Chapter XVIII: Vegetable Tanning Materials (1)

As has been stated in the previous chapter, our knowledge of the chemistry of tannins is not sufficiently advanced to render possible any strictly chemical classification, while an additional complication arises from the fact that very different tannins may coexist in the wood, bark, fruit, galls, etc. of the same plant. It therefore seems best to follow the example of Prof. Bernardin in his ‘Classification de 350 matières tannantes,’[130] and arrange the plants under the orders of the natural system of botany, as has already been done by von Höhnel[131] and A. de Lof.[132] In the following pages, only those materials which from their high percentage of tannin, or from some other cause, are of commercial interest or value, are included, as the tannins are so widely distributed in the vegetable kingdom, that any exhaustive list would be quite out of the question.

[130] Gand, 1880.

[131] ‘Die Gerberinden,’ Berlin, 1880.

[132] ‘Matières tannantes,’ Halle aux Cuirs, Paris, 1890. See also
‘Agricultural Ledger,’ 1902, No. 1 (Government Printing Office,
Calcutta, 6_d._), by Mr. D. Hooper, which contains much valuable
information.

Tannins are not confined to any particular part of the plant, though they are usually most abundant in barks and fruits. Insect-galls are often very rich in tannin, usually gallotannic acid; while in several cases woods are of commercial importance from their cheapness, though the percentage of tannin they contain is not generally high. The function of tannins in the vegetable economy is not well understood. In some cases they are probably a waste product of plant-life, and may help to ward off attacks of insects. They usually exist as cell-contents, and as vegetable cells have frequently thick and impermeable walls, and the diffusive power of tannins is low, much time is required for extraction, unless the cells have been previously crushed or broken.

It would be beyond the scope of this text-book to describe in detail the structures of the tannin-yielding parts of plants; but barks are of such general importance, that some particulars seem desirable.

The detailed structure of bark varies greatly in different trees, though its general principles remain unaltered. One of the best short accounts of these is given by Prof. H. Marshall Ward on page 199 of his little book on ‘Timber and some of its Diseases,’[133] and further information may be found in Van Tieghem’s ‘Traité de Botanique’ and other works on structural botany.

[133] Macmillan & Co.

With regard to detailed structure of various tanning barks, ‘Die Gerberinden’ of von Höhnel[134] is one of the best authorities.

[134] ‘Die Gerberinden,’ Berlin, 1880.

The inner surface bark of a young tree, or twig, consists of a layer of soft and living cells resting on the outer surface of the wood, and called the _cambium_. These cells multiply by division (cp. p. 12) and produce from their inner surface the successive annual layers of wood, and on their outer a fibrous tissue called the bast (_phloem_), consisting of lengthened cells, and tubes with perforated divisions (sieve-tubes) which convey sap, and mostly run in the direction of the branch, but are crossed transversely by cells in a line with the medullary rays of the wood. All these cells when first produced in the cambium-layer have thin and soft cellulose walls, but the inner layer forming the wood becomes lignified, or hardened, by deposits of lignine on the interior of the cell-walls, while their contents of living protoplasm disappear. The outer layer forming bast remains much softer and more fibrous, and retains its vitality longer. The outer surface of the young branch is covered by a thin layer of flat cork-like cells forming the epidermis, developed from the growing tissue of the bud, beneath which is a layer of growing cells frequently called the _cork-cambium_. This produces, on its inner side, a layer of soft, juicy, thin-walled cells (_parenchym_), which are living and capable of growth, and contain protoplasm and often chlorophyll, to which the green colour of young twigs is due. This layer at first rests on the bast. On the outer side, the cork-cambium produces corky layers beneath the epidermis. The section of an oak-twig is shown in Fig. 43.

As the tree grows, it is obvious that the corky epidermis which grows in thickness, but not in breadth, must become distended and finally ruptured. In some cases the surface is renewed by fresh corky layers constantly developed below it, and then the bark remains smooth and unfurrowed, as in the beech and young oak, or in the birch, from which thin corky layers are continually peeling; or it may produce a thick layer of cork, as in the cork-oak. In many cases, and especially in older trees, the outer or primary layer of cork-cambium ultimately dies for want of nourishment, and a fresh cork-producing layer is developed in the still living parenchym. As cork is practically air- and water-proof, the new layer cuts off from its source of nourishment and kills all the parenchym exterior to it. In some cases this peels off, as in the Oriental plane (_Platanus_), but usually it forms a constantly increasing coat of dead tissue forming the “ross” or “crap” (Ger. _Borke_), which as it cannot increase in breadth, becomes deeply fissured as the tree becomes old. In some cases the new growing layer or secondary cork-cambium forms a complete coating parallel with the first, but more often it consists of a series of arcs convex towards the tree and cutting the primary cork-cambium at various places, so as to divide the tissue outside itself into scales. Later on the process repeats itself, new arcs forming inside the first, and cutting off further portions of the parenchym. In this way the cork-forming layer gradually sinks deeper and deeper into the bark, till it frequently passes even into the bast-layer, and very complicated arrangements of tissue result, in which corky layers from the secondary cork-cambium are interspersed with bast-cells and sieve-tubes.

As a rule the outer and dead part of the bark contains but little tannin, though to this there are exceptions, as, for instance, in the hemlock and Aleppo pines. It always contains a large proportion of dark colouring matters (reds, phlobaphenes, p. 297).

Cork consists of thin, and often roughly cubical cells, which are filled with air, while tannin is usually contained in somewhat similar cells with thicker walls. The walls of many vegetable cells are perforated with fine holes, and become thickened by internal deposits of hard ligneous matter which sometimes almost fill the entire cell (“stone-cells”). Bark-cells often contain starch-granules, frequently of peculiar and characteristic forms (which are easily recognised by the blue colour produced on treating the preparation under the microscope with a drop of a solution of iodine in potassium iodide), as well as crystals of oxalate of lime and other matters. These, and the form and arrangement of the cells as seen in sections under the microscope, form useful marks of recognition of the various barks. Tannin is most easily detected by staining, before cutting sections, with a solution of ferric chloride in absolute alcohol.

Apart from microscopic characteristics, the external appearance of barks, both to the naked eye and by the aid of a lens, forms a valuable means of recognition. The arrangement of the bast and corky layers, the remains of epidermis, or the form and character of the fissures, and of the lenticels or small corky protuberances which take the place of stomata in the epidermis, should be observed.

Space does not permit of any detailed account of the structure of fruits, wood and leaves, which are also cellular structures in many respects resembling the bark. The cuticles of leaves, and especially the stomata or breathing pores, and the hairs are often very characteristic. (Cp. Plates III. and IV., and p. 272.)

