It is a common misconception that salt-drying a hide completely is a necessity to “lock the hair” … it is a method of curation for storage, in that enough moisture is removed to prevent decay. However, there are benefits to salting – primarily the removal of non-tannable globular proteins… and for the in-shop tanner “wet-salting” is adequate, as fully rehydrating salt-dried skins presents challenges within itself. “Wet-salting” is the process of salting a hide heavily for 1-2 days only, but not hanging it to dry out. Can a hide be pickled green without salting, and tanned? …yes… but leather of higher quality is obtained by salting.
Animal Skin Composition
The composition of animal skins is as follows:
- Water – 65% NOTE: a 10 lb. average raw deer cape consists of 6.5 lb. of water (over 3 quarts!)
- Proteins – 33% (30% preferred, 3% not-preferred) a. 30% Fibrous proteins / “structural substances for leather making”: – collagen (skin structure for tanning), elastin, keratin (epidermis, hair) b. 3% Globular proteins / “non-structural components of skin” – Glycosaminoglycans (most relevant is Hyaluronic Acid) – Albumines / Globulines / Various Proteides & Melanines
- Mineral matter 0.5%
- Fatty substances NOTE: a 10 lb. raw deer cape will contain 5 oz. of non-structural proteins total
Hyaluronic Acid
Hyaluronic Acid is a chain of carboxylate groups – the molecule has a large exclusion volume in the form of a gel, making it difficult to remove it from the fibre structure, even though it is not bound to the collagen, but merely entangled in it. Salting allows the chain structure of hyaluronic acid to contract in volume, causing the molecules to be released from the pelt “during rehydration”. However, as salting in global leather is ecologically counterproductive due to salt waste-water disposal issues – non-structural protein attempted removal is during the soaking process with enzymes. Note: It is the experience of global leather tanners, that “tanning of residual non-structural proteins does exist and for some applications could actually be a benefit for functional leather requiring a firmer handle”.
Side Effects of Leaving Non-Structural Proteins in Tanned Skins
The side effects of leaving non-structural proteins in skins is “stiffer leather of reduced softness”. Salting does produce whiter tanned leather. Will skins that have globular / non-structural proteins left in “still tan”…yes. Quality measure of softness is improved by removal of non-structural proteins… very applicable for decorative and/or functional skins.
The vast majority of fluid drainage you see from salting a skin is simply water… (water = 65% skin composition) …it is not globular / non-structural proteins. The globular proteins (3% skin composition) are removed / washed / released from the skin during the rehydration step, as a result of salting. You must adequately rehydrate a skin to fill the fibre structure back up with water for chemical transport as the primary purpose (see below), and cleanse / release the non-structural proteins as a secondary function.
Regarding rehydration of wet-salted skins: “Rehydration function is to fill up the fibre structure with water, to return it to its original flaccid condition, ensuring that all elements of the hierarchy of structure are wetted to equilibrium, with the purpose of facilitating movement of dissolved chemical throughout the pelt section – and to remove dirt, salt and some soluble proteins”. Only when a skin is completely wetted and filled with water can chemicals be transported throughout the fibre structure. If this does not happen, chemical processing will be non-uniform (leaving rawness in skin, etc.). Salting causes dehydration, causing the fiber structure to collapse – even wet-salted skins are significantly dehydrated. Wet-salted skins must be rehydrated completely prior to pickling (simply soak in plain cold water until fully relaxed).
DO NOT ADD SALT to a rehydration soak for salt-dried skins…it only slows the process. Rehydration is the function of introducing “salt-free water to a water-free skin”. The longer hides are in a rehydration soak, the more time is available for bacterial re-activation… as water temperature increases to 60 deg F, bacterial risks increase.
The exception of salt-addition to a rehydration soak is “air-dried skins” … see Tanning Instructions page “REHYDRATING, DEODORIZING, PICKLING, DEGREASING” for complete instructions and tips, including grease and/or freezer-burned skins, etc.
