Bourne Archive: Muspratt: Leather Finishing

http://boar.org.uk/aaiwxw3MusprattL8Finishing.htm    Latest edit 27 Dec 2010


 

The Bourne Archive

 


Muspratt’s Chemistry, Theoretical, Practical & Analytical (ca. 1859)


Extracts Concerning Leather, 8: Finishing Processes.


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. 521-524

Finishing Processes.—Such are the principal methods followed in the tanning of hides, but after this operation had been performed, the hides are not in a fit condition for the market; they have still to be submitted to several additional processes. When thoroughly tanned, the hides are taken out of the layers or bloomers, and washed slightly, or brushed with water to remove extraneous matters, without, however, detaching the bloom which is so much prized, and is regarded as a kind of guarantee of quality of the leather. They are then hung up to dry in a spacious and well-ventilated apartment, from which the sun’s rays are excluded. Much care is necessary in the drying: if conducted too rapidly, the hide is liable to crack on the grain side, owing to the rapid and unequal contraction of the inner and outer parts; on the other hand, it must not be so slow as to allow the leather to get mouldy. In winter, a medium temperature is maintained in the drying-room, or loft, by means of steam-pipes, judiciously placed so as to give out their caloric to the air of the place. When partially dried, the leather intended for soles and other similar uses where great impermeability is required, is submitted to a process of condensing it and destroying in great measure the porosity which it retains after the tanning has been executed. Formerly, this was done by hammering the leather on a smooth table with heavy mallets or hammers, almost in the same way as the shoemaker hammers his leather upon the lapstone 1; but subsequently machinery was substituted. In many concerns, the compression is effected by means of rollers of brass surmounted by a carriage, loaded with ten to thirteen hundredweight 2.

Sterlingue was the first to introduce a hammer moved by machinery for condensing the leather, but in his invention numerous requisites were found wanting, so that it was often necessary to finish the leather by hand with the hammer or mallet. In 1840, Debergue patented another apparatus, in which a roller attached to a weighted lever, with other regulating mechanism, was the compressing agent. The hide or side of leather being placed upon a strong table faced with brass, and having either a plane or concave surface, was submitted to the action of the roller. Cox and Wiltse have lately introduced machines which are analogous in their action to the preceding, but are simpler in their construction. Another machine was introduced by M.M. 3 Iran and Scellos in 1852, of which reports speaks favourably; but as the object of all these machines, of which there is a great variety, is simply uniform compression, either by beating or rolling; and as the details of their construction would too much extend this article, the Editor forbears to enlarge upon a subject purely mechanical.

After the rolling or hammering, as the case may be, the sides of leather are hung up again to dry, and left with occasional changes of position, till all the moisture is removed as far as possible. After this, the sides are in some instances stored by laying them one above another, and weighted; their position being changed from time to time before they are sent to the market. This storage is supposed by many to improve the quality and color of the leather, but others regard it as unnecessary trouble.

Currying.—Such are the processes through which heavy or sole leather passes. With upper leather, or that prepared from lighter skins, a different course is pursued—the rolling or hammering being dispensed with, and another set of operations termed currying being practised. In some cases, the finishing of the leather to suit the shoemaker, coach 4, and harness manufacturer, becomes a distinct and independent branch of business from that of the tanner; but in many establishments, the two are carried on together, so that the finished leather issues from the tannery.

To curry leather, so as to confer qualities which adapt it for the boot and shoe maker, the carriage and harness maker, and other particular applications, requires its subjection to various processes, which, independently of the tanning, according as they are well of imperfectly accomplished, affect in a great measure the suitability and lasting qualities of the leather. From the difference of treatment which becomes necessary to confer these qualities, result the numerous designations by which the finished material is known. Thus, a distinction is made between smooth of sleek leather, tallowed, oiled, and waxed leather, with several other varieties. In the variety and quality of their curried leather, the manufacturers of Great Britain are far behind their French brethren. The heavy upper and harness leather produced by the British currier cannot be excelled; but when a light article is required for summer wear, and which should retain its suppleness, the inferiority of the British leather, compared to the French, becomes apparent to the wearer after a very short time.

The role of operations required for this finishing may be enumerated—the dipping or soaking, the beating, stretching, oiling, tallowing, waxing, dyeing, and polishing.

