Editing Tanning (leather) (section)

== Beamhouse operations ==
The steps in the production of leather between curing and tanning are collectively referred to as beamhouse operations. They include, in order, soaking, [[liming (leather processing)|liming]], removal of extraneous tissues (unhairing, scudding and fleshing), [[deliming]], [[bating (leather)|bating]] or puering, drenching, and pickling.<ref>{{ cite web | title = Etherington and Roberts Dictionary | date = 2011-03-10 | access-date = 2011-10-14 | publisher = Foundation of the [[American Institute for Conservation]] | url = http://cool.conservation-us.org/don/dt/dt0257.html | url-status = live | archive-url = https://web.archive.org/web/20110225143300/http://cool.conservation-us.org/don/dt/dt0257.html | archive-date = 2011-02-25 }}</ref><ref>{{ cite web | title = 3. Tanneries, Description of the Tanning Process | access-date = 2011-10-14 | publisher = [[Food and Agriculture Organization]] | url = http://www.fao.org/WAIRDOCS/LEAD/X6114E/x6114e05.htm#b1-3.1.%20Description%20of%20the%20tanningprocess | url-status = live | archive-url = https://web.archive.org/web/20110822080512/http://www.fao.org/WAIRDOCS/LEAD/X6114E/x6114e05.htm#b1-3.1.%20Description%20of%20the%20tanningprocess | archive-date = 2011-08-22 }}</ref>

=== Soaking ===
In soaking, the hides are soaked in clean water to remove the salt left over from curing and increase the moisture so that the hide or skin can be further treated.

To prevent damage of the skin by bacterial growth during the soaking period, [[biocide]]s, typically [[dithiocarbamate]]s, may be used. Fungicides such as [[TCMTB]] may also be added later in the process, to protect wet leathers from mold growth. After 1980, the use of [[pentachlorophenol]] and [[mercury (element)|mercury]]-based biocides and their derivatives was forbidden.<ref>[http://www.greenpeace.org/eastasia/campaigns/toxics/science/eleven-flagship-hazardous-chemicals/ "Hazardous Chemicals in Clothing"] {{Webarchive|url=https://web.archive.org/web/20181009172019/http://www.greenpeace.org/eastasia/campaigns/toxics/science/eleven-flagship-hazardous-chemicals/ |date=2018-10-09 }}  ''GreenPeace.org''.  Retrieved 2018-11-13.</ref>

=== Liming ===
{{unreferenced section|date=November 2020}}
{{Main|Liming (leather processing)}}

After soaking, the hides are treated with [[milk of lime]] (a basic agent) typically supplemented by "sharpening agents" (disulfide reducing agents) such as sodium [[sulfide]], [[cyanide]]s, [[amine]]s, etc.

This:
* Removes the hair and other keratinous matter
* Removes some of the interfibrillary soluble proteins such as mucins
* Causes the fibers to swell up and split up to the desired extent
* Removes the natural grease and fats to some extent
* Brings the [[collagen]] in the hide to a proper condition for satisfactory tannage

The weakening of hair is dependent on the breakdown of the disulfide link of the amino acid [[cystine]], which is the characteristic of the [[keratin]] class of proteins that gives strength to hair and [[wool]]s (keratin typically makes up 90% of the dry weight of hair). The hydrogen atoms supplied by the sharpening agent weaken the cystine molecular link whereby the [[covalent]] [[disulfide bond]] links are ultimately ruptured, weakening the keratin. To some extent, sharpening also contributes to unhairing, as it tends to break down the hair proteins.

The [[isoelectric point]] of the collagen (a tissue-strengthening protein unrelated to keratin) in the hide is also shifted to around pH 4.7 due to liming.

[[File:ManCleaningAnimalSkin.jpg|alt=Man cleaning animal skin in Marrakech, Morocco (2019, September)|thumb|Traditional hand scudding in Marrakech, Morocco]]
Any hairs remaining after liming are removed mechanically by scraping the skin with a dull knife, a process known as scudding.

=== Deliming and bating ===
{{Main|Deliming}}

The pH of the collagen is then reduced so the enzymes may act on it in a process known as deliming. Depending on the end use of the leather, hides may be treated with enzymes to soften them, a process called [[bating (leather)|bating]]. In modern tanning, these enzymes are purified agents, and the process no longer requires bacterial fermentation (as from dung-water soaking) to produce them.<ref name=ns>{{cite web|url= https://www.newscientist.com/letter/mg17523585-200-pure-dog-dung/|title= Letters: Pure dog dung|last1= Covington|first1= Tony|date= 31 August 2002|website= [[New Scientist]]|access-date= 6 January 2016|url-status= live|archive-url= https://web.archive.org/web/20160413062202/https://www.newscientist.com/letter/mg17523585-200-pure-dog-dung/|archive-date= 13 April 2016}}</ref>

=== Pickling ===
Pickling is another term for tanning, or what is the modern equivalent of turning rawhide into leather by the use of modern chemical agents, if mineral tanning is preferred. Once bating is complete, the hides and skins are treated by first soaking them in a bath containing common salt (sodium chloride), usually 1 [[quart]] of salt to 1 [[gallon]] of hot water.<ref name="Kephart1916">{{cite book |author-last=Kephart|author-first=H.|author-link=Horace Kephart |title=Camping and Woodcraft; A Handbook for Vacation Campers and for Travelers in the Wilderness |edition=18 |volume=2 |publisher=The Macmillan Company |location=New York|year=1916|page=322 |isbn=978-1-149-75236-4 |language=en|url=https://archive.org/details/campingwoodcraft0000keph/page/n7/mode/2up |oclc=2191524}} (reprinted in 1957)</ref> When the water cools, one fluid ounce of [[sulfuric acid]] is added. Small skins are left in this liquor for 2 days, while larger skins between 1 week and as much as 2 months.<ref name="Kephart1916"/>

In vegetable tanning, the hides are made to soak in a bath solution containing vegetable tannins, such as found in [[gallnuts]], the leaves of [[sumac]], the leaves of certain [[acacia]] trees, the outer green shells of [[walnut]]s, among other plants. The use of vegetable tanning is a process that takes longer than mineral tanning when converting rawhides into leather. Mineral tanned leather is used principally for shoes, car seats, and upholstery in homes (sofas, etc.). Vegetable tanned leather is used in [[leather crafting]] and in making small leather items, such as wallets, [[handbag]]s and clothes.