Valuable hints may also be obtained from the chemical reactions which are described on p. 70 _et seq._, L.I.L.B.

BOTANICAL LIST OF TANNING MATERIALS.[135]

[135] The percentage of tannin given where the source of information
is not stated must in many cases be regarded as uncertain, many
analyses having been made before the introduction of modern methods,
but those quoted as having been done in the author’s laboratory are of
recent date and have been made by the latest methods.

CONIFERÆ, Pines, Cypresses, mostly containing Catechol tans, yielding reds.

_Abies excelsa_, Lam. (_Pinus Abies_, _Pinus Picea_, _Picea vulgaris_, Link.), Norway Spruce. Fr. _Faux sapin_; Ger. _Fichte_, _Rottanne_. The source of the so-called larch-extract, and a principal tanning material of Austria. Contains 7-13 per cent. of a catechol-tannin and much fermentable sugar, and on this account is useful for swelling and colouring, but does not tan heavily. English and Scandinavian bark does not seem much utilised. Best bark 2-8 mm. thick; smooth, yellow inside, with reddish-brown ross outside. For detailed description of structure see von Höhnel, ‘Die Gerberinden,’ p. 35.

_Abies pectinata_, Silver Fir. Fr. _Sapin_; Ger. _Edeltanne_, _Silbertanne_, _Weisstanne_. Used to a limited extent, but apt to be confused with spruce. Contains 6-15 per cent. iron-blueing tannin. Used in Styria, Austria, Russia. Without “ross,” but silver-grey and smooth outside. (Von Höhnel, ‘Die Gerberinden,’ p. 40; ‘Gerber,’ 1875, p. 375.)

_Abies (Pinus, Tsuga) canadensis_, Hemlock Fir (Fig. 44). The principal American tanning material, and source of hemlock extract; averages 8-10 per cent. of a catechol-tannin, but variable, 18 per cent. reported, possibly from a different species. Abundant in Canada and the Northern and North-western States of America. The bark of old trees, which is principally used for tanning and extract-making, is 2-4 cm. thick, smooth and yellow within, greyish and deeply fissured without. The ross, which is red and thick, contains a considerable quantity of tannin, with much dark-red phlobaphen. It does not differ in structure from the inner living and yellow “flesh.” The bark is easily recognisable by its well-marked concave lamellæ of cork, cutting off successive layers of “ross” of several millimetres in thickness. (Von Höhnel, _ibid._, p. 42.)

--Hemlock Fir (_Tsuga canadense_).]

[136] Bastin and Trimble’s American Coniferæ, American Journal of
Pharmacy.

_Abies alba_ (_Picea alba_), White Spruce, North America. In character of tree and bark very similar to Norway spruce.

_Larix europæa_ D.C. (_Abies_ or _Pinus Larix_), Larch. Fr. _Mélèze_; Ger. _Lärche_. Contains 9-10 per cent. pale catechol-tannin, mild and suitable for light leathers. Used, especially in Scotland, for basil tannage.

_Pinus halepensis_, Aleppo Pine. An important tanning material of the Mediterranean coasts. The outer bark, stripped like cork from the living tree (_Scorza_ or _Cortegia rossa_), is a deep red tannage, and contains about 15 per cent. of tannin very similar to hemlock. It is largely used in the island of Syra. The inner and fleshy part of the bark, only obtained when the tree is cut, is _Snoubar_ or _Snobar_ bark, containing up to 25 per cent. of lighter coloured tannin. This bark is reddish brown, and pretty smooth on both sides, except for shell-like depressions on the outer surface. The “scorza rossa” is dark red-brown internally, grey and irregular outside, frequently very thick, and divided into successive layers of 1-2 mm. thickness by cork lamellæ. (Von Höhnel, _ibid._, p. 44.) In appearance the tree resembles the Scotch fir.

_Pinus tæda_, America; _P. Laricio_, Austrian Pine; _P. maritima_, Mediterranean; _P. Cembra_, Alps, Tyrol, 3-5 per cent.; _P. sylvestris_, Scotch Fir. Ger. _Kiefer_; Fr. _Pin sauvage_, 4-5 per cent. _P. longifolia_ Roxb., India, 11-14 per cent.

_Juniperus communis_, Juniper. Bark used in Russia.

_Podocarpus elongata_ and _Thunbergii_, Cape of Good Hope; _Geelhout_, Yellow woods.

_Phyllocladus trichomanoides_, New Zealand; _Tanekahi_, _Tarsekahi_, _Kiri-toa-toa_, “Golden Tan.” Used in dyeing glove-leather. Tannin, 30 per cent., gives green blacks with iron.

_P. asplenifolia_, Tasmania, Celery-topped Pine; 23 per cent. _Phyllocladus_ belongs to Yew family.

LILIACEÆ.

_Scilla maritima_, Squill. Tannin stated from 2-24 per cent. More valuable for pharmacy.

PALMÆ.

_Areca catechu_, Betel-Nut Palm of India. Yields a species of cutch of no importance for tanning.

_Sabal serrulata_, Saw Palmetto of Florida (Trimble). (“Dwarf” palmetto is _S. Adansonia_.) Palmetto root has been much talked of as a tanning material; and makes a light-coloured leather.

An extract is now made from the roots of the Saw Palmetto, which grows freely in the Southern States of America, and is especially abundant on the east coast of Florida. The plant is an evergreen, the stem growing flat along the ground, being held in place by numerous roots each the size of a pipe-stem. The leaves are fan-shaped and ribbed, and two to three feet in diameter. In its hardihood the palmetto resembles a weed, as the leaves may be cut off quite close to the stem without damaging the plant, which will grow freely on poor sandy land which is worthless for other purposes. The average yield is stated to be about 10 cwt. to the acre, but in good seasons and with rich land, over a ton per acre has been obtained.

The air-dried leaves contain about 13 per cent. of tannin, but the results obtained by different chemists vary from 5 to 20 per cent. Possibly these variations are caused by the different amounts of moisture in the various samples.

The leaves must be treated with a solution of caustic soda, to remove the glossy siliceous shield which covers them and prevents their being easily extracted. After the tanning matter has been extracted the remaining fibre can be profitably disposed of to paper and rope manufacturers.

As the supply of palmetto is very large it is likely that it will, to a considerable extent, substitute the employment of gambier, and in the United States the extract has already met with a considerable sale. Samples of the extract examined by the Author analysed from 16-22 per cent. of tanning matter, and several per cent. of mineral matter, and produced a very soft and mellow leather of good colour. The extract contains noticeable quantities of common salt, and organic salts of soda which leave sodium carbonate on ignition.