Soaking and beating.—As the tanned hides or skins come from the tanner they are dray and hard, but before they can be submitted to any of the finishing operations of the currier, they must be rendered soft and pliable. This is done by soaking them in water, or sprinkling them with that liquid, and allowing them to rest till they become sufficiently moist. When this happens, the side is placed upon a rectangular hurdle of basket-work, and trodden with the feet—the operator wearing heavy wide shoes for the purpose—or else beaten with an instrument called a mace. The working is continued carefully so as not to tear or injure the leather, till it becomes sufficiently soft and pliant, and when this is effected, it is placed upon the horse 5, and worked with the cleaner. There are various kinds of this tool, but, in all cases, the sharp edge of the instrument is so formed as to prevent it entering too far into the leather. By its use, the inequalities and overtanned parts on the flesh side are shaved off, and an uniformity of thickness and regularity of surface produced.

Stretching.—By the preceding treatment, the grain of the leather is brought out, but the desired equality is not attained till after it has undergone the stretching and pommeling. The former is executed by means of tools of steel, copper, or brass, or of hard stone or glass, the latter materials being resorted to when a better and smoother surface is desired. After the shaving on the horse or beam with the knife, the skin is further soaked in cold water and scoured, then placed with the flesh side under upon a smooth stone or mahogany; and the workman, grasping the wooden handle of the stretcher in both hands, holds the tool almost perpendicular to the side of leather, and forcibly scrapes the surface, especially where there appears to be inequalities. By continuing this action, usually from the tail to the head part, over the entire surface, and using water freely in sprinkling, the whole is brought to an equal thickness at the same time that it is stretched, and the bloom of the leather more developed; density and smoothness are also acquired.

Pommeling.—The partially prepared hide is now submitted to the last operation called pommeling or boarding. The pommel is of hard wood, having a smooth or fluted or channelled side, the former being Pommelernext to the hand, and the latter for acting on the leather. Fig. 351 shows this instrument with the strop intended to confine the hand of the workman, so as to give him greater power for applying it. According to the fineness of grain required, so do the grooves on the pommeler or crippling board become finer and sharper. Usually the grooved side of this instrument forms an arc of a circle, and the surface next the hand being straight, forms as it were the chord of that arc. This construction allows of considerable body in the middle part, whilst at the extremities it is thin. In using it the workman doubles his hide with the grain side in contact, and strongly rubs it on the flesh side, applying the force outwards, and then reversing it. This mode of working is ordinarily termed the graining, to distinguish it from the bruising, which takes place when the operation is performed on the grain side of the leather. In many instances the hides are again damped, and worked under the slicker of stretcher, and then dried and placed one upon another, and weighted for some time in order to complete the process; or they may be whitened by rubbing the flesh side with a sharp knife on the horse, after the action of the grainer, in which state, provided the leather be treated with oil or tallow before the last workings, it is known as finished russet. Leather prepared in this fashion is called stretched leather, but it may be made into sleek or grain leather by a few further operations, and a very slight modification of those described.

Grained Leather.—To render the hides—which are in most cases cut in two and pared— pliant and even in body, they are drawn over a flame from straw or such material, the flesh side being that so exposed. Fatty matters, which vary with the choice of currier, though tallow or mutton suet is preferred, dissolved if required by heat, are then laid on the flesh side in a coating, by means of a brush or mop, to the extent of three or four and a half pounds for cow hides, and one for thick calf skins. After this dubbing, as it is called, the hides or skins are doubled up, the hair side being inwards, and remain so for some hours, or in some instances one or two days; they are then refolded, and put into a tub containing water, where they soak for eight or ten hours, being worked during this time more or less, so as to detach the fatty matter on the surface of the leather. When the whole of the pieces assume a uniformly white appearance, they are taken from the soak, and beaten with a mace, afterwards worked on the pommel till the grain is brought out, then cleaned with the sharp knife on the horse or beam, and finally, any creases removed by working and polishing them with the brass and glass slicker. This being done, the leather is slightly sprinkled with water, and, if necessary, cleaned and then hung up to dry. When it is required to color the product, the latter operation is not carried too far, so as not to necessitate its moistening again. The operator now rubs on the hair side, by means of a brush, a solution consisting of a sesquiacetate of iron 6, prepared by digesting scrap-iron in sour lies or beer, or of logwood, nut-galls, gum, and copperas 7.

