Editing Tanning (leather) (section)

=== Chrome tanning {{anchor|Mineral tanning|Chrome tanning}} ===
[[File:PPH Drum.jpg|thumb|A modern electric tanning drum in Germany]]

[[Chromium(III) sulfate]] ({{chem2|[Cr(H2O)6]2(SO4)3}}) has long been regarded as the most efficient and effective tanning agent.<ref>Wilson, J.A. The Chemistry of Leather Manufacture. The Chemical Catalog Company, Inc. New York 1923.</ref><ref name=Coving/> Chromium(III) compounds of the sort used in tanning are significantly less toxic than [[hexavalent chromium]], although the latter arises in inadequate waste treatment. Chromium(III) sulfate dissolves to give the hexaaquachromium(III) cation, [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup>, which at higher pH undergoes processes called [[olation]] to give polychromium(III) compounds that are active in tanning,<ref>Harlan, J.; Feairheller, S.; Adv. Exp. Med. Biol. 1977, 86A, 425.</ref> being the [[cross-link]]ing of the collagen subunits. The chemistry of [Cr(H<sub>2</sub>O)<sub>6</sub>]<sup>3+</sup> is more complex in the tanning bath rather than in water due to the presence of a variety of ligands. Some ligands include the sulfate anion, the collagen's carboxyl groups, amine groups from the side chains of the [[amino acid]]s, and masking agents. Masking agents are [[carboxylic acid]]s, such as [[acetic acid]], used to suppress formation of polychromium(III) chains. Masking agents allow the tanner to further increase the pH to increase collagen's reactivity without inhibiting the penetration of the chromium(III) complexes.

Collagen is characterized by a high content of [[glycine]], [[proline]], and [[hydroxyproline]], usually in the repeat -gly-pro-hypro-gly-.<ref>Heidemann, E.;  J. Soc. Leather Technol. Chem., 1982, 66, 21.</ref> These residues give rise to collagen's helical structure. Collagen's high content of hydroxyproline allows  cross-linking by [[hydrogen bonding]] within the helical structure. Ionized carboxyl groups (RCO<sub>2</sub><sup>−</sup>) are formed by the action of hydroxide. This conversion occurs during the liming process, before introduction of the tanning agent (chromium salts). Later during pickling, collagen carboxyl groups are temporarily protonated for ready transport of chromium ions. During basification step of tanning, the carboxyl groups are ionized and coordinate as ligands to the chromium(III) centers of the oxo-hydroxide clusters.

Tanning increases the spacing between protein chains in collagen from 10 to 17 Å.<ref name=Gustav>Gustavson, K.H. "The Chemistry of Tanning Processes" Academic Press Inc., New York, 1956.</ref> The difference is consistent with cross-linking by polychromium species, of the sort arising from olation and oxolation.

[[File:Possible Chromium(III) Tanning Mechanisms.tif|thumb|Possible chromium(III) tanning mechanisms]]
Before the introduction of the basic chromium species in tanning, several steps are required to produce a tannable hide. The pH must be very acidic when the chromium is introduced to ensure that the chromium complexes are small enough to fit between the fibers and residues of the collagen. Once the desired level of penetration of chrome into the substance is achieved, the pH of the material is raised again to facilitate the process. This step is known as basification. In the raw state, chrome-tanned skins are greyish-blue, so are referred to as ''wet blue''. Chrome tanning is faster than vegetable tanning (taking less than a day for this part of the process) and produces a stretchable leather which is excellent for use in handbags and garments.

After application of the chromium agent, the bath is treated with sodium bicarbonate in the basification process to increase the pH to 3.8–4.0, inducing cross-linking between the chromium and the collagen. The pH increase is normally accompanied by a gradual temperature increase up to 40&nbsp;°C.<ref>Heidemann, E.; Leather. Ullmann's Encyclopedia of Industrial Chemistry,2005. {{doi|10.1002/14356007.a15_259}}</ref> Chromium's ability to form such stable bridged bonds explains why it is considered one of the most effective tanning compounds. Chromium-tanned leather can contain between 4 and 5% of chromium.<ref name=Gustav/> This efficiency is characterized by its increased hydrothermal stability of the skin, and its resistance to shrinkage in heated water.<ref name=Coving>Covington, A. "Modern Tanning Chemistry" Chemical Society Review 1997, volume 26, 111–126. {{doi|10.1039/CS9972600111}}</ref>