Leather

Lubrication of leather

Is it necessary?

The usual answer for museum objects is NO!

The main purpose of applying dressings to leather in museums, galleries and libraries is to prevent the leather hardening if the relative humidity fluctuates widely.

If too much lubricant is applied, the leather repels moisture and eventually becomes hard and brittle—the very effects that the application of the dressing was meant to prevent.

Most mould infestation is caused by the presence of too much lubricant.

Some dressings can also darken leather and cause increased stickiness. A sticky surface collects and holds dust, and is very hard to clean.

Lubricants do not provide protection against acidic pollutants.

Consider these points when deciding whether or not to apply a dressing to leather. If the leather is in its required shape, does not need to flex and is in a relatively stable environment, then lubrication is usually not necessary.

As leathers age, their ability to absorb fats and oils effectively is reduced. The amount of fats and oils needed in archaeological and older leathers is less than in their modern counterparts. Lubricating aged leather only causes more problems.

Dressings should not be used to 'feed' leather or as a way of improving its appearance. These approaches inevitably lead to over-lubrication and the development of some of the problems outlined above.

If you want to improve the appearance of a leather object which originally had a polished surface, it is better to use a wax polish. Because this is primarily a surface treatment there should be minimal impact on the leather itself.

When should lubricants be used?

Lubricants are really only necessary if:

• flexibility needs to be restored to an object; and

• the leather is displayed in an environment which experiences repeated and extreme fluctuations in relative humidity.

It is important to realise that museum objects rarely need to be flexible, because they are generally not used. They are usually stored and displayed. If the storage environment is stable, there is little need for lubrication.

To restore flexibility to hardened leather, it is necessary to rehumidify or condition the leather before lubrication. Various procedures may be used (Calnan, 1984). These include:

• sponging the surface with an alcohol solution diluted with water or a water-based moisturiser;

• covering the leather with damp sawdust overnight; and

• placing the leather in a humidity chamber.

If an object needs to be reshaped then humidification often will be enough.

The next step in restoring flexibility to leather is the actual lubrication itself.

Types of lubricants for leather

The fats and oils which lubricate the leather can be applied either in a water-based emulsion or dissolved in organic solvents. Fats and oils dissolved in an organic solvent are known as solutions or dressings.

Vegetable oils are not used as frequently as fats, because in the long term they are more prone to oxidation which results in the oil yellowing and hardening; this is followed by loss of the lubricating properties.

A water-based emulsion is best if tests show that:

• water does not discolour the surface; and

• the surface absorbs water.

Applying oils in emulsion increases the likelihood of oils remaining evenly distributed throughout the interior of the leather. If an emulsion cannot be applied, use a solvent-based dressing.

Guidelines for the use of lubricants

Only apply lubricants to leather which is:

• deformed—the lubricant is used to make the object more flexible and to assist in reshaping the object;

• extremely dry; or

• cracked due to shrinkage.

Always test the lubricant in an inconspicuous area before use.

When the leather is generally in good shape, but is dry and hard, applying a commercial dressing/wax such as Fredelka is useful.

If the dryness is only a surface condition, or if the leather is very thin—for example, book covers and car seats—then a preparation such as British Museum Leather Dressing adequately restores surface-oil content.

Apply it sparingly, using a soft cloth. It can be used on leather with metal attachments or decorations. The beeswax in this dressing forms a thin film on the leather surface which can be polished.

Some surface finishes resist the penetration of oils and fats into the leather, whether you use an emulsion or a dressing. If this happens, it is best to rub an oil emulsion into the flesh side—or underside—of the leather, to encourage penetration.

Although many commercial leather dressings are available, these may not be suitable for museum objects because they are designed for leather objects which are being used. Static museum objects have different needs because flexibility is usually not an important consideration.

Do a careful assessment before applying any treatment to the leather of a book cover. In most cases it is preferable to store the books under the best possible conditions.