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This post gives some insight into the process (1) and chemistry (2) of the wet scraping brain tanning and smoking method of deer hides. In May 2010 I took part in a brain tanning workshop with Kfir Mendel and his company ‘Two Wolves’. Kfir specialises in the so-called ‘primitive technologies’, i.e. replication of technologies used before the invention of machinery to process natural materials, & he recently presented at the 2nd Annual Reconstructive and Experimental Archaeology Conference.

My aim for the workshop was to gain familiarity with material properties of hides prepared by this method, and to get better at material identification and understanding conditions of historic artefacts made from hide.

I learned a lot from Kfir and I highly recommend taking a workshop with him. He is a great teacher who tans skins  for a living and knows everything about producing a beautiful and long lasting skin.

Hide of a whitetailed deer (Odocoileus virginianus) was tanned at the workshop.

Terminology of the leather processing industry is used in this post (Covington  2009). The fibre structure of the skin is anisotropic, the skin is uneven in thickness. The thickness of deer skin ranges between 2 and 3 mm and measures from 0.9 to 1.3 m2. The grain layer takes 1/6 of the total thickness. The fibres of the corium are course and loosely interwoven, resulting in stretchy leather, suitable for clothing . This tanning is method is not without some health hazards (Wescott, D. 1999). For a detailed account of the structure of mammalian skins and the difference in structure between the animal types, see Kite & Thomson (2005).

1. Process in brief:

1. Wet scraping or ‘hydrate and flesh’. Manually by scrapping, this step removes the flesh, that is fat and any meat adhering to the pelt.

2. Bucking, or ‘unhairing and liming’. ‘Liming’ opens up and swells the collagen fibres and breaks down non structural non fibrous proteins, mucins. Swelling opens up and loosens the fibres, removes hairs and helps with oil penetration.

To swell, the hide is soaked in a pickle of alkaline solution (pH up to 12),  either of wood ashes, hydrated lime, or 
commercial lye. All sorts of recipes exist to that end. Takes from 1 up to 5 days for thicker skins.

3. Grain removal or ‘split’. Removes: 1. epidermis, 2. the grain layer or dermal surface/papillary surface; 3. the juncture between the grain layer and the dermis/corum. Leaving grain behind will result in uneven oil penetration and patchy appearance of the hide post smoking.

4. Rinse and acidify or ‘de liming’, this removes the alkali with acidic solution to reduce the swelling of the fibres. Ammonium sulphate or cider or fresh water are used to that end.  This step adjusts the pH to the isoelectric point above 5.0 ideally to around 7, where swelling is minimum. Time: from several hours to several days.

5. Dressing by brain solution. Unsaturated oils such as pig or deer brains, or eggs, or oil are used. The oil has to be forced into the hide and in this process it is done by repeating the steps numerous times. Hypodermis can be removed from the flesh skin at this step. The skin is wrung between the dressing and opened up with the help of a stalking post.

6. Stretching and working the skin. Any holes in the hide can be repaired (by sewing) before the skin is put on the frame for stretching. Softening can be done by frame, cable, staking post, completely by hand, or with various other tools. Keep the fibers moving to prevent the fibres bonding on drying and thereby stiffening the hide. The hide I worked on took 5 hours to stretch and dry. Rotate so that both sides, flesh and grain, are buffed. The cable method produces a more supple hide but it is cumbersome with larger hides.

7. Smoke over a fire pit.

2. Chemistry of the brain tanning and smoking method

The whole process (brain tanning and smoking) can be defined as an aldehyde type of tanning (Covington 2009, Suparno/undated). Aldehydes (R-CHO, a carbonyl centre bonded with hydrogen and an R group) are emitted during wood burning.  Some aldehydes have reactivity towards collagen and prevent putrefaction of meat and skins. The other source of aldehydes is the unsaturated fats in the oil used to dress the hide, these are highly reactive and when exposed to air break down into aldehydes, esters, alcohols, ketones, hydrocarbons. The type of aldehyde produced by oil oxidation and wood burning is acrolein, CH2=CH-CHO, it has unsaturation and aldehyde functionality open to polymerisation, it is toxic and currently not being used by the leather industry.

Discussing the chemistry of the aldehyde tanning, Suparno writes:

‘Although the cross-linking reaction of collagen with aldehyde, especially formaldehyde, has been studied for a long time, the mechanism is still not completely clear.  Most researchers accept that aldehydic compounds react with the free amino groups of lysine and form cross-links.

