Post 350 – by Gautam Shah
First preparation for leather begins when raw hides are washed to clean the blood and other tissue materials. After the tanning, the leather is treated for specific surface qualities. The leather passes through several wet processes for colouring and oiling. After drying the hide is mechanically processed for specific look or feel.
Commercially available leathers are of basic four classes.
1 Full-grain leather is a hide without sanding, buffing, etc., nominally employed to remove or level out the natural surface imperfections. The grain structure nearly remains intact allowing to breathe while retaining the strength and durability. Some furniture upholstery, shoe soles, leather shuttle buffers (such as in weaving looms), belting, etc. use the full-grain leather.
2 Top-grain leather Many of the hides are split and shaved (at certain sections) to level out the surface to even thickness, before and after tanning, but for better quality the process is redone. The repeat process makes the leather thin, soft and pliable. Its surface may have less breath-ability.
3 Imprinted grain leather is a product over which grains are embossed or pressed. The surface imperfections are corrected by splitting, sanding, and then grains are embossed. After these processes the leather is dyed. Heavy pigmentation also cover up micro irregularities of the surface.
4 Split leather is created by separating the layers (process similar to veneering of wood) of leather. The top layer, if too thin, the bottom is reinforced with synthetic materials, woven or knitted fabrics or other splits of leathers. The splits are also top layered with synthetic polymer products. Splits are also used to create suede leathers. By-cast leather is a split leather with a layer of polyurethane lamination that is then embossed. It is a leather in only look aspect (and not the feel); otherwise, its surface is completely layered in plastic.
Heavy leathers are finished by coating the grain surface with a finishing compound, and finally by brushing it under a revolving, brush-covered cylinder. The grain surface of light leathers is buffed, or sandpapered, to correct imperfections in the skin. Buffing the flesh side of leather raises the nap and produces the popular leather known as suede. For smooth finishes, most light-leather is seasoned, or treated with a mixture of such materials as waxes, shellac or emulsified synthetic resins, dyes, and pigments. Pigments are used sparingly to avoid a painted look.
Leathers are surface treated in many different ways. Impregnation is achieved by padding, dipping, brushes coating, spraying, rolling, screen printing etc. Impregnation systems are water or solvent based, and help strengthen the crust while saturating the colouring and coating. Fillers are also water and solvent based, used to for achieving surface uniformity of colour and gloss while adjusting the feel. Auxiliaries are many different types, used to optimize the performance of the finishing systems and for special effects. Modifiers improvise the feel quality of leathers, while altering physical qualities of the leather. Dulling agents help adjust the gloss at the top coat. Topcoats are surface covering systems applied to leathers to impart specific transparent, translucent or opaque (solid) colour, shade, or decorative (metal, pearl, etc.) effects.
Leathers are surface altered by chemical and mechanical treatments. Chemical surface treatments include bleaching, staining, and other coatings. Leathers are infused with such materials as Epsom salts, oils, and then lubricated with hot emulsions of soap, grease, and sometimes wax. Mechanical surface treatments are: pressing -for leveling and compacting, rolling -for firmness and gloss, and embossing -to achieve granular or wrinkled textures.
Leather Substitutes, are synthetic substances that look, feel or behave like a leather due to its one of the dominant qualitative advantage, cost, uniformity or easy availability. Replacement products are also used by people who do not wish to use an animal product. These synthetics include such plastics as polyvinyl chloride and non woven fiber impregnated with binders. These materials lack leather’s porous quality, pliable nature, and resilience.