_Cocos nucifera_, the Cocoa-nut Palm, also contains tannin in roots.

CASUARINÆ.

_Casuarina equisetifolia_ L. (_laterifolia_ Lam.); _Filao_ bark, Reunion; _Tjamara laut_, Java; Casagha or Tinian Pine, Ceylon. Widely distributed in Southern Asia, bark used for tanning and dyeing. Tannin gives blue-blacks with iron. Several other species very similar in structure and properties. (Von Höhnel.) Hooper found 11-18 per cent. of tannin.

MYRICACEÆ.

_Myrica Gale_, Sweet Gale, or bog-myrtle.

_Myrica (Comptonia) asplenifolia_, U.S.A.; “Sweet Fern.” Covers millions of acres in Michigan. Yields 40 per cent. of “extract.” Leaves 4-5 per cent., roots 4-6 per cent. tannin, according to season (Trimble). Has been much talked of, but in Prof. Trimble’s opinion is not likely to prove of much importance.

_Myrica nagi_ (Hind. _Kaiphal_), India, contains 13-27 per cent. of tannin in the bark, and a colouring matter, myricetin, identical with that of sumach.[137] Leather tanned with it is of a somewhat reddish colour which is much brightened by sumaching, and converted into a pale yellow by treatment with alum. It promises to be a valuable tanning material.

[137] Perkin and Hummel, Trans. Chem. Soc., 1896, p. 1287.

BETULACEÆ.

_Alnus glutinosa_, Common Alder. Fr. _Aulne_; Ger. _Erle_. Contains 16-20 per cent. iron-green tannin, with much red colouring matter; old barks as low as 10 per cent. Colour develops during and after tannage. Used alone it gives a red, hard and brittle leather, but with galls, valonia, etc. it produces a satisfactory tannage. Its principal use is to furnish gunpowder-charcoal, and it is possible the bark might be obtained from powder-factories, if the use of gunpowder is not superseded by nitro-compounds. (Von Höhnel.)

_Alnus maritima_, _Hannoki_, Japan; and _A. firma_, _Minibari_. Fruits (_yashi_) contain 25 per cent. tanning matter (iron-blueing), and little colouring matter. Used in Japan for dyeing and tanning. _A. nepalensis_ and _A. nitida_ used in India. Several other species of _Alnus_ contain tannin.

_Betula alba_, White or Common Birch. Fr. _Bouleau blanc_; Ger. _Birke_. Inner bark used in Scotland (in conjunction with larch for tanning sheep-skins), Norway, Russia, etc. It contains only 2-5 per cent. of iron-greening tannin, and much fermentable sugar. By far the most important use of birch bark in tanning is to produce the birch-bark tar used to give scent and insect-resisting power to “Russia” leather (_Youft_; Ger. _Juchten_). The outside bark consists of thin layers of cork, often white with a crystalline deposit of betulin, which when distilled yields the odorous oil. The distillation is a dry one, and tarry products accompany the true oil, and at first give a strong empyreumatic smell to the leather, which it loses by keeping, while the true “Russia” odour remains. This “ageing” may be hastened by hanging the leather in a hot stove. If the oil is distilled in a current of steam, or with petroleum ether, the tarry matter passes over, while the matter giving the true odour remains in the retort (p. 372).

_Betula lenta_, American Black Birch. The bark and twigs distilled with water yield an essential oil, which is almost pure salicylate of methyl, and largely substituted for oil of wintergreen (_Gaultheria procumbens_), with which it is chemically identical. Used for perfumery, and as a rheumatism remedy. Often erroneously spoken of as the source of “Russia” oil. A mixture of a trace of wintergreen oil with sandal-wood oil considerably resembles the “Russia” scent (p. 373).

CUPULIFERÆ.

_Castanea vesca_, True or Spanish Chestnut. Fr. _Châtaignier_; Ger. _Kastanie_. Abundant in Italy, South of France and Corsica, where it forms great forests. Bark said to be nearly as strong in tannin as oak (up to 17 per cent., de Lof), but not much used in tanning.

Wood only contains 3-6 per cent. tannin, but is the source of the valuable chestnut extract, first employed for dyeing, and introduced as a tanning agent by Aimé Koch. The strength of extract is of course very variable, even for the same density (see p. 339), but it usually contains from 28 to 32 per cent. of tannin.

The tannin gives blue-black with iron, but is not identical with either oak-bark or gall tannins, but apparently a mixture, or possibly a methylated derivative of the latter, and identical with oakwood tannin, or so nearly so as to be indistinguishable; it may also be identical with divi tannin. Decolorised chestnut extracts, sometimes mixed with quebracho and other materials, are often sold as “oakwood” or “oak-bark” extracts. The extract gives a firm leather, with a good deal of bloom if used strong, and a more reddish tint than valonia. The extract often contains dark colouring matters, and the colour of leather tanned with it is readily darkened by traces of lime derived from calcareous waters or imperfectly delimed hides. Like all wood-extracts it tans rapidly, the colour penetrating first, and the tan following, but, according to Eitner, it does not, alone, make full or solid tannage, perhaps from want of acid-forming matters, but answers particularly well in combination with spruce-bark. It is largely used in England for sole-leather in combination with valonia, myrobalans and other materials.

The higher the temperature of extraction, the more colouring matter is contained in the extract in proportion to tannin matter and the greater is its viscosity. Much colouring matter remains undissolved if the extract is dissolved in cold water, but there is, in addition, a loss of tanning power, the colouring matter being also capable of combining with hide. It has in fact been used for tanning by dissolving it in solutions of borax or alkaline salts. By improved methods of manufacture the colouring matter has been much reduced.

The chestnut is an important food tree, the nuts forming a considerable part of the food of the inhabitants of Corsica and Sardinia, and even of Italy.

Oaks.

Almost all species of oak contain useful quantities of tannin in the bark, and probably in the wood. Most if not all oaks yield catechol-tannins with, probably, some mixture of ellagitannic acid.

_Quercus robur_, Common Oak. Fr _Chêne_; Ger. _Eiche_. It is frequently separated into the two subspecies:--

_Quercus pedunculata._ Commonest oak of lowlands, England, Ireland and Scotland. Acorns in bunches or spikes on a stalk ¹⁄₆ inch long, hence Ger. name, _Stiel-Eiche_. Leaves sessile or short-stalked. In favourable situations, _said_ to yield about 2 per cent. more tannin than _Q. sessiliflora_, but this is doubtful. It is the commonest oak in Slavonia, and the source of commercial oakwood-extract.

