The first most important thing after flaying an animal you intend to have processed is to properly handle it and perform preservation on the skin, that it does not putrefy between flaying and tanning. There is usually a gap between flaying and tanning, since most butchers do not happen at the tannery. There are many ways to do this preservation, but dry salting is the only method we accept for custom tanning.
Definitions
- Flaying – Removing the skin from the animal
- Salt – Granulated Sodium Chloride (table salt)
- Rawstock – An untanned skin, which may or may not be preserved
- Green skin – An untanned, unpreserved skin, basically right off the animal
- Salted stock – Preserved rawstock using the dry salt method
- Collagen – Protein structure that makes up the bulk of skins, which is primarily the target of the tanning process.
- Flesh side – The inside part of a skin, opposite to where the hair follicles are.
- Nontannables – Components (mostly fats) of a skin that are chemically unable to be stabilized through tanning, and must be removed before tanning occurs, usually through mechanical means and chemical dissolution.
On Putrefaction
Organic materials are generally made up of fats and proteins. Proteins are chains of molecules arranged in a way to perform a function, such as forming the fiber of collagen, or connective tissue. Enzymes are also proteins, that do work to other molecules, proteins, fats, and generally anything.
Skins putrefy when they are under microbiological attack, such as from bacteria and fungi. The first threat is the threat of bacteria, which can grow and multiply and render the skin useless within 24 hours depending on handling and climate.
Bacteria and fungi excrete enzymes that break down the proteins that make up the skin, which is called decomposition or putrefaction. This is because the bacteria and fungi are digesting and eating the skin. The greater amount of bacteria, the more damage occurs to the skin.
The goal of preservation is to kill or inhibit the growth of the bacteria and fungi through making the skin inhospitable, and eventually the tannage will make the proteins that make up the skin unreactive to microbiological enzymes, thereby preventing putrefaction.
General methods of inhibition of microbiological growth
The growth of microbiological organisms (mostly bacteria) responsible for the decomposition of skins can by inhibited to varying degrees by the following methods:
Handling Practices
Initially this is the most important method. The goals of proper handling is to avoid direct contact with contaminating surfaces, prevent unneeded hydration, and to quickly cool the skin.
After flaying, the ideal short term handling is to immediately drape the skin flesh-side-out over a rail in a shady, dry area.
If possible, do not let the flesh touch the ground or let the wool side curl in to touch the flesh side. The purpose is not to introduce extra bacteria to the skin. There is already bacteria there, but it is beneficial to reduce the diversity and quantity if possible.
If possible, do not spray the skin with water, as the extra hydration will aid the growth of bacteria and significantly shorten the amount of time until the skin may be handled without short term preservation.
If possible, do not let the skins fold unto themselves or touch each other – it is imperative to allow the skins to cool to ambient temperatures. Folded/piled skins remain close to body temperature for a long period of time, which cause accelerated bacterial growth.
Temperature
The optimum temperature range for bacterial growth is between 70-90 degrees Fahrenheit. The skin may be stored at a temperature higher or lower than the bacteria can survive. They are never stored at high temperatures because the heat will break down the proteins and “cook” the skin – damage that is unrecoverable. The most effective method is refrigeration, or Chilling.
Dehydrating
The rate of bacterial growth is largely determined by the water content in the skins. After flaying, the skins are “damp” – but not “wet”. Preventing them from getting wet will buy more time, and drying them will actively preserve them for longer periods of time.
More information on Drying/Dehydrating can be found below.
Antimicrobial agents
Poisoning the bacteria and fungi with chemicals is an effective way to significantly “buy time” until the skin can be further preserved or processed. This method is sometimes combined with other methods to increase the effectiveness of preservation.
Short Term Preservation
These preservation methods will generally not last longer than 72 hours, and should only be used as an intermediate method before long term preservation or if the skins will be able to be processed within a short time.
It is important that the skins do not get wet from plain water, otherwise these methods may be shortened to 36 hours or less.
Chilling
Placing the skin in refrigeration will preserve the skin for a short period of time.
Antimicrobial Spray
After flaying, a bactericidal/antimicrobial agent can be sprayed onto the flesh side, which will allow for the skin to be able to be held at ambient temperature and humidity for up to 72 hours.
Short-salting
This method is simply the beginning steps of dry salting, but you expect to process the skin within 72 hours.
Long Term Preservation
These preservation methods will keep the skin in good condition for a period longer than 30 days if they are done properly and the storage environment is suitable.
Salting (Dry)
This method is considered the best long term preservation method due to it’s reliability. We have detailed instructions on how to perform salting in another post: Rawstock Preservation Instructions
Dry salting’s primary preserving action is dehydration of the skin, which inhibits microbial growth.
Dry salted skins that are properly stored can last for an indefinite period of time.
Salting (Wet Brining)
This method is mostly used within the tannery in order to have a somewhat relaxed skin that is ready to be processed almost immediately. The skins are submerged in a saturated salt solution, usually with the addition of an antimicrobial agent.
As with the dry salting method, although the skins are “wet” – they are in essence dry to microbiological life due to the salinity and factors with the sodium ions that make up the salt that are beyond the scope of this document.
Brined skins if properly monitored and stored can last for periods of 6 months or greater.
Pickling
This method is similar to Brining, with a reduction of salt and the addition of acids that lower the PH of the solution, which not only inhibits microbial growth, but also performs an essential step as a prerequisite to the actual tanning.
In this case, the salt does not act primarily as a dehydrant, but as an electrolyte that allows for more effective dissolution of Nontannables.
Pickled skins can last several weeks if the solution is monitored and maintained. It is possible that damage will occur over longer storage periods.
Chilling
Rawstock are chilled just above freezing. Alternatively if the skin has some salt content, it can go below freezing for longer storage periods. It is good practice to avoid the formation of ice crystals, as these damage the structure of the skin and degrades the quality somewhat. Chilled skins can last in storage for several months.