Part 5 of the Manufacture of leather described the processes carried out immediately after the tanning of the skins up until the point where the skins would be retanned and dyed. This instalment will look at the processes carried out in the dyehouse as the leather goes through its final wet processing before entering the finishing department.

Dyehouse Processing

Following the shaving of the leather, the skins then enter the dyehouse for further processing. Generally the first process that happens to the skins is "clearing". Clearing involves the removal of iron deposits from the leather that are produced during the grinding/sharpening of the shaving knives. Iron is normally removed by the addition of oxalic acid, although other proprietary chelating agents are now also used. This process can, depending on the amount of iron present, take some time, so generally the skins are left in oxalic acid overnight, after which the liquor is washed away.

Following the clearing of the leather, the skins are then "stripped". The process of stripping involves the addition of alkali to the skins. When the pH of the leather rises above 6-7, the vegetable tannins become loose and can be removed from the skin. The aim of stripping is to remove the tannin from the grain of the leather. This enables the dyer to work on a clean surface free from any discoloration that might have occurred during the initial tannage or drying, and also compensates for variations in tan colour between batches of skins. After the skins have been washed off from the alkaline bath, the pH (around pH 5.5) is then suitable for the retannage, dyeing and re-oiling of the skins.

The Retannage

The aim of the retannage is to produce a specific leather-type with the properties required from general crust leather. In our circumstances, where our main production is for bookbinding leathers, this usually involves the addition of vegetable tans and fatliquors to further refine the crust leather. This is not always the case however, as a wide variety of different end products can be produced from one raw material. In the case of the Indian goatskins we buy, we currently use different retannages in order to produce leathers as diverse as a shrunken grained goat - our Chieftain Goatskin, aluminium retanned archival leathers, embossed goats and bagpipe leathers. This is however by no means a complete list of what can be produced from this material. In the following discussion we will concentrate on the production of two leathers, a smooth leather and a shrunken grain leather.

The first addition to the drum at this stage is further vegetable tan, which will generally be sumac or myrabolams for top end bookbinding leathers. This results in a fresh level tan surface for the future dyeing of the skins. After the tan has been running for around 30 minutes, the dye and fatliquors are added.

Dyeing and Fatliquoring

The history of the dyeing of leather goes right back to ancient times, when natural dyes such as dyewood extracts laked with metal salts were used. The processes for using these products were complicated, and the range of colours was limited. With the advent of the "aniline" (synthetic) dyes at the end of the last century, dyeing became simpler and it became possible to dye virtually any shade. Currently we use anionic dyes that are descendants of the original aniline dyes, having improved physical properties (especially lightfastness) but at the same time being safer to produce.

In the production of leather, fatliquoring is usually the last operation in the aqueous phase before drying. This process is generally carried out using either fish oils or synthetic oils that have been emulsified to allow their use in aqueous solutions. Like the retannage, it is of decisive importance for the quality and properties of the leather. The fatliquoring process largely determines the mechanical and physical properties of the leather. If the leather is dried without fatliquoring, it becomes hard and tinny, because the fibres are not lubricated.

The function of the fatliquoring is to separate the fibres in the wet state so that they do not stick together too much during drying.

quate lubrication and elasticity of the fibres after fatliquoring is essential to give a soft supple handle. Too little fatliquor will reduce the tensile strength, whilst too much can lead to poor adhesion of the finish coats.

After the dyes and fatliquors have been given sufficient time to penetrate the leather, they are then fixed to the leather by acidifying the leather to around pH 3 using formic acid. The use of formic acid is important as this enhances the effect of the dyes.

Finally the skins are lifted from the drum, "horsed up" and dried under tension, either, as in our case on, sheets of glass or more usually on toggle frames.

Shrunken Grain Leather

The only alteration major to this basic process in our tannery occurs with the production of our Chieftain and Clansman Goatskins. In both these cases the skins arrive in already tanned in their countries of origin - Chieftain from India and Clansman from Nigeria. With these skins, the skins are stripped twice and with considerably stronger concentrations of alkali. There are two reasons for carrying out this operation. Firstly the strong stripping ensures that we have removed as much of the unknown original tannage as possible, and secondly, in order to produce a shrunken grain one ought to start with a raw skin. Unfortunately this is not possible when using Indian and Nigerian Goats, since, as mentioned in the previous article, they are tanned in their country of origin in order to prevent rot. By putting the leather in a stronger stripping process this brings the skins back to a near raw condition, and allows the shrinking process to take place.

Following the above-mentioned strong strip, the leather is in an alkaline state. The drum is drained of its float, and a very astringent, acidic syntan is then added that draws the grain of the leather. The skins then thoroughly retanned using sumac or myrabolams and carry on through the processing listed above, however at the end the skins are hung to dry to ensure that the grain is not pulled out of the skin.

In the next edition of Skin Deep we will discuss archival leathers in depth. The ageing properties of different types of tannages will be studied and points raised in what to look out for when purchasing quality bookbinding leathers.