_Q. sessiliflora_, Ger. _Traubeneiche_. Common in hilly districts, and scattered throughout the country. Acorns in bunch on the branch, or with very short stalk; leaves on stalk ¹⁄₂-1 inch long.

Of English barks, Sussex and Hampshire are considered the best, and contain up to 12-14 per cent. of tanning matter; a coppice bark from Wastdale, Cumberland, is however, recorded to have yielded 19 per cent. tanning matter (Hellon).

Probably each of the two varieties of oak gives best bark where it thrives best (v. Höhnel).

Belgian bark is sometimes equal to English, and contains 10-12 per cent. tanning matter. Dutch bark as exported is generally inferior and not cleaned; Swedish is bright, but very poor.

Oak-bark contains a tanning matter, quercitannic acid, giving green-blacks with iron salts, and possibly containing both catechol and pyrogallol groups, but its constitution is not fully understood. It yields both red anhydrides and ellagic acid; and gallic acid has been obtained by the action of hydrochloric acid, though not by fermentation in the tannery. The tannin is not a glucoside, but the fact that a sugar, lævulose, is also present in the bark has led some observers to erroneous conclusions regarding the constitution of the tannin. The unpurified infusion of the bark of _Q. robur_ gives a blue-black with iron-salts, from the presence of a colouring matter; but those of most other oaks give green-blacks.

Most tannin is contained in the living part of the bark. The yield diminishes in trees over twenty-five years, and coppice barks, from absence of ross, are often strong, and also contain less colouring matter and more fermentable sugar.

Warm and rich soils seem to yield the best barks.

The brighter the colour of the fresh cut “flesh,” the better the bark. Dark brown inner side shows that bark has been exposed to rain, which deteriorates strength and colour; but a very light colour is thought by some to indicate poorness in tannin. White lichen is said to be a mark of poor bark, and probably indicates a damp and unfavourable situation.

Oaks are generally cut when the sap is rising (15th April to 15th June), and the buds open and new soft cells begin to grow, for the bark is then more easily detached.

Experiments in France have shown that the bark of timber felled at other seasons may be loosened by steaming, and it is said there is no practical loss of tannin. Superheated steam, produced in a small boiler in the woods, is used.

The bark is peeled with tools of various forms, the branch and knotted places being loosened by beating with a mallet. The bark must be peeled immediately the tree has been felled.

The peeled bark, in pieces up to three feet long, is laid on hurdles sloped in such a way that the rain runs off as much as possible, and in this way it is dried, but in wet seasons is much damaged. Bark so dried in the woods often retains 40-50 per cent. water, and must be stacked or stored so as to allow of further drying.

English bark is sometimes sold in “long rind,” and sometimes “hatched” or chopped in pieces about four inches long. Belgian and Dutch barks are generally hatched. Belgian tree bark is “cleaned” (and cleanings often mixed back with bulk), Dutch bark is not cleaned. Much sand and dirt is contained in most Continental bark: screenings of Belgian bark yielded a black liquor, and contained so much sand that they would not even burn!

_Oak-bark extract_ is occasionally offered for sale, but is not usually genuine or of good quality, except that of the American chestnut-oak, _Q. prinus_, from which an excellent extract has been manufactured in the Alleghanies. Factitious extracts often contain myrobalans and quebracho.

_Oakwood_ contains only a very small percentage (from 2-4 per cent.) of a tannin, practically identical with that of chestnut, but different to that of oak-bark. It is stated by de Lof to reach 9-14 per cent. in old heart-wood; but this is doubtful. The wood retains the tannin in its interior for a long time. Wood of a Roman bridge built at Mainz 55 B.C. is stated by de Lof to have still contained 2·14 per cent. tannin in 1881 A.D. A good deal of imitated oakwood extract is undoubtedly made from chestnut wood, and unfortunately no very satisfactory way of distinguishing it is known, though oak-bark extract can be distinguished from oakwood by giving a precipitate at once, even in dilute solution, with bromine-water, while the wood gives a brown precipitate only after long standing. Precipitation by bromine-water is a general characteristic of catechol tannins, and hence a mixture of quebracho (a cheap catechol tan) with chestnut would simulate oak-bark in this respect. If a few drops of the non-tannin solution or an alcoholic extract from the “total soluble” of extracts containing quebracho or other catechol tannins be treated with concentrated sulphuric acid in a test-tube, a deep crimson will be produced, especially at the surface of the acid, which remains pink on dilution with water. With pyrogallol derivatives, such as genuine oakwood, a yellow or brown only is produced (J. Hughes). The test is very delicate. Another distinction is that bark extracts contain perceptible traces of manganese, but this cannot be relied on as many wood extracts also contain some, probably derived from the twig and branch bark which is used along with the wood. Oakwood extract is now manufactured on an enormous scale in Slavonia, and is used both by sole- and dressing-leather tanners, chiefly to increase the strength of the layer liquors. The extract is also used to increase the weight of leather after tannage by mopping on the flesh-side. All the best oakwood extract manufacturers contract to sell on analysis and colour estimation, and good Slavonian oakwood extract generally contains 26-28 per cent. of tanning matter, giving a tintometer measurement of 4-5° red, and 20-25° yellow, when a solution containing ¹⁄₂ per cent. of tanning matter is measured in a 1 cm. cell. For particulars of the manufacture of concentrated extracts see p. 337.

_Q. cerris_, Turkey Oak. Ger. _Zerreiche_. Common in southern Europe, a fine tree, but bark inferior to _Q. robur_. Fig. 45.

_Q. pubescens._ Fr. _Chêne velu_; Ger. _Weiss-_ or _Schwarzeiche_. In mountain districts and scattered in Southern Europe, about equal to _Q. robur_.

_Q. ilex_, Evergreen Oak. Fr. _Chêne vert_, _Chêne yeuse_; Ger. _Grüneiche_, _Steineiche_; Span. and Ital. _Encina_. South Europe, Algeria. Said to be somewhat stronger in tannin than common oak, yielding 5-11 per cent. of a rather darker coloured tannin, but well adapted to sole-leather. Good bark is smooth outside, without fissures, short in fracture.

_Q. Suber_, Cork Oak. F. _Chêne liège_; Ital. _Sughero_, _Suvero_. (Figs. 46, 47.) The outer bark is cork; the interior bark contains 12-15 per cent. of tannin which is redder than that of ordinary oak. Trees at first produce an irregular cork, sold as “virgin cork” for ferneries, etc. After this is stripped, later growths are more uniform, and fit for use; tanning bark is only obtained when the tree is cut down. Bark is rough but pale-coloured on both sides and about 1 cm. thick; interior like ordinary oak, but more strongly furrowed. Produced chiefly on Mediterranean coasts, and formerly largely used in Ireland.

