four months, but it may be prolonged at will, by allowing a period of twenty or thirty days to elapse between the addition of fresh ooze, or the filling of a new pit. The ooze used in the foregoing operation is prepared in a series of vats similarly constructed, or they may be placed one above another, so that that the liquid in the higher may flow into the next beneath, as in the lixiviation of soda 8 or of kelp—see Iodine 9—only that each is furnished with a coil of steam-piping, for the purpose of heating and concentrating the liquors, with a view to the removal of gallic acid and other substances. The liquor emptied from the tan vat is mixed with the ground bark or other material in the first of these, and after a while, the solution is drawn off to the lower one, where it becomes further enriched. In this way the liquid may be brought to any degree of density necessary for the operation, and much of the extractive and gallic acid may be removed.
Another patent process by Cagswell, recommends the process of filtration, as a means of causing the tannin of the ooze to unite with the matter of the hide in a shorter time than is ordinarily required. The prepared hides are to be laid in sawdust, in such a way as to form a kind of basin, and into this the rich ooze is poured. Gradually it finds a passage through the cells of the skin, and passing out at the other side, runs through the sawdust into a tank beneath of adjacent to the floor, but whilst going through the hide, the tannin is retained. By keeping the hollow of the hide filled with fresh ooze, the gelatine-fibrous tissue in a very short time becomes saturated with tan. There are some drawbacks, however, to this method; for instance, the porosity of the hide is very unequal, and as the filtration is greater where most porous, these parts are liable to be overtanned, when the others may not be sufficiently so. Again, the edges are never finished in this operation, and consequently, the hides after the body has been made into leather, must be subjected to ooze in the pit. The great space, too, necessary to carry on an extensive trade by this means, is not among the least objections to it.
preceding methods in which the tanning agent is left free to react upon the
skins, several others have been patented in which mechanical force, in concert
with other means, has been employed to effect the tanning in a shorter time.
The simplest of these are such as merely cause the agitation of the hides and
tan-liquor in the vat, so as to effect a more thorough and rapid contact of the
combining substances. Of several on this principle, that of Mr. Squire of
Some erect a roller over the mouth of the pit, and string the hides together by stitching in the form of an endless web or belt, which is wound in several coils loosely round the roller, and then placed so as to be partly submerged in the ooze of the pit. By turning the cylinder, the immersed portion becomes saturated with the liquid; and as it rises again on the roller, the weight of the suspended hides exercises a force by which considerable quantities of the liquor are squeezed out, and the hides descend at the other side in a condition to absorb fresh liquor. This alternate absorption and expression of the liquor is very conducive to rapid tanning; but the time is still further abridged by a process patented in 1837 by Cox and Herapath, which consists in passing the hides between two rollers fixed longitudinally over the pit. In order to avoid injury to the hides from contact with the metallic surface of the rollers, and to allow the solution to pass away more readily, the lower cylinder is covered with horse-hair cloth, and the upper one with folds of flannel. Both rollers revolve, and the upper on may be weighted to any degree, so that the hides passing between them lose all absorbed water and re-enter the same pit, or another containing a stronger ooze, in a condition to imbibe the solution more readily. It is stated that sole leather may be thoroughly tanned by this method in one or two months, and kips for upper leather in three or four weeks. It should be remarked however, that although with care an average quality of leather may be prepared in this manner, yet if too much pressure be exerted, the process of tanning will be rather retarded than otherwise, because the elasticity of the tissue will be destroyed.
Tanning by Hydrostatic Pressure.—Spilsbury, in 1831, was the first to introduce the method of permeating the hides by hydrostatic pressure. His mode consists in securing any holes or damaged parts in the hide, and then to stretch it on a suitable frame, having iron hooks for fastening the edges of the skin all round. Upon the frame so furnished and another is laid, and screwed tightly, so as to allow no room even for water to percolate between the two frames. Another hide is then fixed upon the second frame, and its edges secured in the same manner by a third frame, the whole three being bolted together by means of clamps. In this way a kind of water-tight compartment is formed between the two hides. The frames are then placed upright, and the interval between the hides is filled with ooze. As the skins form the side walls, it is evident that their whole surface will be exposed simultaneously to the liquor, and as the latter by the force of its own weight penetrates them, the tannin enters into combination, and they are gradually converted into leather. An arrangement of pipes and stopcocks permits the confined air to escape in the first instance, and afterwards the spent liquor, after it has percolated through the hides. Of course the rapidity of the tanning by this, as by other processes, depends more or less on the thickness of the hides, the strength of the ooze, and the amount of pressure exerted.
