Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, Sichuan, P.R. China
This paper discusses the characteristics and problems of leather making with yak hide. The paper addresses the following issues: histological structure of the hide, defects of the raw hide, the key techniques of leather making with yak hide, and the essential special leather chemicals.
Keywords: Characteristic, hide, leather making, problems, yak
There are about 14 million head of yak in China, constituting 90% of the global yak population. The yak rearing area in China covers Qinghai, Tibet, Sichuan, Gansu, Xinjiang, Inner Mongolia and Yungnan Provinces. The yak hide is one of the main sources of raw hide for the manufacture of leather. The annual production of yak raw hide is estimated at more than 1.5 million hides.
Early in 1950s, the yak raw hide was used to make leather in Sichuan and Qinghai. With almost half-century of experience, the leather making techniques have developed considerably. In some areas such as Ya' an, Aba, Ruoergai of Sichuan Province, Xining of Qinghai Province, Lanzhou of Gansu Province and Lhasa of Tibet, there are some tanneries able to make a large number of products from yak hide. These include corrected grain leather, shoe upper leather and garment leather. However, the utility of yak hide is, to date, not sufficiently developed. Not many high quality products of yak leather are available in the market.
In this review, the characteristics and problems of leather making with yak hide are discussed. A major purpose of the review is to promote the development of yak hide tanning processes.
In China, the yak hide is produced mainly in the south-western and north-western cold highland areas, especially in the minority nationality regions. Because of the backward economies and the difficulties in transport, natural (air) drying is the current preservation method used for most yak hides in these areas. One of the consequences of this is the prevalence of a hide defect known as dry-crack, which accounts for 55% of all defects. The second defect is hide rot and insect damage, which are 21 and 14% of all defects, respectively. To solve these problems, the use of salt-curing as a preservation method is suggested. The proportion of defects associated with this method is only 29%, compared with air-drying (Gao 1992).
Another defect of the hide is warble hole caused by warble fly infection. This causes damage to quality of a large number of hides. There is hardly any hide not infected. The damage caused by the warble fly consists of holes covering 6% of the whole hide. Fortunately, there are methods to prevent damage by warble, and these have been applied with much success. The hide damage can be successfully reduced; it is possible to make higher-grade leather with the yak hide. Another aspect related to the improvement of quality of leather products from yak hide has to do with the histological structure of yak hide. Huang (1992) studied this aspect. The results show that the yak hide has a dense cover of long hair. The sebaceous (fat glands) and the sweat glands in the hide are fairly developed and are present in large numbers. The hide is characterised by a great differentiation between the papillary layer (grain layer) and the reticular layer. The thin collagen fibres in the papillary layer of the hide are compactly woven, but the reticular layer is much looser. Therefore, to produce high quality soft leather from yak hide, the hair roots in the papillary layer should be removed as completely as possible and the differences between the two layers should be eliminated or reduced and more attention should be paid to removing the rich fatty materials from the sebaceous glands.
The special problems summarised above in terms of defects and, most specifically the histological structures of yak hide aside, the leather making processes based on yak hide are similar to leather making with cattle hide. The processes of making shoe upper leather from yak hide is as follows: soaking, fleshing, degreasing, de-hairing and liming, re-fleshing, splitting of pelt, de-liming, bating, chrome tanning, splitting blue, shaving, re-tanning, neutralising, dyeing and fat-liquoring, filling, drying, dewing, vibration staking, coating finish, ironing, grinding, measuring and storing. Degreasing is a very important process in the case of yak hide because the sebaceous glands of yak are very well-developed; degreasing affects the processes that follow, and, if not done properly, may cause the cracks of the rain on finished leather. For the same reason, it is important for leather making with yak hides that the processes of de-hairing and liming are carried out with great care. The new technology of de-hairing with enzyme, sodium sulphide and sodium hydrosulphide is very effective and should be used whenever possible.
One has to contend with the natural defects of yak raw hide. However, it is also easy to cause defects if the hide is not properly processed. For example, loose grain, surface-crack can easily be caused by defective or poor processing. Therefore, it is important to consider the development of special leather chemicals for yak hides while, at the same time, the leather making processes are studied continuously and improved. To solve the problem of loose grain, it is necessary to do the filling with SCC acrylic emulsion filler and/or MSF micro-latex filler. The filler penetrates into the region between the grain layer and the reticular layer, thereby helping to stick the two layers together.
The yak raw hide has inherent defects and man-made scars. To increase the grade of the leather, ending agent is widely used in the coating finish. Through the mending and the mechanical processes, buffing and pebbling, a perfect result can be obtained.
Yak hide is an important natural resource in China and must be utilised sufficiently. It is important to develop the economies of the minority nationality regions and to improve the tanning industry in these areas.
Gao X. J. 1992. Study on shoe upper leather-making with yak hides of Qinghai-Tibet plateau of China. In: International conference of leather science and technology held in Chengdu, P.R. China, 25–29 October 1992. pp. 72–82.
Huang Y. Z. 1992. Study on the structure of yak hide of China. In: International conference of leather science and technology held in Chengdu, P.R. China, 25–29 October 1992. pp. 105–117.