_Q. pseudosuber_, African Oak. Fr. _Chêne faux liège_. Algeria. Not stronger than English oak, but with more colouring matter, hence strikes quickly through leather. Bark very thick.

_Q. Mirbecki._ Fr. _Chêne Zeen_. Algeria. Rapid growth. Bark contains 8 per cent. of tannin.

_Q. Tozæ._ Fr. _Chêne tauzin_. Pyrenees and S. France. Bark contains 14 per cent. of tannin.

_Q. coccifera_, Kermes Oak. Fr. _Kermes_, _Garouille_ (Fig. 48). South Europe and Algeria. Root bark is called “rusque” or “garouille”; averages 10-18 per cent. tannin, but trunk bark does not exceed 11 per cent. This tree is the food of the kermes insect, used for dyeing scarlet before the introduction of cochineal. Garouille is principally used in the south of France, giving a firm sole-leather of a disagreeable odour and dark brown colour.

_Q. Ægilops_ (and probably other species--_Q. macrolepis_, _græca_, _Ungeri_, _coccifera_), Valonia. Fr. _Valonée_; Ger. _Valonea_, _Ackerdoppen_, _Orientalische Knoppern_. Best Smyrna contains up to 40 per cent., Greek 19-30 per cent., Candia valonias up to 41 per cent., and Caramanian (probably not _Q. Ægilops_) 17-22 per cent. of tannins which are at least principally pyrogallol derivatives and which give blue-blacks with iron, no precipitate with bromine-water, and which deposit a great deal of bloom consisting of ellagic acid.

_Q. Ægilops_ (Fig. 49) is said to be most abundant in the highlands of Morea, Roumelia, Greek Archipelago, Asia Minor and Palestine, while _macrolepis_ forms large forests in many parts of Greece, and especially on the lower slopes of Mount Taygetos. In Asia Minor the fruit ripens in July-August, when the trees are beaten and acorns left on the ground to dry. They are afterwards gathered, and carried on camels to stores in the towns, and thence by camel and rail to Smyrna, where they are placed in heaps 5-6 feet deep in large airy stores, and allowed to ferment and heat for some weeks, when the acorn, which contains but little tannin, contracts and falls from the cup, and is used for feeding pigs. This fermentation is risky, and if carried too far the cups become dark-coloured and damaged. The acorn contains a considerable amount of fermentable sugar.

When ready for shipment, the valonia is hand-picked, the largest and finest cups (prima) going to Trieste, the second selection to England (Inglese), and the remainder, known as “natural,” also coming largely to England. The “Inglese,” although inferior in appearance to the very large selected cups, is, of course, less costly, and gives an equal yield of tannin.

In 1887, Smyrna exported about 23,000 tons to England, and 16,000 tons elsewhere, principally to Austria, Germany and Italy. The largest known crop is stated at 70,000 tons in Asia Minor, and 14,000 in Greece, but the average yield is considerably less than this.

The beard contains considerably more tannin than the cups, sometimes over 40 per cent. It is often sold separately at the same or a lower price, and in Smyrna is known by the Turkish name _tirnac_ (Ital. _trillo_).

In Greece the best valonia is collected (in April?) before the cup is matured and while it still encloses the acorn, and is known as _chamada_ (It. _camata_ and _camatina_). The colour of these kinds is excellent and the percentage of tannin high. Mainly used by dyers, but often worth attention for tanning where colour is important. In camatina the acorn is completely covered in the cup, while in camata it is partly exposed.

The next quality, _rhabdisto_, is beaten down by sticks in September-October (hence name), while after the first rains the fruit falls and turns black, and is called _charcala_. It contains but little tannin, and is not generally collected.

Sometimes valonia is attacked by a sort of honeydew, probably caused by an aphis, which renders it very sticky, and perhaps more liable to heat, but does not in itself damage its tanning properties.

The lighter the colour, the heavier the weight, and the thicker the scales of the beard, the better the quality usually proves, but analysis is the best guide. Caramanian valonia is very inferior.

The tannin contained in valonia is especially suitable in the manufacture of sole-leather. It deposits much bloom, and if used as a dusting material, has the characteristic of making the leather solid and compact, but leaves the grain somewhat rough and hard to work. In mixture with gambier and other materials, it is an excellent tannage for dressing leather, and with proper management deposits little or no bloom (cp. p. 231).

_Q. infectoria_ (Fig. 50) is the source of the “Turkish” or Aleppo galls. Galls are caused by insects, principally of the genus Cynips, or gall-wasps, which lay their eggs in different parts of plants, and in some way cause an abnormal growth of the bud, leaf, or other part.

_Aleppo galls_ are developed from the young shoot of the oak, are best before the insect has escaped, and contain in this stage up to 50 or 60 per cent. of gallotannic acid. When the insect has developed and escaped, the galls are of course perforated, much lighter, and more porous. These galls and those of _Rhus semialata_ are the principal sources of the tannin of commerce.

The _Q. infectoria_ also bears a large gall like an apple, “Apples of Sodom,” or “rove,” caused by a different insect. This, in a crushed condition, has been somewhat largely used as a tanning material, and contains 24-34 per cent. gallotannic acid.

English oaks have several species of galls and oak-apples, but they do not seem to be of much value for tanning.

_Knoppern_ are galls produced on the immature acorns of various species of oaks, principally _Q. Cerris_ in Hungary, and were formerly largely used there for tanning, as they contain up to 35 per cent. gallotannic acid. They are now less abundant, and have been largely replaced by valonia, sometimes called _orientalische Knoppern_. Like all purely gallotannic materials, they naturally give a soft and porous tannage, ill-adapted for sole-leather purposes, which has led to the Austrian practice of drying, or rather stewing, the leather in very hot and damp stoves, which make it hard and brittle.

_Chinese_ and _Japanese galls_ are the product of the action of an aphis on a species of sumach, and will be mentioned again under sumachs (_Rhus_).

_Djaft_, _dchift_, _jift_, or _jaft_ is a material apparently of Eastern origin, and said to be derived from an oak of Kurdistan. Dark red scales or fragments, origin uncertain, very astringent and darkish tannage, liquor when spilt dries whitish, apparently from crystallisation of some sort. It contains a large amount of tannin. It appears very irregularly in commerce and the writer would be glad to obtain further samples and details of origin. He once used 6 or 7 tons successfully in sole-leather tannage. It has also been attributed to a shrub allied to the _Cæsalpinias_ (p. 286).

The most important American oaks are--_Q. prinus_ (_castanea_, _monticola_), the Chestnut or Rock Oak (Fig. 51). About equal to our oak in strength, bark very thick, and infusion strongly fluorescent, especially in presence of ammonia. Source of chestnut-oak extract. The most important tanning oak-bark of the United States.

