Volume 8, Number 5
Oct 1984

Experts Agree Potassium Lactate Ineffective

Potassium lactate and, more recently, potassium citrate, have been widely used in the United States at least since the 1960s for treating leather to protect it from attack by sulphuric acid. Since 1980, however, the grapevine has spread doubts about its effectiveness, and there were murmurs that repeat applications were necessary. Now, finally, the work that apparently inspired the rumors has been published. It comes from research centers in England and the Netherlands, both of which sent representatives to the recent FAIC Leather Refresher in Harpers Ferry, Virginia: Betty Haines of the Leather Conservation Center, and H.A.B. van Soest and Pieter Hallebeek of the Central Research Lab in Amsterdam.

The publications are The Conservation of Bookbinding Leather (available for j30 from the British Library, Reference Division, Publications Office, Great Russell St., London WC1B 3DG), and "Chemical Solutions of Problems Encountered in the Conservation of Old Leather" by van Soest and Hallebeek, in ICOM Committee for Conservation Preprints, 7th Triennial Meeting, Copenhagen, 10-14 Sept. 1984. v.2, p. 84.18.16-84.18.18 (available from ICOM, 1 rue Miollis, 75732 Paris, France, or from ICCROM, 13 via di San Michele, 00153 Rome, Italy).

The van Soest-Hallebeek case against potassium lactate, and the solutions they found to the buffering problem, were put as follows.

When sulphuric acid, formed in the leather as a result of contact with polluted air, interacts with the protein of the leather, it forms an end product, ammonium sulphate, which can accumulate to 11% of the weight of the leather. It dissociates and serves as a reservoir for sulphate ions, which can cause the pH to decline to values as low as 2.1. The amount of ordinary buffer salts (lactates, citrates or tartrates) necessary to counteract this amount of acidity in the leather, as well as that in the environment as time goes by, is impossibly high: 10 liters of 2% solution per square meter of leather. (For a 7% solution, such as we commonly use in the United States, that would be about three liters per The authors say, however, that concentrations higher than 2% do not penetrate the leather more than superficially.)

The authors discovered a buffer that is twice as effective as potassium citrate, and which does not have to be applied in aqueous solution (in fact, should not be used in aqueous solution because it darkens the leather to an extent greater than ordinary water would): imidazole. The formula is C₃H₄N₂ or HNCHNCHCH, and the dictionary says it is pronounced îm îd âz ôl or -åzôl This is a very cheap, common chemical, white crystalline base that reacts as a weak acid. It is used in agriculture to control insects, though it is not a poison. It is a structural antagonist, a type of antihistamine. It is hygroscopic, so it acts as a humectant in the leather. [The authors dc not say whether this makes the leather more vulnerable to mold.] It forms insoluble salts with iron, copper and similar metals, which keeps them from catalyzing the oxidation of sulphur dioxide. The method of application and the concentration is determined by the condition of the leather.

Even though imidazole is a more effective buffer than potassium lactate and other such salts, in order to counteract the acidity in deteriorated leather and control buildup of ammonium sulphate over the years, repeat applications are necessary. The number of repeat applications would depend again on the condition of the leather, presumably. Tests and calculations would have to be made on each object treated.

This short paper ends with a warning that neat's foot oil is not always pure, but may be adulterated with rape seed oil, soya oil, or even, for all we know, a drying oil. The iodine value should be in the 67-81 range if it is pure. Other values for neat's foot oil are given so that shipments can be tested.

Betty Haines's comments will be covered in a subsequent article.