Excerpts from NPS Museum Handbook, Part I (1996)
There are many problems associated with the use of "saddle soap" on objects made from animal skin products. With the best of intentions, this commercial product has been inappropriately applied to just about every form of skin material in the past.
"Saddle soap" was not developed as a cleaner, but as a 19th century leather conditioner. Its basic components of neatsfoot oil and cod or sperm oil were emulsified with soap in water to produce an emulsion fat-liquor introduced during early tanning. As a conditioner, saddle soap is considered obsolete by tanners today. Its application has caused considerable permanent damage to skin and leather objects since its components cannot be easily rinsed out and adequately removed (as manufacturer instructions often suggest). Saddle soap effectively softens and emulsifies surface oil and dirt, however it usually distributes them deeper into the material. The mixture's high moisture content presents a hazard to aged skin materials that should not be wetted, as well as light colored vegetable and/or alum tanned leathers. Commercial formulations of saddle soap differ in their ingredients, some containing abrasives and even colorants. Saddle soap quality fluctuates greatly among manufacturers.
Perhaps most importantly, leather care experts now suspect that the surface cracking on many older skin and leather objects may well be due to past "saddle soap" application. Avoid it.
Saddle soap is actually a very poor cleaner. It must first dissolve its own oils, limiting its capacity to dissolve dirt and oils in the leather. Saddle soap is also notoriously alkaline and alkalinity actually damages leather. Alkalinity can abrade both the hide itself and the stitching which binds it. Another problem arises during application. Most saddle soaps instruct the user to work the lather into the leather. Since loosened dirt is suspended in the lather, it is pushed back into the leather's pores.
Excerpt from NPS Museum Handbook, Part I (1996)
While skin materials have a great affinity for water, inappropriate levels of atmospheric moisture or direct wetting usually cause serious damage. The direct wetting of skin products initiates deterioration because these materials have only a limited degree of water resistance.Objects made of full-tanned leather are also highly susceptible to stiffening and darkening from wetting.
When skin material is subjected to either excessive moisture or high humidity in conjunction with heat and acid conditions, its chemical structure is attacked, causing shrinkage and embrittlement. If allowed to continue, the skin will lose its structure and become gelatinous.
In vegetable tanned and chrome-tanned leather the absorption of unnecessary moisture increases hydrolysis of the protein chains that form the collagen fibers. This will cause the fibrils to shrink, stick together and eventually become brittle and hard. (Hydrolysis is chemical decomposition in which a compound is split into other compounds by reacting with water.) This reaction is not reversible.
Oil-tanned leathers have a different reaction to excess water, the hydrolysis of the protein chains that form the collagen fibers is not as drastic. The leather will shrink but it will retain most of its handle (tactile touch) and flexibility and can usually be stretched back to most of its original size. Of course, the more often oil-tanned leather comes in contact with water, the more the protein chains are hydrolysized, leading to irreversible shrinkage of the fibers and eventual brittleness.
Place an ample pinch (appx 1 tablespoon) of glycerin soap flakes into the lather bowl. Add approximately 1/4 cup water slowly, whisking with your soap brush or other implement until a think lather forms. This should be the consistency of shaving cream. You may not have to use all the water. The lather and only the lather should be applied to the leather with a soft brush or a damp cotton cloth and rubbed in a circular motion.
- Scrubbing is not recommended unless you need to remove mud or other glop.
- Rinse and squeeze out brush/cloth before reapplying lather. Remove all residual soap with a damp cloth. There is no need to saturate the leather with water.
- Always clean out your soap bowl and soap brush after an evening of use. Dirt, grim and grit remaining in the soap and brush are a detriment to the next batch of leather you will be cleaning.
- It is best to let leather dry after cleaning so that excess moisture is not trapped under layers of conditioner or polish.