_Q. alba_, or “white oak,” is perhaps the most widely distributed and abundant of any of the American oaks, and very closely resembles the European _Q. robur_.

_Q. tinctoria_ or _nigra_, Black or Quercitron Oak. Poor as a tanning material, but used for dyeing yellow, and for modifying the colour of hemlock tannages. The dyestuff, _quercetin_, is closely allied to that of fustic, and gives yellows with alum and tin mordants.

A good deal of information is given by Trimble[138] on American oaks and other tanning materials.

[138] ‘The Tannins,’ vol. ii., Lippincott, Philadelphia, 1894.

Important Indian oaks are _Q. glauca_, _Q. lamellosa_ and _Q. incana_; bark of last said to yield 22 per cent. of tannin.

SALICACEÆ, Willows.

The bark of various willows, especially _Salix arenaria_ and _Russeliana_, is used for tanning in Russia, and for Danish glove-leather. Some contain up to 12-14 per cent. of an iron-blueing tannin. They impart a strong odour to leather, but different to that of birch-tar oil, and the scent of genuine Russia leather is due to a combination of both. In many cases the bark peeled off osiers for basket-making is employed. A Russian willow (species unknown), in the form of thin bark of osiers or small branches, gave 9·5 per cent. tannin when examined in the Leather Industries Laboratory of the Yorkshire College; and willow barks certainly demand more attention than they have received in England as tanning materials for fine leathers. _Salix caprea_ has been used in France for glove leathers, but is weaker than _S. arenaria_.

Poplars belong to the same natural order, and have been used for tanning, but their barks at the most contain 2-3 per cent.

POLYGONACEÆ, Docks.

Most members of this family contain tannin.

_Rumex hymenosepalum_, Canaigre, Gonagra (Cana agria), Red Dock, wild pie-plant (Fig. 52). Common in sandy alluvial plains of Mexico and Texas, and considerably resembling rhubarb. Its tuberous roots resemble those of the dahlia, and contain, when air-dried, 25-30 per cent. of a catechol tannin, probably allied to that of mimosa. Undried, the roots contain about 68 per cent. of water and only 8 per cent. of tannin. When well harvested by slicing thin and rapidly drying, it gives leather a bright orange colour, and, it is said, considerable weight and firmness, and is thus specially suitable for use in retanning and finishing light goods and harness leather. Besides tannin, the root contains a yellow colouring matter, and about 8 per cent. of starch, of which the granules are very variable in form and size, but mostly oval or elongated. They do not stain readily with iodine till they have been well washed, or treated with dilute sulphuric acid. Both the starch and tannin are contained in large and somewhat thin-walled cells, and the sliced material is easily extracted at low temperatures. Greater heat gelatinises the starch, and extracts a darker colour. The best temperature for extraction is between 30° and 50° C. (see p. 348).

The root is most readily grown from tubers or portions including the crown, as the plant seeds sparingly. Sandy soils, subject to inundation or irrigation, seem best suited to its culture. In California and Arizona the growth begins in October or November with the winter rains, blooming about the end of January, while the leaves die down in May and the roots remain dormant during the summer. It is not important at what time the roots are harvested, and they seem to improve in percentage of tannin up to the second year, after which they become darker and deteriorate.

The harvested crop should be sliced into thin pieces and rapidly dried at a low temperature, or still better, converted at once into extract. This is already done on a considerable scale at Deming, New Mexico. The residue after extraction is used in America as cattle-food; and might no doubt be also applied to the production of alcohol.

Planting takes place in autumn, in rows, say 30 inches apart, with 10 inches between each root. Roots for “seed” should be kept in the ground or stored in dry sand. This should yield a crop of 10 tons per acre in an average season.

_References._--Report U.S. Commissioner of Agriculture, 1878, pp. 119 _et seq._; Trimble, Am. Jour. of Pharmacy, p. 395, 1889; Canaigre, Bull. No. 7, Arizona Agr. Expt. Station, 1893; ‘Canaigre or Tanner’s Dock,’ Bull. No. 105, University of California, Berkeley, Cal.; ‘Canaigre Tannin,’ Trimble and Peacock, Philadelphia, 1893; ‘Report to the German Leather Trades Association,’ by V. Schroeder, 1894; ‘Il Canaigre,’ E. Andrieis, Turin, 1899.

_Rumex maritima_, or _maritimus_. Central Europe, England, Ireland. It is said by de Lof to be found in California, where it is used by the Indians for tanning; but he probably confounds it with canaigre. De Lof found its roots, wet, to contain 6 per cent. and after drying, 22 per cent. of tannin, together with starch and an acid allied to malic.

Several English docks contain tannin; the writer had a sample of leather tanned with dock-root (very possibly _R. aquaticus_), many years old, but still soft and close in texture, and of excellent quality.

_Polygonum amphibium._ Said to grow on thousands of acres (?) on the lower Missouri. Roots contain 22 per cent., branches 17 per cent. of tannin. _P. amphibium_ is a common English and European plant, with spikes of pink flowers, growing in marshes and ponds. Probably this is the _Polygonum_ analysed by Fraas, who found 20-26 per cent. tannin.

_Polygonum Bistorta._ Common in damp places in England. Bistort, Snakeweed, called “Eastermer giants” in Cumberland, where the young leaves are used for making herb-puddings. Fraas found 16-21 per cent. tannin in the roots.

Other species are known to contain much tannin. Perkin found a red colouring matter in _P. cuspidatum_, a native of India and China, commonly grown in gardens as a foliage plant (Journ. Chem. Soc., 1895, p. 1084). _P. tinctorium_, used as a source of indigo in China and Japan.

_Coccoloba uvifera_, Seaside Grape of West Indies; source of West Indian kino. Whole plant rich in tannin.

LAURACEÆ, Bay Family.

_Persea_, or _Laurus lingue_. Bark used in Chili for tanning Valdivia leather. (According to Arata, _Laurus caustica_.) A tree 25-30 feet high and 2 feet in circumference. Bark rough outside, and whitish, with an aromatic smell and taste, brittle and easily ground, contains 17-19 per cent. of a catechol-phloroglucol tannin, greening iron salts (Journ. Chem. Soc., 1881, p. 600). About 60,000 heavy hides are tanned yearly with this bark in Valdivia and district, and mostly sent to Hamburg. The hides are thick and scarcely tanned through, colour fair, leather soft and porous.

_Persea Meyerina_ N. and _Laurus Pneumo_. Said to be also used in Chili.

SANTALACEÆ.

