Leather can be thought of as a non-woven mass of fibrous collagen protein that is derived from the skin of animals and that has been rendered biorefractive. An animal skin is made biorefractive and thereby converted into leather by the tanning process. The tanning process permanently incorporates what amounts to biocide into the collagen protein. Other desirable characteristics are imparted to the leather through fatliquoring, coloring, dressing, waterproofing, shaping, and working.
Fatliquoring is the process of incorporating fats, greases, and oils into the body of the leather. The presence of fats, greases, and oils in leather lubricates the fibers and tends to waterproof the leather. Without enough internal lubrication, the fibers will abrade and break due to friction, and the leather may crack.
Leather is given a color by means of dying or pigmenting. Depending upon the means employed, the coloration can be incorporated through the entire thickness of the leather, or it can be concentrated at and near the surface.
Dressing is a surface finish that is given to the material. The surface finish is a continuous organic matrix distinct from the protein fibers. That organic matrix can range from a latex paint to a wax. Dressing always refers to that which is the outmost layer of organic matter of the leather. Thus it is possible for a dressing to be “dressed,” since it is possible to apply a wax polish to a surface of painted leather. Prior to wax polishing, the organic matrix that is the paint outer layer of the leather was the “dressing.”
Working the leather refers to the repeated flexing of the leather in order to reduce stiffness. This is an entirely mechanical process.
HOW TO THINK OF CHEMICALS FOR LEATHER CARE
All chemical treatments of leather fall into one or more of these categories: substitute fatliquors, surface dressings, waterproofers, and cleaners.
HOW CLEANERS WORK
Cleaners are chemicals or chemical preparations that aim to remove foreign matter from the leather. The trick with cleaners is that they have to be able to remove the foreign matter without themselves permanently changing the appearance of the leather being cleaned. Invariably, the matter that has to be removed from the leather is a solid or semi-solid, and the cleaning chemical is almost invariably a liquid. Cleaning is achieved either by dissolving the solid matter or by lifting the matter from contact with the leather through a process of preferential wetting. Preferential wetting means that the leather prefers to be in contact with the cleaning liquid more than with the solid contaminant.
In either case, dissolved solid or lifted contaminant, a cloth or sponge is nearly always necessary to complete the removal of the foreign matter from the leather. The foreign contaminating matter is transferred from the leather to the cleaning cloth.
The cleaner preparation also has to be removed from the leather to completely restore the leather to its previously clean state. Cleaner that is not itself removed can become a foreign contaminant. Cleaner is removed either by rinsing, transfer to a cloth or sponge, by evaporation, or a combination of all three processes.
Rinsing is the process in which excess water is used to dissolve the leather cleaner and carry it off. In effect, rinse water substitutes for the cleaner chemical in the leather. Water, weakly bound in the fiber mass, then itself evaporates. This process of removing cleaner is effective provided the leather itself is not adversely affected by getting wet.
If the leather to be cleaned is dressed, that is has a continuous matrix of organic matter on top of the fibers, then other dressing agents can be used to clean the leather product. In effect, it is the surface dressing that is to be cleaned, not the deep mass of protein fibers. Water and other leather dressing agents are useful in removing contaminants from dressed leather. The processes of dissolution and preferential wetting occur here also, as well as the transfer of solid contaminants from one body to another. Thus, cleaning, waxing, and polishing can occur as part of a single process.
SILCONES AND LEATHER
Silicone is a kind of synthetic oil, and the term silicone refers to a homologous series of organic chemicals that are based upon a backbone of alternating silicon and oxygen. Organic side chains, most often methyl groups, are bonded to the silicon atoms. The chains are terminated with methyl groups, making them non-reactive. The chains also can be terminated with hydroxyl groups or with hydrogen, making them reactive.
Silicone oil is a lubricant, and when it is able to penetrate into the mass of fibers it acts as a substitute fatliquor. Silicone imparts water-repellency, as well as gloss and a pleasant “hand” or feel to leather. Because silicone wets the leather fibers so well, a moderate amount of silicone is able to substitute for fatliquor without impairing the “breathing” of the leather. “Breathing” means that water vapor is able to pass though the mass of fibers. Leather can also be “stuffed” with grease or wax that blocks the passage of water vapor through the mass of fibers. The use of silicone enables the leather to be made water repellant to a degree without the loss of this quality of “breathing.” Stuffing leather with grease makes it waterproof, but renders the leather unable to “breathe.”
Silicone is usually an important component of leather polishes, especially ones that require buffing to bring out the shine. The silicone helps smooth out the wax crystals into a continuous glossy matrix, and also contributes to gloss and water repellency.
Silicone is a very useful product in the care and cleaning of leather. It finds use as a fatliquor, in cleaners, and in dressings. The drawback to the use of silicone is that silicone has a very low surface tension, and the function of other, water-based leather treatment chemicals can be impaired because silicone prevents the other product from wetting the leather. Leather goods