Leather - Forms

Forms

Several tanning processes transform hides and skins into leather:

• Vegetable-tanned leather is tanned using tannin and other ingredients found in vegetable matter, such as tree bark prepared in bark mills, and other similar sources. It is supple and brown in color, with the exact shade depending on the mix of chemicals and the color of the skin. It is the only form of leather suitable for use in leather carving or stamping. Vegetable-tanned leather is not stable in water; it tends to discolor, so if left to soak and then dry it will shrink and become less supple, and harder. In hot water, it will shrink drastically and partly gelatinize, becoming rigid and eventually brittle. Boiled leather is an example of this, where the leather has been hardened by being immersed in hot water, or in boiled wax or similar substances. Historically, it was occasionally used as armor after hardening, and it has also been used for book binding.
• Chrome-tanned leather, invented in 1858, is tanned using chromium sulfate and other salts of chromium. It is more supple and pliable than vegetable-tanned leather and does not discolor or lose shape as drastically in water as vegetable-tanned. It is also known as wet-blue for its color derived from the chromium. More esoteric colors are possible using chrome tanning.
• Aldehyde-tanned leather is tanned using glutaraldehyde or oxazolidine compounds. This is the leather that most tanners refer to as wet-white leather due to its pale cream or white color. It is the main type of "chrome-free" leather, often seen in automobiles and shoes for infants.
  • Formaldehyde tanning (being phased out due to its danger to workers and the sensitivity of many people to formaldehyde) is another method of aldehyde tanning. Brain-tanned leathers fall into this category and are exceptionally water absorbent.
    • Brain tanned leathers are made by a labor-intensive process which uses emulsified oils, often those of animal brains. They are known for their exceptional softness and their ability to be washed.
  • Chamois leather also falls into the category of aldehyde tanning and like brain tanning produces a highly water absorbent leather. Chamois leather is made by using oils (traditionally cod oil) that oxidize easily to produce the aldehydes that tan the leather to make the fabric the color it is.
  • Rose tanned leather is a variation of vegetable oil tanning and brain tanning, where pure rose otto replaces the vegetable oil and emulsified oils. It has been called the most valuable leather on earth but this is mostly due to the high cost of rose otto and its labor-intensive tanning process.

• Synthetic-tanned leather is tanned using aromatic polymers such as the Novolac or Neradol types (syntans, contraction for synthetic tannins). This leather is white in color and was invented when vegetable tannins were in short supply during the Second World War. Melamine and other amino-functional resins fall into this category as well, and they provide the filling that modern leathers often require. Urea-formaldehyde resins were also used in this tanning method until dissatisfaction about the formation of free formaldehyde was realized.
• Alum-tawed leather is transformed using aluminium salts mixed with a variety of binders and protein sources, such as flour and egg yolk. Purists argue that alum-tawed leather is technically not tanned, as the resulting material will rot in water. Very light shades of leather are possible using this process, but the resulting material is not as supple as vegetable-tanned leather.
• Rawhide is made by scraping the skin thin, soaking it in lime, and then stretching it while it dries. Like alum-tawing, rawhide is not technically "leather", but is usually lumped in with the other forms. Rawhide is stiffer and more brittle than other forms of leather; it's primarily found in uses such as drum heads where it does not need to flex significantly; it is also cut up into cords for use in lacing or stitching and for making many varieties of dog chews.

Leather—usually vegetable-tanned—can be oiled to improve its water resistance. This supplements the natural oils remaining in the leather itself, which can be washed out through repeated exposure to water. Frequent oiling of leather, with mink oil, neatsfoot oil, or a similar material keeps it supple and improves its lifespan dramatically.

Leather with the hair still attached is called hair-on.