Editing Leather (section)

==Production processes==
[[File:Ledertrocknung in Suai.jpg|thumb|Drying of leather in [[East Timor]]]]
[[File:Leather tanning, Fes.jpg|thumb|Ancient leather tanning in [[Fes]], [[Morocco]]]]
[[File:Tanned leather.JPG|thumb|Tanned leather in [[Marrakech]]]]
[[File:Modern-day tannery.jpg|thumb|Modern-day tannery in [[León, Guanajuato|Leon]], [[Mexico]].]]
{{Main|Leather production processes}}

The leather manufacturing process is divided into three fundamental subprocesses: preparatory stages, tanning, and crusting. A further subprocess, finishing, can be added into the leather process sequence, but not all leathers receive finishing.

The preparatory stages are when the hide is prepared for tanning. Preparatory stages may include soaking, hair removal, [[liming (leather processing)|liming]], [[deliming]], [[bating (leather)|bating]], [[bleaching]], and [[pickling]].

[[Tanning (leather)|Tanning]] is a process that stabilizes the [[protein]]s, particularly [[collagen]], of the raw hide to increase the thermal, chemical and microbiological stability of the hides and skins, making it suitable for a wide variety of end applications. The principal difference between raw and tanned hides is that raw hides dry out to form a hard, inflexible material that, when rewetted, will [[Putrefaction|putrefy]], while tanned material dries to a flexible form that does not become putrid when rewetted.

Many tanning methods and materials exist. The typical process sees tanners load the hides into a drum and immerse them in a tank that contains the tanning "liquor". The hides soak while the drum slowly rotates about its axis, and the tanning liquor slowly penetrates through the full thickness of the hide. Once the process achieves even penetration, workers slowly raise the liquor's pH in a process called basification, which fixes the tanning material to the leather. The more tanning material fixed, the higher the leather's hydrothermal stability and [[Shrinkage (fabric)|shrinkage]] temperature resistance.

Crusting is a process that thins and lubricates leather. It often includes a coloring operation. Chemicals added during crusting must be fixed in place. Crusting culminates with a drying and softening operation, and may include splitting, shaving, [[dyeing]], [[Whitening (leather processing)|whitening]] or other methods.

For some leathers, tanners apply a surface coating, called "finishing". Finishing operations can include oiling, brushing, buffing, coating, polishing, embossing, [[Glaze (painting technique)|glazing]], or [[Tumble finishing|tumbling]], among others.

Leather can be oiled to improve its water resistance. This [[currying (leather)|currying]] process after tanning supplements the natural oils remaining in the leather itself, which can be washed out through repeated exposure to water. Frequent oiling of leather, with [[mink oil]], [[neatsfoot oil]], or a similar material keeps it supple and improves its lifespan dramatically.<ref>{{cite book|author=NIIR Board of Consultants|title=Leather Processing & Tanning Technology Handbook|date=2011|publisher=NIIR Project Consultancy Services|isbn=9788190568593|page=323|url=https://books.google.com/books?id=hbKkCwAAQBAJ&q=why+oil+leather&pg=PA324|language=en}}</ref>