Several coatings with such a composition are applied, the leather being partially dried after each, folded up, and worked under the stretching and polishing iron in order to give the finish. Before being sent out, however, it is rubbed over once or oftener on the grain side with a piece of woollen stuff dipped in sour beer or barberry juice, worked by the pommel and slicker, and finally, exsiccated. This kind of leather is much used by the saddler, trunk, harness, and coach makers. Of course, very many modifications in the order of application and execution of the foregoing treatment of the hides are practised by different parties, but the preceding general view of the operations will indicate the nature of the business.

Oil Leather.—Calf and cow leathers are curried in oil very extensively. The preliminary dipping, treating or working, et cetera, are applied, in order to give the substance more suppleness. Fish oil is recommended, but the French prefer the material usually employed for scouring skins that are intended for chamois leather, and which consists of fish oil and potassa. It is thicker than the ordinary oil, and becomes more rapidly absorbed; besides it gives greater pliancy and softness, and less is required than of the fish oil alone. Neat’s foot oil is likewise used, and it is said to add materially to the good qualities of the leather, but a proportion of fish oil must be mixed with it. When applying the dressing or oily matter, the skins should be slightly moist, so that, as they dry, the oleaginous portion may be gradually and thoroughly absorbed. Should they be too dry, the oil would be soaked too quickly, yet not properly combined so as to render it sufficiently resisting to water or moisture; on the other hand, excess of moisture must be guarded against, as in this instance too much oil would be expended, and the time would be prolonged in proportion. A coating of oil is uniformly spread over the grain and flesh sides, after which the hides are hung up to dry, the direct rays of the sun or too violent a draught being guarded against. In summer time the drying is effected in twelve hours, though in winter it sometimes requires three or four days. When dry, they are again fulled 8, and a further coating of fish oil mixed with the fatty scourings already mentioned, only in smaller quantity than in the previous case, laid on, and the process is repeated after the drying of the leather as before. Finally, the grain side is cleaned by rubbing it well with a brush and a weak potassa lie. The coloring of the grain or flesh side, as the case may be, follows these operations, and is effected by employing the dye already alluded to. After receiving one coat, the leather is pommeled crosswise; another layer of blacking applied; dried and pommeled, or stretched, according as it is deemed necessary; rubbed over with a cork pommel to give a smooth surface, and finished by giving a light coating of oil on the hair side.

Varnished Leather.—During the last few years a considerable quantity of a light glazed or varnished leather, intended for the manufacture of patent boots and shoes, has been introduced into the market. The French are celebrated for the beauty and superior quality of their manufactured product. This kind of leather is simply tanned kips 9, calf-skins, or split ox-hides of superior quality, well curried, and then passed through two other operations—one intended to render the leather impermeable to the varnish, and the other the laying on of the varnish. In currying leather intended for this manufacture, it is necessary that the fatty or oleaginous matter be laid on as thinly and evenly as possible, in order to insure the uniformity of the article being prepared.

Such curried skins are taken and rubbed over, either on the flesh or grain side, and a coating of boiled linseed oil mixed intimately with ochre, ground chalk, or some such substance, and dried; the coatings are renewed after each desiccation till it has received three, after which the surface is well rubbed over with a pumice-stone tool till it appears even, and the fatty and other matter is compressed into the pores of the leather. When this is done, other layers of a thinner material than the above, but of the same nature, are applied, and after drying, rubbed into the skin as before; by which means the adjacent part of the skin becomes thoroughly saturated with the fat, and an even surface is given to it. After this part of the work is executed, the prepared surface is coated over two or three times with a composition made with boiled linseed oil, but without mineral matters; subsequently adding lampblack and as much turpentine as will make it flow freely. This is laid on evenly with a fine brush, and allowed to dry after each coat, till a sufficiently black and shining surface is given to it; for which purpose it is rubbed over with a piece of woollen stuff and the finest kind of pumice-powder or Tripoli 10.

