As a natural product leather has a lot to commend it, apart from well known arguments about the initial growth stage of the product.
Craftspeople who start using leather may wish to need know more about this natural and useful material. Chrome Tanned and Vegetable Tanned are frequent descriptors. In practice there may not seem to be much difference. Here are a few things I have learned. I am not expert, please correct me if I am wrong, and I will try not to lay too much emphasis on my own leanings (which do seem to be pointing one way). I am anxious to cover environmental and some usage issues. Please note my use of the the word bookcrafters. Bookbinders will know all this as part of their training ~ one of the reasons why a bookbinding apprenticeship lasts five years.
Leathers come from animals ~ bull ~ cow ~ calf ~ goat ~ sheep ~ pig ~ and others. Vegans and some vegetarians need not read further. Crafters need to learn about the properties of these different leathers, and how they will suit their intended use. In general the bigger animals are made of tougher ~ harder to work with ~ stuff.
After slaughter all hides have to be cleaned. The resulting gunge of fleshy tissues needs to be disposed of with respect to the environment. Chucking it in the nearest river cleans up one place, but transfers troubles downstream, and instream.
After scraping the skins clean, the hairs have to be removed. This invariably involves soaking in nasty chemicals. No other way has yet been discovered. For disposal of these chemicals ~ see the paragraph above.
Traditionally the skins then have to treated so that the structural elements within them ~ that were once part of a living being ~ have to be preserved in a useful state. If this is not done the skin would slowly harden and also start to rot away ~ with associated nasty smell ~ and loss of strength. Compare the cellulose in the fibres of wood ~ a dead vegetable ~ preserved to make paper ~ and the collagen of animal skins ~ preserved to make leather. The water in the collagen has to be removed. Simple drying is not sufficient. The water is chemically bound into the leather, and has to be chemically replaced by something which keeps the collagen supple and useful.
Centuries ago ~ maybe 80 centuries ago ~ it was found that the bark of trees contained chemicals that were released into water and then into skins. Given time this worked useful wonders on the skins. The chemical was tannin ~ hence tanning. In Europe the most effective source of natural tannin is the bark of the oak tree. Many other trees ~ such as chestnut ~ might be used, according to availability. The timescale for this is one of months ~ even years ~ rather than days.
Modern production methods cannot afford such a waiting time, and other chemicals have been found that do the job more quickly ~ a day or two, rather than weeks.
Chromium is an essential ingredient of the "modern" tanning process. In its elemental state it is a hard silvery metal, frequently used to provide a polished surface on less expensive metals ~ chrome-plated car parts. In some forms ~ and miniscule quantities ~ it is an essential part of the human diet ~ broccoli and red wine being notable in this regard. Without going into too much chemistry the chromium we are talking about ~ in diet and tanning ~ is in the form of salts.
There is recent suggestion that the chromium salts in leather upholstery ~ which are different from those in foodstuff ~ can enter the body through the skin in sufficient quantity to worry some people. The evidence is strong enough ~ or the user concern is sufficient for one car manufacturer to stop using such a product.
Much of this is of no importance to the bookcrafter ~ useful to know and bear in mind. More important to all craftspeople is the knowledge that vegetable tanned leathers do not take dyes particularly well. They are restricted to a few 'natural' or 'earthy' colours. Most leather merchants offer a wide range of colourful leathers ~ but they are chrome-tanned.
Decorative work on leathers is ~ I am told ~ more difficulty on chrome tanned leathers. Embossing and carving are best undertaken on vegetable tanned material.