===Tanning methods===
{{Main|Tanning (leather)}}
Tanning processes largely differ in which chemicals are used in the tanning liquor. Some common types include:
* '''{{visible anchor|Vegetable-tanned leather}}''' is tanned using [[tannin]]s extracted from [[vegetable matter]], such as tree [[tanbark|bark]] prepared in [[bark mill]]s. It is the oldest known method. It is supple and light brown in color, with the exact shade depending on the mix of materials and the color of the skin. The [[tan (color)|color tan]] derives its name from the appearance of undyed vegetable-tanned leather. Vegetable-tanned leather is not stable in water; it tends to discolor, and if left to soak and then dry, it shrinks and becomes harder, a feature of vegetable-tanned leather that is exploited in traditional shoemaking. In hot water, it shrinks drastically and partly congeals, becoming rigid and eventually brittle. [[Boiled leather]] is an example of this, where the leather has been hardened by being immersed in boiling water, or in [[wax]] or similar substances. Historically, it was occasionally used as [[armor]] after hardening, and it has also been used for [[book binding]].<ref>{{cite book |last1=Various |title=The Vegetable Tanning Process - A Collection of Historical Articles on Leather Production |date=September 17, 2020 |publisher=Read Books Limited |isbn=9781528764384 |url=https://books.google.com/books?id=c_P_DwAAQBAJ&dq=vegetable+tanned+leather&pg=PT52 |access-date=June 14, 2021 |archive-date=23 May 2023 |archive-url=https://web.archive.org/web/20230523032907/https://books.google.com/books?id=c_P_DwAAQBAJ&dq=vegetable+tanned+leather&pg=PT52 |url-status=live }}</ref><ref name=methods /> 
* '''Chrome-tanned leather''' is tanned using [[Chromium (III) sulfate|chromium sulfate]] and other [[chromium]] [[salt (chemistry)|salt]]s. It is also known as "wet blue" for the pale blue color of the undyed leather. The chrome tanning method usually takes approximately one day to complete, making it best suited for large-scale industrial use. This is the most common method in modern use. It is more supple and pliable than vegetable-tanned leather and does not discolor or lose shape as drastically in water as vegetable-tanned. However, there are environmental concerns with this tanning method, as chromium is a heavy metal; while the trivalent chromium used for tanning is harmless, other byproducts can contain toxic variants. The method was developed in the latter half of the 19th century as tanneries wanted to find ways to speed up the process and to make leather more waterproof.<ref>{{cite journal |last1=Mengistie |first1=E |last2=Smets |first2=I |last3=Van Gerven |first3=T |title=Ultrasound assisted chrome tanning: Towards a clean leather production technology. |journal=Ultrasonics Sonochemistry |date=September 2016 |volume=32 |pages=204–212 |doi=10.1016/j.ultsonch.2016.03.002 |pmid=27150762 |doi-access=free }}</ref><ref name=methods>{{cite web |title=Tanning--leather manufacturing |url=https://www.britannica.com/technology/tanning |website=Encyclopedia Britannica |access-date=June 14, 2020 |archive-date=17 April 2021 |archive-url=https://web.archive.org/web/20210417055207/https://www.britannica.com/technology/tanning |url-status=live }}</ref>
* '''Aldehyde-tanned leather''' is tanned using [[glutaraldehyde]] or [[oxazolidine]] compounds. It is referred to as "wet white" due to its pale cream color. It is the main type of "chrome-free" leather, often seen in shoes for infants and automobiles. [[Formaldehyde]] has been used for tanning in the past; it is being phased out due to danger to workers and sensitivity of many people to formaldehyde. [[Chamois leather]] is a form of aldehyde-tanned leather that is porous and highly water-absorbent. Chamois leather is made using oil (traditionally cod oil)<ref>{{Cite journal |last1=Sahu |first1=Bindia |last2=Jayakumar |first2=Gladstone Christopher |last3=Alla |first3=Jaya Prakash |date=2022-07-01 |title=Recent trends in oil tanning and its applications - A way forward towards cleaner approach in chamois leather making |url=https://www.sciencedirect.com/science/article/pii/S0959652622013683 |journal=Journal of Cleaner Production |language=en |volume=356 |pages=131755 |doi=10.1016/j.jclepro.2022.131755 |s2cid=248213393 |issn=0959-6526}}</ref> that oxidizes to produce the aldehydes that tan the leather.
* '''Brain tanned leathers''' are made by a labor-intensive process that uses emulsified oils, often those of animal brains such as deer, cattle, and buffalo. An example of this kind is [[buckskin (leather)|buckskin]]. Leather products made in this manner are known for their exceptional softness and washability.
* '''Alum leather''' is transformed using [[aluminium]] salts mixed with a variety of binders and [[protein]] sources, such as flour and egg yolk. Alum leather is not actually tanned; rather the process is called "tawing", and the resulting material reverts to rawhide if soaked in water long enough to remove the alum salts.