Leather making is one of the oldest crafts performed by humankind. Leather making is one of the oldest crafts performed by humankind. Although there is world-wide trend to exploit alternative materials derived from other sources, leather still finds widespread use.

Tanning is the most important step in leather production. It is typically carried out in an aqueous environment in rotating drums. During tanning, collagen will fix the tanning agent to its reactive sites, as a result stopping the putrefaction phenomenon. Tanning can be classified into three groups: mineral tanning, vegetable tanning, and other organic tanning (aldehyde, quinone, oil, and synthetic tanning).

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Mineral Tanning

Ono Suparno, Dept. of Agroindustrial Technology, IPB University, Bogor
Four elements play a significant role in the modern leather tanning industry, i.e. chromium(III), aluminium(III), titanium(IV), and zirconium(IV), of which chromium(III) is the most important. Nowadays, more than 90% of the world’s leather is tanned with chromium, which is a consequence of the easy processing, the broad achievability and the excellent properties of leather. Tanning using Cr(III) sulfate can achieve shrinkage temperatures above 120oC. However, it also has considerable potential for environmental pollution.

The interactions of collagen with chrome have been extensively investigated since the end of the nineteenth century. The fundamental reaction is the formation of complex bonds with the ionised carboxyl groups of aspartic and glutamic acid residues on collagen fibres.

Other mineral tannages (Al(III), Ti(IV), and Zr(IV)) have similar reaction mechanisms to chromium, although reaction is dominated by electrovalent bonding, thus much lower shrinkage temperature is obtained than with chrome. The maximum shrinkage temperatures of leather tanned with Al(III), Ti(IV), and Zr(IV) salts are 79, 90, and 97oC respectively. The development of titanium and zirconium tannage is relatively new. Empirically, the chemistry of Ti(IV) is dominated by the titanyl ion TiO2+ and the species in the tanning agent are chains of (Ti-O)n. Zirconium salts are characterised by eight-coordination and high affinity for oxygen, resulting in a tetrameric core structure; the basic unit of structure is four Zr(IV) ions at the corners of a square. The tanning powers of titanium and zirconium are similar and both are better than aluminium.

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Oil Tanning

Ono Suparno, Dept. of Agroindustrial Technology, IPB University, Bogor
Oil tanning is a very old way of imparting the properties of finished leather to skins. In modern oil tanning for chamois leather, the flesh split of the sheepskins are used as having desirable open fibre structure. After the usual beamhouse processes, they are brought to the isolectric point, e.g. pH 4.5.

Penyamakan minyak adalah penyamakan kulit menggunakan minyak, biasanya minyak ikan, untuk menghasilkan kulit samak minyak (chamois leather). Metode tradisional pembuatan kulit chamois adalah mengimpregnasi kulit domba split basah dengan minyak ikan dalam fulling stocks dan kemudian menggantungnya dalam stoves hangat untuk oksidasi minyak.

Minyak yang teroksidasi tersebut memiliki kemampuan menyamak kulit. Kedua proses tersebut dapat diulang sampai kulit tersamak dengan memadai. Kelebihan minyak dari kulit dihilangkan dengan pengepresan hidrolik dilanjutkan dengan pencucian akhir dalam air alkalin hangat. Kulit tersebut kemudian digantung untuk pengeringan dan kemudian dilanjutkan ke finishing. Dalam finishing, kulit diwarnai dengan bahan pewarna (dye) untuk meningkatkan keindahannya atau untuk keperluan mode (fashion). Umumnya, warna diperoleh dengan cara menggunakan pewarna asam atau premetallised yang menghasilkan warna-warna cerah.

Dasar penyamakan minyak modern adalah mengoksidasi minyak ikan yang sudah diaplikasikan pada kulit setelah penghilangan kapur (delimed pelt) dengan bantuan oksigen atmosfir pada kondisi terkendali. Bahan penyamak gliserida tak jenuh yang biasa digunakan adalah minyak cod dan minyak sardine. Asam-asam lemak tersebut memiliki sampai enam ikatan ganda dalam rantai alifatiknya yang memberikan produk reaksi dari oksidasi dan polimerisasi untuk memberikan efek penyamakan minyak pada kondisi penyamakan normal.

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