Why do modern leather bindings decay? Is it possible to obtain a leather for bookbinding purposes as good and as durable as the leather produced from the 16th to 18th century? These are the two problems which the Committee on Bookbinding Leather appointed by the Society of Arts set themselves to investigate. Fortunately we are able to solve both problems.
In the olden days all skins were tanned with a liquor made from either oak bark or sumach, and in some cases a mixture of the two. The skins used by the tanner were usually obtained direct from the butchers. After soaking and cleansing they were then limed for a period sufficiently long to loosen the hair. After the removal of the hair and superfluous flesh and fat, the skins were washed in several changes of fresh soft water to remove the excess of lime, the process being assisted by working the skins over on a beam with a blunt knife. When in suitable condition they were brought into sour, old tan liquors. There was no hurry, the skins being slowly tanned in weak infusions, and when the process was complete, the leather was simply washed free from superfluous tan, dyed with wood or other vegetable dyes, rinsed free from excess of dye-stuff, and dried out. The leather was afterwards softened by stretching, and polished or glazed by brushing the skin over with oil, soap, beeswax, or a solution of some moss. Such leather lacked the high finish, the regular colour, the bright shades of modern leather, but it lasted fifty or one hundred years with hard wear, and, under favourable conditions, appears to be almost imperishable.
The finish, or general appearance, of leather continued to improve up to about 1850, but after that date some of the bindings examined showed signs of rapid deterioration in quality. This deterioration increased on bindings subsequent to 1870, and probably 75 per cent. of the leather used for bookbinding during the last twenty years either has already decayed or will do so within a comparatively short time.
Now to deal with the answer to the first question, Why do modern leather bindings decay? The chief causes are as follows:—
1. The introduction of tanning materials other than oak and sumach, stronger in tanning, and more rapid in their action. Many of these tanning materials are unstable, and the leather produced disintegrates on exposure to light and air.
2. The use of dried and cured skins of variable soundness imported from abroad. Goat, calf, and sheep skins are imported into this country 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.
4. The use of sulphuric or other mineral acids for the purpose of developing the depth of colour during the process of dyeing.
5. The shaving and splitting of skin for producing an even substance.
6. Printing and embossing grains upon leather, together with other methods of finishing now in common use.
7. The stripping, scouring, souring, and re-tanning of East India leathers (Persians).
8. The removal of the natural grease or nourishment of the skin.
These eight causes, although by no means exhaustive, are, however, the chief factors in the deterioration of modern leather; and in as few words as possible I will explain the reason of their introduction and effect.
As the industry advanced there were found in different parts of the world tanning materials other than oak and sumach, some of which were two, three, or five times as rich in tannin as oak, thus making infusions of greater strength, and consequently of quicker action, with the result that leather which formerly required from three to six months to tan was turned out in almost as many days, or, at the most, in as many weeks. It was not realised, however, that these different tanning materials contained tannins of different qualities, capable of producing quite a different leather to that produced by means of oak or sumach. The experts who spent months testing and investigating this question came to the unanimous conclusion that the speed of tannage or the strength of the tanning liquor had very little to do with the wearing qualities of the leather produced, but found that some of these new materials contained tannins of a different chemical nature from that of sumach, and that they produced leather of an unstable character, some of the leathers undergoing change in a few hours on exposure to strong light and air. On the other hand, some of the new tanning materials produced permanent leathers practically equal to sumach. Those tanning materials, therefore, which contained tannins of the catechol series, including the tannins of hemlock, larch, quebracho, mangrove, gambier, and turwar, were condemned, as all these materials produce a leather which on exposure to light and air turns first a red shade of colour and afterwards develops what is now well known by bookbinders and librarians as the red decay, where the leather crumbles off on application of friction. On the other hand, tanning materials of the pyrogallol class, such as sumach, galls, divi-divi, myrobalans, oak, and chesnut, produce a leather which is practically unaffected by light or air. Further, it was found in every case where authentic bindings dating from the 13th century onwards were examined the tanning material used was one of this pyrogallol series. On the other hand, leathers which showed the red decay were in every case found either to have been tanned with tanning materials of the catechol series, or were rotted with acid.