Drake, instead of using frames, directs that the hides should first be submitted to an incipient tanning, and then sewed together in pairs, so as to form a kind of sack. A small aperture is left at the shoulder or neck part for the introduction of a funnel for filling the sewed hide with the tanning juice; and to prevent the distension to which the hides would be liable on being filled, he places each pair in an upright frame of suitable size. The bag being filled with the ooze, the liquid percolates through the pores of the skins, being deprived of the tannin in its passage, and trickles down into a receptacle beneath the frame, whence, if rich in tannin, it is pumped back into the bag in order to be again exuded. Towards the close of the operation, when the skins become firm and hard, so that the confined fluid exudes very slowly, the atmosphere of the room is heated to about 150°, with a view of promoting the permeation of the ooze, and the process is continued till the hides give indications of being thoroughly saturated with tannic acid, which is known by their beginning to darken in places, and by the passing through of the ooze without losing strength. The contents of the bag are then withdrawn, the stitchings cut open, and the leather subjected to the finishing processes, which will be presently described.
By the improved methods of Chaplin and Cox, the hides are sewed up into bags, leaving a small aperture for the introduction of a branch pipe from the main, connecting them with a tank which holds hot ooze. On turning a tap, the ooze flows into the bags by means of the branch pipes; and provided the connection and sewing be secure, the pressure exerted may be varied at pleasure, according to the height and quantity of liquor in the vat. As the hides are necessarily expanded more or less under the force so exerted, frames answering to the size of the bags are used to keep them from bursting or being too much expanded. The plan of encasing the sewed hides in canvas of flannel, has been patented as an improvement on the practice of placing them in frames or boxes of wood.
All these methods by infiltration are subject to the disadvantage arising from the unequal permeability of the hides at different parts, and from the weakening of their structure consequent on the prolonged distension.
By Exhaustion of Air.—Knowlis and Duesbury, to avoid the evils which hydrostatic pressure occasioned, had recourse to infiltration produced by the exhaustion of rarefaction of the air in the vat, in order to tan the hides more regularly. For this purpose the hides are suspended within an air-tight vessel from cross beams furnished with hooks. To economise room, they are arranged as closely as possible without touching, and weighted beneath to keep them extended. The top of the vessel is movable, and the pipes for exhausting the air by the pump, or for forcing air into the apparatus, are fixed at the sides near the top. When ooze has been introduced till the hides are covered, and the lid is firmly screwed down, the air is exhausted, and the contents left to their natural reactions for twenty-four hours or more. The air is then admitted and the ooze drawn off. During three hours the hides are now left in the vessel in contact with the air, till in fact they become saturated with it, and when this period has elapsed, the tan liquid is again introduced, and the exhaustion renewed. In this way, the alternate filling of the vat with liquor, exhaustion, and exposure to air, are continued till the leather is fully formed, taking the precaution to employ stronger ooze as the operation proceeds.
By Endosmosis.—The process patented by Turnbull in 1845 embraces a combination of the exosmotic and endosmotic principles. The hides sewed into bags, are filled the ground tanning material and water, or a weak ooze, and the bags are then immersed in a vat filled with a strong extract of catechu, freed as much as possible from catechuic acid by solution in cold water and filtration. The inequality of density between the weaker liquid within the bags, and the stronger solution contained in the vats, causes a motion by which the exterior penetrates, and the interior solution exudes, both parting with tannin to the gelatino-fibrous tissue, and causing its saturation in a very short time without the intervention of any mechanical force. To expedite the operation, the liquid in the vats is rendered denser by dissolving in it salts or sugar in the proportion of fourteen pounds to one hundred gallons. During the tanning, it is necessary to keep the bags filled by occasional additions of water or weak ooze, and likewise to maintain the density of the liquor in which they are immersed, as this causes the endosmosis or infiltration and consequently the exosmosis or exudation, to be more rapid.
By Acupuncture,—Schnyder, some few years ago, obtained a patent for a quick process of tanning, effected by exposing a more extended surface of hide to the tan-liquor than is ordinarily done. His method is to puncture the hides with steel points either by hand or machinery, with an instrument having from one to three hundred points to the square inch, and afterwards to expose the skins so indented to the action of the tan-juice, with or without pressure, as may be most convenient. Either the flesh or hair side operated upon with the steel points; and, notwithstanding that such perforations permit the liquor to enter into the interior portions of the skin, still, as the tanning approaches completion, they close up, and the leather is as capable of resisting as that made from hides which have not been subjected to the same operation.
According to Morfit, this method is more worthy of attention than most others, owing to the combination being so readily effected between the skin and tan that the ooze has not time to acidify before it becomes exhausted, and so the relaxation and expansion of the fibres, the enlargement of the grain, and consequent brittleness of the leather which acids produce, are obviated. Also, because by this method the formation of leather takes place in the heart of the hide as readily as at the surface, the overtanning of the latter, by which it becomes rigid and brittle under the old system of prolonged exposure on the tan vat, is done away with.
