Volume 5, Number 2
Apr 1981

Method for Preparing Leather for Slurrying

Editor's Note: Since the pH of new leather affects its permanence, conservation laboratories often test with a pH meter their incoming leather shipments or samples. The readings thus obtained take on more importance when we consider that accelerated aging, generally the best way of estimating permanence, is not a practical possibility in most cases. There are several difficulties in the use of accelerated aging of leather.

The first difficulty is the time required: it may take weeks to get an answer. Second, the PIRA test is not as good as we thought it was¹ . The third difficulty involves expense. The best method of aging leather, with the gas chamber², calls for equipment that is too expensive and specialized for the kind of conservation research laboratories we have in America today. The last difficulty is that no standard test exists for accelerated aging of leather. Because this makes it hard to compare results from one study to the next and between labs, both research and routine applications are inhibited.

Perhaps one day a way will be found to make accelerated aging more feasible and reliable. In the meantime, pH is frequently chosen as a predictor of permanence because of its demonstrated correlation¹. However, simple surface readings may be misleading. Almost all leather has a finish of some sort on the grain side, which may cause the pH to vary between the two surfaces by a full point or more. To get reliable readings, a slurry must be prepared, usually with a blender.

Adrian Sclawy's position is that even this slurry may not give an entirely accurate reading, since it leaves the fibers mostly intact. His freeze-and-fracture method, described below, has yielded readings about 0.2-0.3 point different either way from those obtained by the regular (unfractured) slurry method.

The following method has been used to slurry leather in order for the internal pH value to be taken.

Method:

1. Select and weigh a leather specimen approximately one inch in size.
2. Soak the leather specimen in distilled water until saturated.
3. Place the leather on the stage of the specimen freezing apparatus. A carbon dioxide cylinder is attached to the freezing stage.
4. Turn on the CO₂ gas and cover the stage with a beaker in order to freeze the leather specimen. (The temperature of the stage reaches approximately -42°.)
5. When the specinen is completely frozen turn off the CO₂ and remove the specimen.
6. With a hammer or mallet strike the leather specimen until it becomes soft, before it thaws. This shatters the fibers.
7. Refreeze the leather specimen and repeat Step #6.
8. Use a blender to completely slurry the leather specimen. For each gram of leather used, add 100 ml. of distilled water to the slurry.

Experiment:

A piece of new calfskin with a surface pH of 3,9 was slurried using the above described method. The freezing apparatus used was an American Optical #930 apparatus. A pH meter with a glass electrode was used for both measurements. The prepared slurry had a pH of 4.9.

1Betty Haines, "Deterioration in Leather Bookbindings--Our Present State of Knowledge," Brit. Lib. J. 3: 59-70, 1977. Summarized in ANL, Dec. 1978, p. 28-29.

2C. W. Beebe, R. W. Frey, and N. V. Hannigan, "A Comparison of Gas Chamber Tests of Bookbinding Leather With a Long Time Atmospheric Exposure," JALCA 51: 20-31, 1956.