CHAMBERS'S INFORMATION FOR THE PEOPLE. 



Cordovan leather resembles morocco, but differs 

 from it in that, in the latter, the grain is pro- 

 duced artificially by means of rollers, while in the 

 former the natural grain is preserved. It is tanned 

 with sumach and dyed. 



Tawing, This method of making leather is 

 chiefly applied to the skins of sheep, goats, and 

 calves, although sometimes the skins of horses and 

 cows are tawed. 



The process consists in treating the hides, pre- 

 pared as for tanning, with a solution of alum and 

 common salt In this solution they lie for an hour ; 

 they are then taken out and piled upon one another 

 wet. In this condition they remain for two or 

 three days ; they are then wrung out and hung on 

 laths to dry. When dry, they are again uniformly 

 moistened, and rubbed with a convex iron, to give 

 pliancy and remove folds. 



Hungarian tawing is a variety of the process, 

 chiefly differing from that above described in 

 that larger and thicker skins are used ; the pre- 

 liminary treatment is shorter and simpler, the alum 

 .and salt solution stronger, and the skins are moved 

 -about in the liquor, in which they remain for eight 

 days. Often the Hungarian leather contains no 

 tallow or oil ; but sometimes it is warmed, rubbed 

 with tallow, and then held over a charcoal fire, to 

 make the tallow soak better into it, and then cooled 

 in the air. 



Whiteglove leather is prepared by tawing from 

 'the skins of lambs and kids. After undergoing the 

 usual tawing process, they are placed in a second 

 t>ath, which contains for 100 skins 9 Ibs. of 

 wheat-flour, the yolks of 50 eggs, 2^ Ibs. of 

 .alum, I Ib. common salt, and a gallon and a 

 half of water. Sometimes, instead of the yolks of 

 eggs, fine olive-oil is used. In this they are left 

 some days, then dried, rolled, pressed, and polished 

 on the grain side. 



Shamoying. For this process the skins are 

 prepared in the same way as for tanning or tawing. 

 "They are then sprinkled with fish-oil, rolled up 

 into balls, and hammered with heavy falling ham- 

 oners for two or three hours. During this time 

 they are occasionally taken out, again oiled, and 

 put back under the hammer. When they have 

 iaken up enough oil, they are taken out, and either 

 ihung up in a warm place or piled up in heaps, 

 where they become warm by the oxidation of the 

 oil. They are next steeped in a very weak luke- 

 warm solution of potash, which saponifies the ex- 

 cess of the oil. The skins are lastly wrung out, 

 dried, and rubbed. From the solution of soap just 

 mentioned, a mixture of fatty acid and soap 

 separates ; this is called ' ddgras,' and is used, as 

 .already mentioned, for oiling tanned skins. 



Parchment and Vellum are not, strictly speak- 

 ing, leather ; they have undergone no chemical 

 change, but are merely thoroughly cleaned and 

 dried skins. 



FATS AND FATTY OILS. 



These are substances occurring in animals and 

 vegetables. They are either liquid at ordinary 

 -temperatures (oils) or fusible solids (fats). They 

 are insoluble in water, but very readily soluble in 

 ether and in bisulphide of carbon. They cannot 

 be distilled without decomposition. When treated 

 with solutions -of fixed caustic alkalies (caustic 

 potash or caustic soda), they are saponified that 



348 



is, they are dissolved, and the solution contains a 

 soap and glycerine. They are greasy to the touch, 

 and leave a permanent greasy spot on paper. 

 The chemical constitution of the fats and fatty 

 oils is explained in the paper on CHEMISTRY. 

 We shall here describe the ways in which they 

 are obtained, and the methods used for purifying 

 them. 



VEGETABLE OILS. These are almost entirely 

 obtained from the seeds of many kinds of plants. 

 The seeds are first ground to a meal, and this is 

 wanned and strongly pressed in coarse hair-cloths 

 between metal plates by means of a hydraulic 

 press. The greater part of the oil flows out, leav- 

 ing what is called the ' oil-cake.' The extraction 

 of the oil may be, and to some extent in practice 

 is, effected by means of bisulphide of carbon, in 

 which oils are very readily soluble. 



The vegetable oils may be divided into the Dry- 

 ing Oils and the Non-drying Oils. The former ab- 

 sorb oxygen when exposed to the air, and are con- 

 verted into a dry ' varnish ;' hence they are used for 

 preparing oil-colours. The non-drying oils, when 

 perfectly pure, are not altered by exposure to air ;* 

 but in the state in which they usually occur, mixed 

 with small quantities of foreign matter, from the 

 vegetable or animal tissue from which they are 

 obtained, they become ' rancid ' that is, acquire a 

 disagreeable taste and smell, and become acid to 

 test-paper. By shaking a rancid oil with boiling 

 water, and then in the cold with a weak alkaline 

 solution, the fatty acids, on the presence of which 

 the rancidity depends, are removed. 



The most important DRYING OIL is Linseed- 

 oil, obtained from the seeds of the common flax 

 (Litium usitatissimum). Heated with oxide of 

 lead, it dissolves about 5 per cent of the oxide, 

 and its drying character is greatly increased. 

 When kept for a long time at a high temperature 

 in contact with air, it is converted into a thick, 

 rapidly drying varnish, much used for making 

 printers' ink. 



The other best known drying oils are : Oil of 

 hempseed, from the seeds of the hemp plant 

 (Cannabis sativa] ; poppyseed-oil, from the seeds 

 of the opium poppy (Papaver somniferutri), used 

 chiefly as a salad-oil ; walnut-oil, from the walnut, 

 fruit of Juglans regia, also used as a salad-oil. 

 Croton-oil and castor-oil, from the seeds of Croton 

 Tiglium and Ricinus commum's, can scarcely be 

 classed among the drying oils, although in thin 

 layers they slowly dry to a tough semi-solid sub- 

 stance. They are both used as purgatives ; croton- 

 oil being an exceedingly powerful purgative, castor- 

 oil a mild and gentle laxative. 



The NON-DRYING OILS consist mainly of olein 

 and palmitin ; some of them also contain stearin 

 (see CHEMISTRY). Olein is an oily liquid ; pal- 

 mitin and stearin are fusible solids ; the former 

 melting at a lower temperature than the latter. 

 Oils containing palmitin and stearin, when cooled 

 to a low temperature, deposit these bodies in the 

 solid form ; when warmed again, the deposit dis- 

 solves in the olein. This may be easily observed 

 in olive-oil, which, in cold weather, deposits a 



* The non-drying oils, when exposed to the air in very thin 

 layers (as, for instance, when rags or cotton-waste, used for 

 wiping up oil, are exposed to air), are oxidised, and during this 

 oxidation, heat is given out to such an extent that it has happened 

 that a pile of rags soaked in oil has actually taken fire. Care 

 should therefore be taken that such oily materials are not allowed 

 ':o accumulate. 