_Osyris compressa_ (_Fusanus compressus_, _Colpoon compressum_, _Thesium Colpoon_), “Cape Sumach,” “_Pruim Bast_,” leaves and bark, Cape of Good Hope. Leaves contain about 23 per cent. of tannin and form a useful substitute for sumach; but the tannin is not identical, and is of the catechol class, resembling gambier.

_O. arborea._ Northern India. Leaves rich in tannin.

_Fusanus acuminatus_ (_Santalum acuminatum_), “_Quandony_.” Australia. 18-19 per cent. tannin, dark coloured.

_Exocarpus cupressiformis._ Australia. Bark contains 15 per cent. tannin.

DAPHNOIDÆ, Spurge Laurels.

_Daphne Cnidium_ L., “_Garou_.” Algeria. Used for dyeing and tanning.

PROTEACEÆ.

_Banksia serrata_, Heath Honeysuckle. Australia. Specimen examined contained 11 per cent. tannin; according to Maiden it reaches 23 per cent.

_Banksia integrifolia._ Queensland. Bark contains 11 per cent. tannin.

_Grevillia striata._ Australia. Bark contains 18 per cent. tannin.

_Leucospermum conocarpum. Kruppelboom._ Knotted Tree. Cape of Good Hope. Said by de Lof to contain 22 per cent. of tannin; but a specimen examined by the Author yielded 10·9 per cent. on analysis.

_Protea mellifera._ Sugarbush. _Suikerbosch._ Cape of Good Hope. Contains 25 per cent. tannin, according to de Lof; but Palmer found 18·8 per cent.

_Protea grandiflora. Waagenboom._ Cape. Contains 25 per cent. tannin (de Lof); 15·9 per cent. (Palmer); 15·6 per cent. (Procter).

_Protea speciosa._ Cape of Good Hope.

_Leucadendron argenteum_, Silver Tree, _Silverboom_, _Witteboom_ Cape of Good Hope. Bark said to contain 16 per cent. tannin (de Lof); a specimen examined by the Author yielded 9·2 per cent.

_Brabium stellatifolium_, _Wilde Amandelboom_, Wild Almond.

PLUMBAGINÆ.

_Plumbago Europea_, Leadwort. Fr. _Dentelaire_. A garden plant in England, native in France; contains much tannin, especially in the root-bark.

_Statice coriaria_, Marsh Rosemary. South of Russia. Roots up to 3 metres long and 2-12 cm. thick; used by Kalmucks for tanning sheep-skins; contain 22 per cent. of tannin (de Lof).

_Statice limonum_, Sea Lavender. Coasts and salt marshes of Europe and America. Richer in tannin than _S. coriaria_; used in France, Spain and Portugal.

Several other species contain tannin. These plants are allied to “Thrift” (_Armeria_).

MALPIGHIACEÆ.

_Byrsonima spicata_, Antilles, “Tamwood.”

_Byrsonima coriacea_, Jamaica, “Golden Spoon.”

_Byrsonima chrysophylla_, etc.

_Malpighia punicifolia_, Nicaragua, “Nancite”; “Mangrutta.” Bark contains 20-30 per cent. of light-coloured tannin.

POLYGALACEÆ, Milkwort Family.

_Krameria triandria_, Rhatany, Peru.

The root is used in medicine, and is stated to contain 40 per cent. of tannin.

Wittstein found only 20 per cent. of an iron-greening catechol-phloroglucol tannin allied to tormentil tannin, in the root-bark, the only active part of root.

ANACARDIACEÆ.

_Loxopteryngium Lorenzii._ Span. _Quebracho colorado._ South America, especially Argentine Republic; the highest proportion of tannin occurring in the wood from Gran Chaco district. Wood contains on an average about 20 per cent. of a red, difficultly-soluble tannin, yielding “reds,” and containing catechol and phloroglucol. The tannin is not very soluble in water, and hence can only be used in weak liquors, but is very astringent, and gives a firm, reddish leather. The wood also contains a catechin[139] and a colouring matter, fustin, identical with that of “young fustic.” It is imported into England, and more largely to Havre and Hamburg, in logs, which are there chipped like logwood, and either used direct for tanning, or made into extract. A very cheap tan. With alum it gives a yellow colour. The extract usually dissolves to a fawn-coloured turbid solution. Many quebracho extracts are now made completely soluble by treatment with alkalies or sulphites (cp. p. 338).

[139] See P. Arata, Journ. Chem. Soc., 1878, A, p. 986; 1881, A, p.
1152; and Perkin and Gunnell, Trans. Chem. Soc., 1896, 1303.

“Quebracho” means “axebreaker,” and is consequently applied to a variety of hard woods. Its specific gravity is 1·27-1·38, and it therefore sinks in water.

_Pistacia lentiscus_, Ital. _Pistacio_, Fr. _Lentisque_. Sicily, Cyprus, Algeria. Small myrtle-like leaves contain from 12-19 per cent. of a catechol-tannin, and are very largely used in the adulteration of sumach. Leather tanned with sumach adulterated with this material darkens and reddens on exposure to light and air, and for this reason its use in many cases is decidedly injurious. In Cyprus and the East it is known as “_Skens_,” Ital. _Schinia_, Fr. _Poudre de Lentisque_, in England, often called Cyprus sumach. (Cp. p. 272.)

_P. orientalis_, _terebinthus_, _vera_, etc., India, Mediterranean. Various aphis galls, 30-40 per cent. tannin. A sample of galls of _Pistacia vera_, “_Gool-i-pista_,” India, recently examined in the Author’s laboratory, contained 30 per cent. of a light-coloured tannin.

_Schinus molle_, “Molle,” Buenos Ayres. Leaves only used; said to contain 19 per cent. tannin.

_S. Aroeira_, Brazil. Said to contain 14 per cent. tannin.

_Rhus coriaria_, Sicilian sumach. Ital. _Somacco_. (Fig. 53.) A shrubby bush, of which leaves and small twigs are used.

Mostly propagated by suckers from older plants, which are planted in rows about two feet apart in early spring, and pruned to 6-8 inches. Bushes begin to bear the year after planting, though the strength is not so good as from more mature plants. Cropping is either by pruning off shoots, or gathering leaves by hand; in the latter case shrubs are pruned in winter. The leaves are dried either in the fields, or on covered threshing floors, where they are afterwards separated from the stems by beating. Some is exported in this state, as “leaf” or “baling” sumach, but most is ground to fine powder under edge-runners. “Ventilated” sumach is winnowed to remove dust and sand, which often contains iron. “_Mascolino_” is the best sumach from Palermo and district; “_feminella_” consists of weaker sorts from other parts, and is generally used for mixing.

