The history of aluminium tannages begins in the Middle East. Alum is a double salt of aluminium and potassium phosphate, and occurs naturally in many warm climates, hence it's early use as a tanning agent. The basic process is simple, consisting of simply immersing raw pelts in a solution of alum, and this process was well established in Egypt by 1600 BC. The production of alum leathers spread throughout the Mediterranean through both Arab traders and the Roman Empire, and had reached Britain by the ninth century AD.

The production of alum leathers became very widespread during the Middle Ages, and was used for the production of bookbinding leathers, gloving leather, ladies shoe leather and fur skins. Whilst there has been a dramatic reduction in the quantities of alum leather produced since the advent of chrome tanning, alum leathers are still used for bookbinding, high quality gloves, furs and cricket balls.

The manufacture of alum leathers is called tawing, and is quite distinct from the tanning process described in earlier editions of Skin Deep. Whilst the aluminium in the alum does combine with the leather fibres, as can be seen by the increased shrinkage temperature of the leather, the leather is sensitive to being washed in water. Unlike "normal" leathers that can stand washing, when alum leathers are thoroughly immersed in water, the tanning salts are washed out, and sulphuric acid is produced. When dried the resulting material is hard and inflexible having the characteristics similar to those of a dried raw pelt. Whilst the problems associated with the washing out the of alum are indeed serious (one only has to remember the damage done in the Venice floods), alum leathers handled correctly are among the most stable leathers ever produced. Many fine examples of Medieval alum tawed leathers are still available in libraries and museums around the world.

Whilst the original alum leathers were produced using only alum, the process was modified fairly early in it's history to include salt, egg yolk and flour. These ingredients give the leather a fuller substance, and also a softer handle. In Medieval times, skins were unhaired and then given a bran drench. The fermentation that resulted produced acetic acid that removed the lime from the unhairing, after which the skins would be scudded to remove any remaining hair and pigment from the skin. The skins would then be placed in a tub containing alum, salt, flour and egg yolk, and would be agitated by hand or using wooden poles over a period of a few days. After being left over a wooden horse, the skins would be hung up to dry, producing a very hard and inflexible material. This crusted leather would be allowed to age for a few weeks to allow the alum to stabilise. The leather was then conditioned using damp sawdust, and then hand staked. The stake was a wooden support, at the upper end of which was a blunt steel knife (see right). The conditioned skin would be laid over the stake, and the staker, holding both sdes of the skin, would forcibly draw the skin over the knife in all directions. This action would stretch the leather fully, and in the process remove the stiffness producing a very soft pliant piece of leather.

The process has changed very little over the centuries apart from the usual mechanisation that has occurred widely within the trade. Nowadays the skins are placed in a wooden drum to increase the agitation during the tawing process, and the hand staking is now carried out by machine (see The Manufacture of Leather Part 9).

The one area however where knowledge has been lost is in the dyeing of alum leathers. Alum tawed skins are peculiar in the way that they dye, and special methods and dyestuffs were used. The leathers were dyed using vegetable dyes and mordanting agents, modern synthetic dyes being of little use. The leather was first washed in an alkaline solution (usually stale urine or ammonia), and then repeatedly brushed or dipped in a dye-wood or vegetable dye liquor. Following this the skins would be given a mordant wash using a metallic salt in order to either enhance the colour or to bring out a special tone. The use of the mordanting also had the effect of making the colours generated more permanent.

Typical dye-woods or vegetable dyes were:-

Oak Bark, Logwood, Sumac, Fustic, Elderberry Juice, Cochineal and Persian Berries.

Mordanting Agents included:-

Copper Salts (blue), Iron Salts (black/dark shades), Tin Salts (red).

Following the dyeing, in the case of dipped skins, the leathers were generally "re-egged" in order to replace the egg yolk lost during the dyeing. The skins would then be dried to the crust state again, prior to being conditioned and staked as before.

Regrettably most of the dye-woods and vegetable dyes are no longer commercially available, however there is no reason why interested binders could not experiment if interested - we look forward to hearing about your results!

References
Proctor, "Principles of Leather Manufacture", Spon, 1922
Reid, "Ancient Skins Parchments and Leathers, Seminar Press, 1972
Sharpehouse, "Leather Technician's Handbook", © LPA, Buckland Press Ltd., 1989
Watt, "The Art of Leather Manufacture", Crosby Lockwood & Co., 1885