Vegetable tanned leathers have always been prone to acid deterioration, and a great deal of energy has been put into improving the performance of bookbinding leathers over the last fifty years. There are two different groups of vegetable tannins - the hydrolysable and condense tannins. Leather tanned with hydrolysable tannins have for a long time had a reputation of greater durability, and it is for this reason that the vast majority of bookbinding leathers are tanned using these tannins.

Work in the early 1930's showed that hydrolysable tannins had a far greater buffer salt content, and it was to check on the buffering capacity of the leather that the PIRA test was invented. A long-term trial carried out between 1935 and 1970 on bound books gave very inconclusive results and it was realised that further research would have to be carried out. The British Leather Manufacturers Research Association together with several libraries and tanneries have been carrying out extensive trials since then to improve the durability of bookbinding leather.

In America in the 1940's it was discovered that vegetable tanned leather retanned with aluminium gave exceptionally good resistance to ageing in a gas chamber containing a moist acid vapour and the BLMRA decided to investigate the use of this leather for bookbinding. Aluminium imparts tremendous stability to vegetable tanned leathers as can be shown from the fact that the retannage with aluminium increases the shrinkage temperature from 75-80 C to 125 C. The increased stability of the tannage can be demonstrated in the gas chamber where leather retanned with 2% aluminium after 24 weeks has retained 60% of its tear strength compared with 20% of vegetable tanned leather and semi-chrome leather. All the leather except the semi-alum leather had lost their resistance to grain scuffing and flexing. The semi-alum tannage also buffered the leather to a far greater extent ending the 24 week trial with a pH 3.0 compared to pH 1.7 for the other leathers, all the leathers starting at a pH of roughly 4. In their conclusion the BLMRA stated that vegetable tanned leather for archival bookbinding should be retanned with 2% aluminium on leather weight.

These results were published in 1984, and since then we have been working on this leather, not only to meet these standards but also to produce a leather that is acceptable to the bookbinder, without the early problems experienced of water-repellency and springiness.