The different varieties of sumach are classed as follows:--

Relative
Market Value.
Sumach for baling 2·5
„ „ grinding 2·3
„ from yearling plants 1·5
„ „ ends of branches collected in autumn 1·0

To prepare these different grades for ultimate consumption, they are ground in mills similar to those employed for crushing olives, that is, in which two large stone wheels follow each other, revolving upon a circular bed, the whole construction being about the same as the Spanish or Mexican _arrastre_. The sumach thus pulverised is passed through bolting-screens to separate the finer from the coarser particles.

After the sumach leaf has been subjected to the first process of trituration, the coarse remaining portions are re-ground and the product added to that which has been already obtained. The still unpulverised residue known as _peduzzo_ is sifted, and the coarser and ungrindable parts are used as fuel, while the finer are mixed with the partially-ground, small, leaf-bearing branches (_gambuzza_, _gammuzza_), and ground again.

Palermo is the principal seat of the sumach trade. The material is generally bought from the small growers by middlemen, who hold it till market conditions are favourable. The quotations are always in _tarì_ of 42·5 centimes per _cantar_ of 79·342 kilos, which are obsolete even in Sicily, and have to be reckoned into _lire_ (francs) and kilos. Consequently 1 tarì per cantar equals 0·53565 lira per 100 kilos.

In 1894, the prices delivered at the mills were about 41-42 tarì for mascolino, 37-38 tarì for femminello, 14-18 tarì for brusca, and 10 tarì for stinco, per cantar; the lira being worth about 9_d._[140]

[140] Cf. ‘Kew Bulletin,’ No. 107, pp. 293-6.

Sumach has been introduced into Australia, and is said to thrive well in the dry plains of the Wimmera district.

Sumach often contains much sand, and sometimes particles of magnetic iron ore, which cause black stains, and may be collected by a magnet, and which dissolve in dilute hydrochloric acid without evolution of hydrogen, to a yellow solution. Metallic iron, which is also attracted by the magnet, dissolves in hydrochloric acid with effervescence to a colourless or green solution.

Good sumach contains at least 25-27 per cent. of tannin. The Author has analysed samples of undoubted genuineness containing as much as 32 per cent. of a tannin, principally gallotannic, with some ellagitannic acid, and a colouring matter (myricetin) identical with that of _Myrica nagi_ (p. 250), which gives yellows with alumina and tin mordants, and is fugitive to light.

Sumach is the best tanning material known for pale colour and soft tannage, and is hence used for moroccos, roans, skivers, etc., and also for brightening leathers of darker tannages, such as mimosa, gambier, the colouring matters of which warm sumach liquors seem able to dissolve.

In the report of the Society of Arts Committee on bookbinding leathers,[141] it is stated on abundant evidence, that sumach-tanned leathers are less affected by light and gas-fumes, and less liable to decay than those of any other known tannage.

[141] Soc. Arts. Journ., 1901, p. 14.

Sumach is frequently adulterated with the ground leaves and twigs of _Pistacia lentiscus_ (“schinia” or “skens”), _Coriaria myrtifolia_ (“stinco”), _Tamarix africana_ (“brusca”), _Ailantus glandulosa_, _Vitis vinifera_ (leaf of the common grape vine) and some other species of the Rhus family, but _Pistacia lentiscus_ is used to a much larger extent than any of the others. _Pistacia_, _coriaria_, and _tamarix_ all contain considerable quantities of tannin, though less than genuine sumach, and of a different chemical constitution.

The most satisfactory method of detecting these adulterants is by microscopic examination, none of the chemical methods proposed being very satisfactory; though, as many of the added matters contain catechol tannins, while those of sumach are purely pyrogallol derivatives, the method proposed by Hughes for the detection of quebracho in oakwood by the reaction of concentrated sulphuric acid (p. 296) might render good service, and any sumach infusion which was rendered turbid by bromine-water would at least be open to grave suspicion.

_Ailantus glandulosa._

_Coriaria myrtifolia._

_Colpoon compressa._

_Rhus cotinus._]

_Pistacia lentiscus._

_Rhus metopium._

_Rhus coriaria._

_Tamarix Africana._]

The most important work on the microscopic structure of the tissues of sumach and its adulterants was done by Andreasch, when during the later stages of his last illness he was obliged to winter in Sicily.[142] His work will well repay study, but unfortunately does not admit of useful abstraction here. A very useful investigation was also made in the Author’s laboratory by Messrs. M. C. Lamb and W. H. Harrison,[143] as regards the treatment and examination of the leaf-cuticles, which renders the detection of mixture comparatively easy. For details, the original memoir must be consulted, but if the suspected sumach be gently warmed for a few minutes with strong nitric acid, its more delicate leaf structure is entirely destroyed, and after washing and neutralising with sodium carbonate the strong cuticles of the leaves of the more common adulterants, “schinia” (_Pistacia lentiscus_), “stinco” (_Coriaria myrtifolia_), “brusca” (_Tamarix africana_), and _Ailantus glandulosa_ are uninjured, and easily recognised. Examination is rendered easier by dyeing the cuticles; safranine, acid green, Bismarck brown, and naphthol yellow being suitable for the purpose. Mr. Lamb’s photographs of the cuticles are reproduced on Plates III. and IV., but if possible, it is most satisfactory to compare the suspected sample direct with known specimens of the adulterants.

[142] ‘Sicilianischer Sumach und seine Verfalschung,’ Wien, 1898.

[143] ‘Sumach and the Microscopic Detection of its Adulterants,’
Journ. Soc. Dyers and Colorists, March 1899.

_R. glabra_, Southern States, U.S.A. (Fig. 54). Very largely used in the States to take the place of Sicilian sumach. A sample collected by the late Professor Trimble, and analysed in the Leather Industries Laboratory, contained 25 per cent. of tannin and produced a leather of very much darker colour than Sicilian.

_R. typhina_, “staghorn” or Virginian sumach, contains 10-18 per cent. of tannin. A sample from same source as above contained 13 per cent.

_R. cotonoides_, U.S.A. The analysis of a sample of this material gave 21 per cent. of tanning matter, and leather tanned with it was almost equal in colour to that from _R. glabra_.

Other sorts found in States: _R. semialata_ (5 per cent. tannin); _R. aromatica_ (13 per cent. tannin); _R. metopium_ (8 per cent.); _R. copallina_, _R. pumila_, _R. canadensis_; _R. toxicodendron_ is the well-known “poison ivy,” a climbing plant which causes a severe and irritating eruption if touched.

_R. glabra_ and _R. copallina_ are chiefly recommended for extended cultivation in the United States.