Decay of Leather. Of late years there has been much dissatisfaction expressed over the decay of modern leathers. In many cases leathers 100 years old or more are in a fair state of preservation, while many leathers—perhaps we should say most leathers—30, 20 or even 10 years old, show signs of decay, and some have rotted away almost entirely. In no way is the decay of modern leathers shown more conclusively than in a long file of the sheep-bound set of the United States Public Documents. The earlier volumes, covering the 15th Congress up to the 36th, will in general be found to be in a good state of preservation. About the year 1860, however, marked deterioration appears. In some cases it seems as if the backs had entirely disintegrated. While a sheep-bound set is the best example of the decay of modern leathers, many libraries have been greatly annoyed by the decay of other leathers which are supposed to be much stronger than sheepskin.
So great was the dissatisfaction with modern leathers that in the year 1900 the Society of Arts in London appointed a committee to investigate modern leathers and to ascertain the best methods for preparing leathers for binding. The report of this committee was published in 1905, and a general knowledge of its contents is necessary for those who wish to know why leathers decay and how they may be preserved. Another valuable work which should be carefully studied is entitled "Leathers for libraries," published by the Sound Leather Committee of the Library Association in England. In a chapter of this book J. Gordon Parker, Ph. D., Director of the London Leather Industries Research Laboratories, tells us that the chief causes of decay are as follows:
1. The introduction of tanning materials other than oak and sumac, stronger in tannin, and more rapid in their action. Many of these tanning materials are unstable, and the leather produced disintegrates on exposure to light and air.
Tanning is the process of changing hides, which are animal matter subject to decay, into material which is strong and flexible and which is not subject to decay. There are two groups of tanning materials. The one called the pyrogallol group, which includes sumac, gall nuts and oak bark, is commended by the Committee of the Society of Arts. The other is called the catechol group and is strongly condemned. Among the materials belonging to the catechol group are hemlock bark and larch, used chiefly in this country, and turwar bark used largely in tanning East Indian goat and sheep skins.
The United States Government Printing Office proposals call for leathers tanned with a pyrogallol tannin, since it has been proved that such leathers prove most resistant to the evil effects of light, heat and gas. If a skin has been tanned with sumac, or sumac in combination with oak bark, it has received the best possible preparation for its life as a bookbinding leather.
It is not possible, neither is it necessary, to have all leathers tanned with sumac. Those leathers which are intended to remain on the shelves for many decades, should be in this tannage. Those which will be discarded in a few years may be tanned in a less expensive manner. Even some of the quick, modern materials, which accomplish in a few hours what once would have taken weeks, are not too poor for leathers which are not to be kept indefinitely.
Even the expert cannot detect skins which have not been tanned and dyed properly, but leathers which last a short time may be suspected either of being improperly tanned or of having some kind of injurious acid in their composition.
2. The use of dried and cured skins of variable soundness imported from abroad. Goat, calf and sheep skins are imported into this country (England) from all over the world; some are simply dried in the sun, some salted, whilst others are cured with various ingredients.
3. The use of infusions of acids and other bleaching agents to produce bright and even shades of colour.
Skins tanned with sumac are light-colored and can be dyed any desired color. Most other tanning materials, however, make darker-colored skins, which can be dyed only with dark colors. It has been found possible to scour and bleach skins with sulphuric acid and to re-tan with sumac.
4. The use of sulphuric or other mineral acids for the purpose of developing the depth of colour during the process of dyeing.