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THE BEGINNINGS OF LEATHER CHEMISTRY F. L. SEYMOUR-JONES, 358 KNICKE~OCKER ROAD. ENGLEWOOD, NEWJERSEY
While color reactions between iron salts and theastringent principle of trees and galls were known to the Greeks, one may fairly start the history of leather chemistry with the Hon. Charles Howard who, about 1675, published a paper in the newly begun Phikxofihical Transactions of the Royal Society of London on "Oak Prepared for Tanning." He pointed out that any part of the oak (besides the bark), or indeed thorns or birch, could be used for tanning, and also gave instructions for the preparation of this material which foreshadowed the disintegrator. In 1754 one Glesditsch, a German botanist, investigated a wide variety of materials-from chestnut bark to rose l e a v e s a s sources of tanning materials. Lavoisier in 1768 also noted the tanning properties of oak leaves. Somewhat prior to this, in 1763, Dr. William Lewis attempted to analyze galls, reporting on the presence of an astringent substance, soluble in water and alcohol, which precipitated gelatin solutions and gave a black coloration with iron salts. Among this early miscellany comes an Irishman, Dr. David Macbride, who in experimenting on a cure for sea scurvy in 1767 was led to the astonishing discovery "that lime-water extracts the virtues of oak-bark more completely than plain water." He cited testimonials from tanners but since lime precipitates tannin the only feasible explanation seems to be that his water had considerable temporary hardness. The connection between curing scurvy and tanning remains obscure. Isolation of Tannin These early chemists were greatly interested in the astringent principle found in all tanning materials. At that time tanning was supposed to be "simply a physical process in which the astringent property of the bark caused the hide to shrivel, harden and become non-putrescible." This astringent principle was extracted from galls and its properties studied by Ruelle, Macquer, Giannoti, and Monnet, the "four academicians of Dijon," in 1777. Scheele in 1787 also extracted the principle, obtaining a mixture of gallic acid and tannin. D i d in 1791 confirmed Scheele's work and also obtained a resinous substance which on evaporation hydrolyzed to gallic acid. D6yeux in 1793 carried out elaborate experiments on galls and concluded that they consisted of extractive matter, a resin, a green coloring matter, gallic acid, and ligneous tissue. The first four were "in strong combination" and "to this body entire and not to any principle in particular" he ascribed the astringency necessary for tanning. Seguin in 1797 recognized that tanning materials contained a particular water-soluble principle which combined with skin to form a compound
insoluble in water. This he regarded as essentially the same as the precipitate obtained by the action of tanning materials on the gelatin from boiled skin. To him is due credit for distinguishing clearly between tannin and gallic acid. The following year Proust introduced the name tannin and isolated i t in an approximately pwe condition by precipitating i t with stannous chloride, afterwards removing the tin with sulfwetted hydrogen. S6guin's Experiments Armand Seguin was an interesting character living in an interesting and adventurous age. He was a personal friend of Lavoisier, which fact probably accounts for the nature of his theory of tanning. S6gnin considered that the steps were: (a) swelling of the skin by acid, ( b ) "disoxygenation by gallic acid," (c) disoxygenation of the skin by the same principle, (d) combination of the disoxygenated skin with the true tanning matter. The "disoxygenation of the skin"apparent1y meant the conversion of i t into a form of gelatin, which he knew gave an insoluble, imputrescihle precipitate with tannin. That Segnin was more than a theorist is proved by a "Report to the Committee of Public Safety on the new methods of tanning leather proposed by citizen Armand Skgnin," written by citizens Lelisvre and Pelletier in 1797. S6guin's chief innovation was the introduction of tan liquors. Previously hides and tan were piled alternately in pits and then water poured on, or else skins were sewed into a bag and filled with tan and water. Seguin leached his tanning materials and worked his hides up through a series of liquors of increasing concentration. This process had previously been used in England-in fact, was patented by Anthony Fay in 1790, whereas S6guin's patent is 1795. But Segnin introduced two further innovations, namely deliming and plumping in 0.07% sulfuric acid prior to tanning, and in rounding the hides-removing the neck and bellies and tanning them more cheaply. His process was favorably reported on as less costly, less laborious, and decidedly more rapid. He foreshadowed a coming industry in suggesting that tanning extracts he prepared and exported to France from the forests of French Guiana and America. Early Chemical Processes The precaution of insisting on practical examination of a scientific prmess before publication was permitted had then (as perhaps it might have now) much to recommend it. In 1788 or 1789 one Saint Real noted that boiling fresh hide extracted animal jelly whereas none was extracted on boiling leather. Hence he deduced the fact that the preliminary wet work-unhairing and swelling the skins in limes, barley infusions or old, spent, acid tan liquors-removed the animal jelly from the hides, thus
