TAN

or TANNIC ACID. ( Tannin , Fr.; Gerbstoff , Germ.) See its preparation and properties described under Galls . The barks replete with this principle should be stripped with hatchets and bills, from the trunk and branches of trees, not le than 30 years of age, in spring, when their sap flows most freely. Trees are also sometimes barked in autumn, and left standing, whereby they cease to vegetate, and perish ere long; but afford, it is thought, a more compact timber. This operation is, however, too troublesome to be generally practised, and therefore the bark is commonly obtained from felled trees; and it is richer in tannin the older they are. The bark mill is described in Gregory’s Mechanics , and other similar works. The following Table shows the quantity of extractive matter and tan in 100 parts of the several substances :— | Substances. | In 480, by Davy. | In about 8 oz., by Biggins. | In 100 parts, by Cadet de Gas sin court. | | White inner bark of old | 72 | | 21 | | Do. young oak | 77 | | | | Do. Spanish chestnut | 63 | 30 | | | Do. Leicester willow | 79 | | | | Coloured or middle bark of oak | 19 | | | | Do. Spanish chestnut | 14 | | | | Do. Leicester willow | 16 | | | | Entire bark of oak | 29 | | | | Do. Spanish chestnut | 21 | | | | Do. Leicester willow | 33 | 109 | | | Do. Elm | 13 | 28 | | | Do. Common willow | 11 | boughs, 31 | | | Sicilian sumach | 78 | 158 | | | Malaga sumach | 79 | | | | Souchong tea | 48 | | | | Green tea | 41 | | | | Bombay catechu | 261 | | | | Bengal catechu | 231 | | | | Nut-galls | 127 | | 46 | | Bark of oak, cut in winter | — | 30 | | | Do. beech | — | 31 | | | Do. Elder | — | 41 | | | Do. Plum-tree | — | 58 | | | Bark of the trunk of Willow | — | 52 | | | Do. Sycamore | — | 53 | 16 | | Bark of Birch | — | 54 | | | Bark of Cherry-tree | — | 59 | 24 | | Do. Sallow | — | 59 | | | Do. Poplar | — | 76 | | | Do. Hazel | — | 79 | | | Do. Ash | — | 82 | | | Do. trunk of Span. chestnut | — | 98 | | | Do. Smooth oak | — | 104 | | | Do. Oak, cut in spring | — | 108 | | | Root of Tormentil | — | | 46 | | Cornus sanguine a of Canada | — | | 44 | | Bark of Alder | — | | 36 | | Do. Apricot | — | | 32 | | Do. Pomegranate | — | | 32 | | Do. Cornish cherry-tree | — | | 19 | | Do. Weeping willow | — | | 16 | | Do. Bohemian olive | — | | 14 | | Do. Tan shrub with myrtle leaves | — | | 13 | | Do. Virgin i an sumach | — | | 10 | | Do. Green oak | — | | 10 | | Do. Service-tree | — | | 8 | | Do. Rose chestnut of Amer. | — | | 8 | | Do. Rose chestnut | — | | 6 | | Do. Rose chestnut of Carolina | — | | 6 | | Do. Sumach of Carolina | — | | 5 | TANNING ( Tanner , Fr.; Gärberei , Germ.); is the art of converting skin into Leather , which see. It has been ascertained, beyond a doubt, that “the saturated infusions of astringent barks contain much le extractive matter, in proportion to their tannin, than the weak infusions; and when skin is quickly tanned (in the former), common experience shows that it produces leather le durable than leather slowly formed.” [66] The older tanners, who prided themselves on producing a substantial article, were so much impre ed with the advantages of slowly impregnating skin with astringent matter, that they employed no concentrated infusion (ooze) in their pits, but stratified the skins with abundance of ground bark, and covered them with soft water, knowing that its active principles are very soluble, and that, by being gradually extracted, they would penetrate uniformly the whole of the animal fibres, instead of acting chiefly upon the surface, and making brittle leather, as the strong infusions never fail to do. In fact, 100 pounds of skin, quickly tanned in a strong infusion of bark, produce 137 of leather; while 100 pounds, slowly tanned in a weak infusion, produce only 117 1 ⁄ 2 . The additional 19 1 ⁄ 2 pounds weight in the former case serve merely to swell the tanner’s bill, while they deteriorate his leather, and cause it to contain much le of the textile animal solid. Leather thus highly charged with tannin, is, moreover, so spongy as to allow moisture to pa readily through its pores, to the great discomfort and danger of persons who wear shoes made of it. That the saving of time, and the increase of product, are temptations strong enough to induce many modern tanners to steep their skins in a succe ion of strong infusions of bark, is sufficiently intelligible; but that any shoemaker should be so ignorant or so foolish as to proclaim that his leather is made by a proce so injurious to its quality, is unaccountably stupid. [66] Sir H. Davy, on the Operation of Astringent Vegetables in Tanning.— Phil. Trans. 1803.