The leather is now ready to undergo the final operation or varnishing, which consists in laying on coats of a composition made of linseed oil and copal varnish, with which any coloring matter intended to show on the leather is incorporated. Generally this is composed of twenty pounds of boiled linseed oil, such as is employed in the first operation, twenty of turpentine, ten of thick copal varnish, and one pound of asphalte, Prussian blue, or ivory black in the finest possible state of division. In making it, the oil and tinctorial matter in fines powder are incorporated, then the varnish is added, and finally the turpentine, the whole being so well stirred as to insure a homogenous mixture. It is then set aside in a warm place fro two or three weeks, after which it is ready for use. It is necessary that the utmost cleanliness in regard to the manufacture be observed, and that no dust or other matters be allowed to float in the atmosphere, either in the apartment wherein the coatings are given, or in that of the drying chamber, as this would injure the smoothness and brilliancy of the article.

The number of varnish coatings applied varies, but five is the ordinary limit, the drying being effected after each in the chamber intended for the purpose, the temperature of which is kept at 160° and under. In this way a brilliant surface is given, and the leather has the property of resisting strains and of bending without the least crack or any injury to the even and perfect surface of the varnish.

It has been stated that the black varnish generally used for boot and shoe leather, is colored with lampblack and Prussian blue or ultramarine mixed with a little Krem’s white, and incorporated with the varnishes, answers well; lakes are used to give a red color; white lead for white; ochres for yellow, and so on, the shade being varied at will. An inferior quality of polished leather is made for belts and coach harness, by impregnating the skins with the linseed oil varnish, mixed at once with the lampblack and Prussian blue, copal varnish and turpentine. The linseed oil varnish is made by boiling, say, five gallons of this material with four pounds and a quarter of white lead and an equal weight of litharge, till the compound acquires the thickness of a dense syrup. For particular uses this is mixed with ochre, powdered chalk, or ground bones, as in the first operation for making a superior dress-boot leather.

Nossiter of Birmingham obtained a patent for making sheep and goat skin leather into this article. He splits it by machinery and, on the new surface produced by the section, forms a grain by polishing with a slicker, and then lays on the varnish. It is said that by this means a better surface is obtained than when the outside, either on the hair of flesh side, is operated upon.

Split Leather.—With reference to the preceding patent it may be here stated, that a considerable number of hides are now split by machinery, generally after receiving a slight tanning, and the divided parts afterwards fully tanned and curried, so as to adapt them for the manufacture of light boots, shoes, and other applications. It is almost unnecessary to remark, that leather which has been subjected to this operation, is much weaker than that which is manufactured in the entire state. A large proportion of cheap and inferior articles are formed of split leathers. Ox hides, kips, and skins, so split and tanned, are chiefly used by boot and shoe makers for the inner soles, by trunk and portmanteau makers, and in carriage facturing; whilst the sheep and goat skins similarly treated, are converted into the finer kinds of glove leather.

Next Page: Fancy and Speciality Leathers.


Footnotes

1. ^    A stone which shoemakers lay in their laps to beat leather upon (OED).

2. ^   A hundredweight (1 cwt) is 112 pounds weight — 50.80 kilogrammes.

3. ^   An abbreviation of Messieurs, more usually seen in English as Messrs.

4. ^   Muspratt mentions coach building from time to time and it may seem unclear as to how leather was significant in this. He treats the harness of the horses separately but in the carriage itself there were four main uses for leather: 1, in the straps by which the body was slung from the springs: 2, the inside of the body was lined and the outside sheathed in leather: 3, the seating was upholstered with leather: 4, the hood was made of leather. Not all vehicles had all these features but all were commonly found.

5. ^   This is a frame, more extensive than the skin to be extended on it.  Strings are tied to the edges of the skin and attached to pins or pegs set into the horse to allow the even stretching of the skin. It is sometimes called a herse, which would be the French name for it. Herse is a French word for a frame-like structure such as a portcullis or a traditional harrow. It is also used for the frame on which votive candles are set in most French churches. In its English form, hearse, it was the framework on which a coffin is set and over which drapes were laid, while it awaited its funeral. The name has nowadays, been transferred to a smart motor vehicle fitted out for the funerary transport of the coffin and body. See Part 9.

6. ^   This is probably iron(II) acetate.

7. ^   These presumably produce acetic acid.

8. ^   This is treading or beating (typically cloth but here, hides) for the purpose of cleansing and thickening (OED).

9. ^   A kip is the skin of a young or small beast (e.g. calf or lamb), as used for leather (OED).

10. ^ Tripoli is a fine earth used as a polishing-powder, consisting mainly of decomposed siliceous matter (OED).


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