Volume 8 - Autumn 1999


The Manufacture of Leather - part 8

by Thomas McNeill

In the Manufacture of Leather - Part 6 we followed the final wet end processing of the skins, where they were retanned and dyed ready for finishing. In the next couple of instalments we will describe the "trials and tribulations" of finishing Hewit's leather to the highest possible standards.

Types of Finishes

As approximately 90% of all leathers produced at J. Hewit & Sons are used for bookbinding, the need for very "fast" finishes (i.e. finishes with resistance to water washing, soap washing, excessive dry rubbing, or dry cleaning solvents) is not required. In fact, leathers with a fast finish are normally disadvantageous to bookbinders, as these types of finishes do not lend themselves to re-dyeing, gold finishing, blind-tooling or polishing.

In most cases, there are four finishes we use in our standard production leathers.

Aniline Finish

Leather with an aniline finish is leather that has been coloured with dyestuffs only. i.e. no opaque finish such as pigment has been used. The aim of an aniline finish is to produce leather, which has a "natural" look. The major drawback of an aniline finish is that since it is non-opaque, a very 'level' dyeing must be achieved in the dyehouse. Aniline finished leathers also show the natural grain faults of the leather, "warts and all". Many types of leather have too many faults and uneven grain to be produced with an aniline finish. Our main stocked leather with an aniline finish is calf, although goats, basils (sheep) and skivers are sometimes available if we are given enough time to select suitable skins as they progress through the tannery. Aniline finished leather has excellent tooling, polishing and re-dyeing properties and lends itself to the application of genuine gold leaf.

Aniline / Casein Polished Finish

The aim of this type of finish is to produce leather, which retains its "natural" look but at the same time achieves a degree of uniformity in colour with the minimum opacity necessary. This is achieved by using a mixture of aniline dyestuffs and pigmented dyestuffs bound together using casein. Pigments, unlike dyestuffs are insoluble in water or solvents and are opaque. Casein, a protein that comes from milk exists as a water-soluble colloid. It is not unlike albumen (known in the bookbinding world as 'glaire'), which is the protein present in egg white. Binding takes place by the deposition of the binder around the pigment due to the loss of water by evaporation and also hydration of the leather fibres. Pigmented dyestuffs would not themselves adhere to leather or form a "film" so, as in the case of ordinary paints; other materials such as casein must be mixed into the finish. For this reason caseins are known as "Binders".

The leather is first sprayed with a lighter shade than required of this mixture, then a transparent 'top-off' coat with a slightly darker shade is applied. The finish is then fixed using Cationic Casein. Cationic Casein is positively charged whereas most caseins are Anionic and are negatively charged. The reaction of the negative and positive charges meeting binds the two together forming an impervious layer.

Until fairly recently this type of finish would be fixed by cross-linking using Formaldehyde but because of today's strict health and safety legislation (e.g. the UK's C.O.S.H.H. regulations), we are no longer allowed to use formaldehyde in bulk productions.

The finished leather is then polished. The combination of the darker mixture and the burnishing effect on the tips of the grain enhances the final colour and gives a very attractive two-tone effect. This type of finish has all the workable properties of aniline finished leathers and is most commonly used in the production of our Chieftain, Clansman and Kinauld Goatskins.

Matt or Glazed Pigment Finish

This is the most common finish we apply at Hewits and with the exception of the leathers described in earlier paragraphs, all our other standard production leathers are finished "Full Pigment". As the name would suggest, this finish contains no aniline dyestuffs. The dyestuffs are all replaced with an opaque inorganic pigment; the mixture is bound together still using a small amount of casein, but the main binder in this type of finish is Acrylic Resin. Acrylic Resin is thermoplastic, which unlike casein forms a continuous film, resulting in a more level finish and better coverage of the finished skins. The small amount of casein contained in the finish ensures the leather does not become "sticky". It also helps to ensure the finished skins do not resemble sheets of plastic, as it is important we ensure they retain the look and feel of leather.

The leather is finally coated with a matt or bright water-based lacquer emulsion, which seals the finish and gives a certain amount of resistance against wet and dry rubbing. None of the finishes we produce at Hewits are completely "Fast". Should you require a particularly matt finish, be prepared for very poor wet and dry rub results. To achieve a very matt finish we are required to add dullers to the lacquer emulsion which has a detrimental effect on the continuous film causing the "sealing" properties of the lacquer emulsion to break down.

Matt or Glazed Pigment Finishes lend themselves well to the application of metallic foil but not genuine gold leaf and because it is a sealed finish it cannot be tooled or re-dyed.

We are often asked to produce some "weird and wonderful" finishes and effects, which we are always happy to attempt. But please be reasonable. Not so long ago we were asked to produce a leather which could withstand 10,000 rub cycles (using British Standard leather testing equipment), without the finish deteriorating in any way. Considering for example that an upholstery leather has only to withstand 300-500 rub cycles, we politely told the customer "miracles we can do but the impossible takes just a little longer", or words to that effect. But please do not let us put you off asking; we enjoy a good laugh now and again!

In the next issue of skin deep we will be covering various aspects of the work undertaken in the finishing department including, spraying finishes, embossing, different methods of softening skins and polishing and glazing techniques.

Tommy McNeill has been working at the Hewit tannery for the past 18 years. He started his time with us in the finishing room, and after a few years moved 'upstairs' to oversee the warehouse as manager, where many of our customers would have had the pleasure of dealing with him on a day-to-day basis. Four years ago, he moved back to the finishing department that he now manages in his capacity as works manager.


Skin Deep - Volume 8 - Autumn 1999

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