Mineral Tanning.—It has been shown in the introductory portion of this article, that gelatine, fibrin, and albumen, are capable of combining with and precipitating metallic oxides and basic salts from their solutions. On this reaction have been founded certain processes for converting skins into what may be called a mineral leather, by impregnating them with suitable metallic bases. This is easily effected by merely by placing the raised and depilated hide in contact with solutions such as bases, when the affinity of the gelatino-fibrous tissue for the mineral compounds results in their combination. Alum has long been known as a compound capable of rendering light skins imputrescible—in fact, of converting them into leather; but this agent has not been used for making ox or such hides into sole or upper leather till lately. The first to draw attention to the manufacture of leather suitable for all ordinary purposes, without the aid of tannin—considered as a branch of trade—were D’Arcet and Ashton; but their attempts, although they suggested the idea, left much for their successors to accomplish.
Bordier, in 1842, obtained a patent for an improvement in this department, which deserves more attention than the methods of his predecessors. On his system the hides are to be washed, depilated, and raised by any effectual method, and then exposed to the mineral constituent, which consists of a subsulphate of sesquioxide of iron 10, prepared by digesting twenty-two pounds of protosulphate of iron 11—copperas—in a mixture of two parts and a quarter of nitric acid, specific gravity 1.33, and three pounds one ounce of sulphuric acid, of 1.848 density. A stone-ware jar may be employed for this purpose, the contents to be heated with steam, and stirred repeatedly. As soon as the red fumes of peroxide of nitrogen—NO4—cease to be evolved, the vessel is to be removed from the heat, and the mixture kept agitated, till, as it cools, it forms a kind of paste. After settling for twenty-four hours, it is to be diluted with water, and some further portion of sesquioxide of iron added. It is again left for several days, with occasional stirring, for the purpose of allowing the sulphuric acid to take up the sesquioxide of iron and retain it in solution. The cleaned skins are then immersed in the bath, and stirred about or handled occasionally, as when working in the tan-pit, till they are made into leather. With light skins this happens in three days or thereabouts, and in six to eight days with thick skins intended for sole leather. During this immersion a combination of a basic salt—a subsulphate of the sesiquioxide of iron—takes place, and an acid salt with uncombined sesquioxide remains in the bath. There is little doubt that the acid liberated in consequence of the union of the base with the skin must have an injurious effect upon the leather, as is the case when tan is used; and if this be so, a method of working in which the uncombined acid might be neutralized as soon as disengaged from the iron base, would be preferable.
Cavalin’s mode is to impregnate the cleansed, depilated, and partially-drained hides, with a solution made of ten pounds of bichromate of potassa and twenty of alum, in one hundred and eighty pounds of water. For this end they are immersed during four days, being handled once in the twenty-four hours, and rubbed if necessary. Much of the success of the finishing process depends on the thorough manner in which the first saturation is performed, and therefore the strength and density of the bath is kept up by the addition of more of the above salts, in proportion as the hides absorb what has been already dissolved. For the second step in the process, the vat is filled with a solution of copperas—protosulphate of iron—using one pound of this salt for every gallon of water. The hides saturated with the aluminochromic solution are immersed in this bath in such a way as not to touch one another, nor the walls of the vat. At an interval of twelve hours they are drawn out, allowed to drain for a short time, and again plunged into the bath; the alternate draining and immersion being thus continued till the leather is formed. It is necessary to maintain the strength of the bath, as the iron enters into combination, by adding more of the salt. In this operation the iron base is peroxidised in the hide by means of the chromic acid, which is itself reduced to the state of sesquioxide, and remains, with the iron and a portion of the alumina base, firmly united with the tissue. The time extends over from five to six days for upper leather, eight or ten for Swedish sole leather, and thirteen to nineteen for thick butts. After the finishing of the operation, the hides are drained, and then thoroughly soaked in water to remove everything that has not entered into combination. Upper leather prepared by this method is alleged to have a body and imperviousness that render it equal for wear to the best tanned leather. The same inventor recommends another method of making leather of skins, by first submitting them to a dyeing process, employing a strong dye beck, in which four ounces of alum are added to the gallon of liquor. Frequent immersion in this solution reduced to the proper strength—as determined by a preliminary test with a bit of prepared skin—is necessary, till the materials assume a light-green yellow color on the grain, and yellow-green on the flesh side; after this they are drained and finished on a bath made by dissolving ten pounds of bichromate of potassa in eighteen gallons of water.
Next Page: Finishing Processes.
5. ^ Asphalt is a matrix of hydrocarbons, notably bitumen, in which is suspended more or less mineral matter such as sand or chalk. Originally, it was taken from tar lakes and tar sands deposits near the Earth’s surface but is now a product of the oil industry. It is often thought of as a tar-based product but bitumen and tar are distinct entities with some similar characteristics. Tarmacadam (tarmac) is a similar product to artificially produced asphalt but made with coal tar.
11. ^ There are various iron oxides. Protosulphate of iron is the salt of sulphuric acid which is formed from combination with the oxide of iron which contains the minimum number of oxygen atoms. It is the Sulphate of iron(II) oxide.