Chrome tannage

Control of chromium penetration, essential to permit tannage of the center of the hide, is accompHshed by pH adjustment. At a pH > 3.0 the reactivity of the hide to the chromium complex is greatiy increased. The pH is therefore raised gradually to the desired point by addition of a mild alkah, usually sodium bicarbonate. The chemistry of chrome tanning involves competing reactions that must be controlled for satisfactory results. 

The chrome tanning is one step in a compHcated series of leather operations leading from the raw hide to the finished products. Chrome tanning is the most important tannage for all hides except heavy catde hides, which are usually vegetable tanned. In heavy shoe uppers and soles, a chrome tanned leather is frequently given a vegetable retan to produce chrome retan leather. 

These facts play an important factor in the dyeing behavior. The isoelectric point of hair is at a pH of ca. 5, whereas untreated collagen has its isoeletric point at a pH of ca. 6, which varies with the kind of tannage. For pure chrome suede it is close to pH 7 or about pH 6 after slightly anionic retanning. Consequently, for good dye fixation the acidification at the end of a dyeing is around pH 3 for fur hair and ca. pH 4 for chrome-tanned suede. 

For accounts of the various methods of chrome tannage, see Procter, Leather Trades Review, 1897, p. 390 1898, p. 400 Cobb Lecture to Society of Arts, 1918 Stiasny, Collegium, 1912, p. 293 Wintgen and Lowenthal, Kolloid. Zeitsch., 1924, 34, 289. 

Use Tannage of white kid suede, glove leathers retannage of chrome leather. 

The predominant tanning technique in the world is chrome tannage (about 85%). As a multi-purpose material chrome tanned leather can be transferred to leather with different properties (to different leather types) by further processing. However, a significant quantity of vegetable tanned leather is manufactured, too. Part-tanned chrome-free - often termed wet white - leather is also produced, in small but growing quantity. 

Hauber, C. and Germaim, H.-P., 1997. The addition of fungicides in chrome tannage and their penetration, absorption and distribution in the wet blue. World Leather May, 75-82. 

Slabbert N P 1981 Mimosa-Al tannages An alternative to chrome tanning. J Am Leather Chem Assoc 76 231 -244... 

Uses Fatliquoring agent for pickle, chrome tannage, rechrome of leathers Features Dry handle exc. stability to electrolytes, acids, hard water, chrome tanning salts exc. lightfastness Properties Yel./brn. liq. pH 5.0-6.5 (10%) 75% act. 

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