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rendering them less compact, easier to swell, and capable of combining with tan more readily. Therefore he heated his hides in changes of water at 60' until no more jelly was extracted, and then tanned a t 60'. Skguin's reporters investigated this and realized the folly of deducing too much from inadequate data by their further experiments which showed that jelly could he obtained by boiling a t any time prior to tanning and that this jelly combined with tan. Other and artificial sources of tanning materials were sought for even a t that date. Pseiffer in 1777 claimed to tan leather by treatmg it with successive portions of the distillate obtained from coal and peat, mixed with water. S6guin looked into this and found that while it unhaired and swelled the skin, it did not tan. Somewhat more reliable was the work of Charles Hatchett who in 1805 announced his "discovery of artificial tannin" to the Royal Society. He digested carbonaceous materialsawdust, bitumen, turpentine, coal, a wax candle, or even a piece of skin -with nitric acid and found that the dark astringent "resin" so produced would tan leather. Tannin Analysis Quantitative tannin analysis dates back to the Rev. George Swayne in 1792, who precipitated tannin with "martial vitriol." He ran into a snag in that the black iron-tannin precipitate passed the filter owing to the "close attachment of the vitriolic acid" to the particles (whereby we have ink and so to destroy this combination he added "mild salt of tartar" with which the acid "has a nearer affinity." Presumably the reverend gentleman could not pass in modem freshman quantitative, since he includes in his results an experiment in which "the filter caught fire while it was drymg." George Biggin in 1799 estimated tannin by precipitating with gelatin and checked his results by hydrometer readings and by precipitation with stannous chloride. Gallic acid he determined calorimetrically by boiling a skein of worsted in a gallic acid ferric sulfate solution. The Work of Davy Humpbry Davy, shortly after his appointment as head of the Royal Institution of London, was in 1801 granted three months leave of absence to learn sufficient about tanning to deliver lectures on "The Principles of the Art of Tanning." This proved so fascinating to his chemical mind that for some years he spent much time in trying "to elucidate this obscure hut most interesting part of chemistry." His work is so comprehensive that it can only be briefly summarized here. Of the quantitative side he discovered that the amount of the gelatintannin precipitate depended on the relative amounts and concentration of the reagents. Hence for analysis he used a standard isinglass solution
and a standard temperature. Gallic acid was determined colorimetrically with ferric sulfate and "extractive matter" by precipitation with aluminum chloride. He determined the amounts of the diierent tanning materials required to produce one pound of leather, thus ranking them by relative tanning value. He proved the adulteration of catechu extract from India. He showed that febrifuge barks (e. g., cinchona) were tannin free. He estimated tannin in tea. He showed that the innermost layer of hark was richest in tannin. Liming he showed to be due to the formation of an insoluble compound between epidermis and lime. The disgusting nature of bating (with dung-a process which still partially survives) led him to try substitutes. Weak solutions of potassium and ammonium carbonates worked well on a small scale but not in a tan yard. Tanning he regarded as a combination between hide, tannin, and extractive matter-a slow process being necessary to get pliable leather. Fifty years later Campbell Morfit of Philadelphia summed up the process, "to tan a skin is to saturate it with tannin in such a manner as to promote the slow combination of this principle with the gelatin, albumen, and fibrine contained in the former, so as to form with them a new compound. This reaction, in the operation of tanning, does not proceed spontaneously, but is the result of a slow process." This is much the same idea as Davy's, and were the modern chemical theory to be put in layman's language the above would almost be adequate.
The Chemistry of Tannin The birth of organic chemistry led to the study of the composition of tannin. Wuttig in 1816 claimed it as an acid, Berzelius by analysis of the lead salt in 1827 gave it the formula C18Hl&. He was the first to distinguish between the two classes of tannins by the color reaction with iron salts. Pelouze in 1834 introduced the etherial extraction of galls for its preparation, a method which with slight modifications is still official in the British Pharmacopoeia. Liebig the same year established the formula as C18HnOl2, which Pelouze, Dumas, and eventually Berzelius accepted. Pelouze noted that atmospheric oxidation of tannin gave gallic acid, while Robiquet in 1839 found the same result could be obtained anaerobically by a ferment in the galls. Larocque in 1841 discovered that alkaloids precipitated tannin and used quinine sulfate in tannin analysis. Stenhouse in 1842 subjected different tanning materials to destructive distillation and found some yielded pyrogallol while others did not. Thus he explained the blue and green colorations with iron salts and settled the much debated question whether all tannins were or were not impure varieties of a single substance. Mulder in 1847, in studying the relationship of gallic to gallotannic acid, established the latter as C I ~ H I ~which O ~ , formula lasted until very recent years.
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Mineral Tannages Alum tanning dates hack to prehistoric times while the chrome process is too recent for mention here. Mmeral tanning was, however, tried a t an early date. Samuel Ashton in 1794 patented a very mixed list of mineral mixtures, including iron, copper, and zinc salts, together with ochre and sulfur. Julius Bordier in 1842 patented "such metallic saline and earthy substances" as combine with hide, rather a broad and indefinite claim. He had, however, the germ of the modern idea of mineral tannage, for he preferred the use of basic sulfate of iron. Such is the story of leather chemistry up to 1850 and further than this we cannot now go.