Collagen-NH2 + HCHO –>  Collagen-NH-CH2OH

The N-hydroxymethyl group is highly reactive and cross-linking may occur at a second amino group:

Collagen-NH-CH2OH   + H2N-Collagen –>  Collagen-NH-CH2-NH-Collagen’

The whole process is equivalent to that in the leather industry called ‘oil tanning’ that produces chamois leather (Covington 2009), which combines dressing the skin with oil and applying aldehyde.  Covington describes the chemistry of the ‘oil tanning’ process as follows:

‘The reactions in the oil tanning process are not completely clear. The active agent is unsaturated oil, preferably cod oil, which can be modelled by linoleic acid, CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO2H, which is known to polymerise but not to form epoxides. It has been generally accepted that the reaction is based on the formation of aldehydic compounds, particularly since the process is accompanied by the release of acrolein, CH2=CHCHO, which has been used as an element of quality control. However, acrolein alone does not make acceptable chamois leather.

Sharphouse (1971) summarised the tannage in these terms:

Fixation of oil or resinous auto-oxidation products to the protein fibre in some intimate sheath-like form. These may be in polymer form and resist removal by alkaline wash waters and common solvents. It is presumed that they account for the differences between aldehyde tannages and ‘full oil’ chamois tannage.

In this way, he defined the outcome of the tannage as a polymer matrix within the collagen matrix; there is no certainty of reaction between the polymer and the collagen, unlike the product of aldehydic tanning. Therefore, the system can be pictured as a matrix of polymerised hydrocarbon chains, holding the collagen fibre structure apart, as an extreme form of lubrication to prevent the fibre structure coming together and sticking. This model does, however, provide a rationale to explain the three most important features of oil tanned leather:

  • Hydrothermal stability: The shrinkage temperature of oil tanned leather is only a few degrees higher than raw pelt. Therefore, the conventional view of tanning does not hold, i.e. there is little interaction between the tanning agent and the collagen. This is an example of the tanning agent having more affinity for itself than for the substrate.
  • Water retention: The effect of keeping the fibre structure apart means that the collagen can be hydrated and hold excess water within the hydrophobic polymerised oil matrix. In this way there is the apparently contradictory situation of a hydrophobic tanning chemistry producing the most hydrophilic leather.
  • The ‘Ewald’ effect: Oil tanned leather is one of a few cases where the leather exhibits a reversibility of hydrothermal shrinking. If the leather is held in hot water, at or above the shrinkage temperature, it will shrink as expected; however, if the leather is immediately placed in cold water it regains about 90% of its original area. The phenomenon can be used to mould the leather into shapes, a shrink fitting process called ‘tucking’. When leather shrinks under hydrothermal conditions, the triple helix structure begins to unravel, hence reversibility only applies in the earliest stages, when the structure can reregister by itself. However, the oil matrix provides a scaffold that mirrors the collagen structure, so the denatured protein can regain much of its original structure, but not quite all of it. ‘ (Covington 2009: 315 – 319). In other words, brain tanned and smoked hide is washable.

© Luba Dovgan Nurse and luba’s conservation, 2000-2016. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Luba Dovgan Nurse with appropriate and specific direction to the original content.

Thanks to Kfir Mendel for comments on the earlier post!

References

 

AIC wiki Animal Skin/Leather.

Covington, A. 2009. Tanning Chemistry: The Science of Leather. Cambridge: RSC Publishing.

Edholm, S. & T. Wilder. 2001. Buckskin: The Ancient Art of Braintanning. Boonville: Paleotechnics.

Kite, M., & R. Thomson (eds).  2005. Conservation of Leather and Related Materials. London: Butterworth.

Richards, Matt. 2004. Deerskin into Buckskin: How to Tan with Natural Materials, a Field Guide for Hunters and Gatherers. Cave Junction: Backcountry Publishing.

Rue, Leonard Lee III. 2004, The Deer of North America. Guilford: The Lyons Press.

Sharphouse J.H. 1971. Leather Technician’s Handbook.  Shoe Trades Publication, 2nd edition.

Suparno, O., no date. Aldehyde tanning, available from : http://web.ipb.ac.id/~ono.suparno/html/aldehyde_tanning.html [accessed 27 June 2010].

Wescott, D. 1999. Primitive technology: a book of earth skills. Gibbs Smith.