Test methods
pH indicator paper (non bleeding) can give a reliable indication of the pH value, and this is important for
assigning the bookbinding to a damage category and treatment.
Several data are needed to provide insight into the background of the damage to leather and parchment: the
pH value, the ammonium content and the sulphate content. These parameters can be determined by the book
conservator himself, but for a correct interpretation of the individual test results and of their correlation, it will often
be necessary to consult a specialist in this field.
Other characteristics can be studied in a laboratory: soluble fat content, tannin composition, shrinking
temperature, total moisture content and total fat. These tests require relatively much material (currently about 0.5
gram) and are therefore seldom used for 'everyday' bookbinding conservation.
pH VALUE
Always wear gloves when determining the pH value so as not to influence the acidity of the material to be
measured.
1/pH indicator paper
Using tweezers, take a small sample (c. 10 mg) from a damaged spot on the leather or parchment binding and
place it on a slide. Add a drop of demineralised water with a Pasteur pipette, wait for one minute, immerse the
indicator strip, and compare it instantly with the reference colours on the box. When choosing an indicator strip,
bear in mind that its measuring range must lie within the expected pH area.
2/pH meter and micro-electrode (extraction)
An aqueous suspension of 0.5 ml is sufficient for a pH measurement. If the required quantity of material is put at
1/20 part of the total suspension, 0.025 gram leather or parchment is sufficient. This quantity can be taken
mechanically (in the form of loose fibres) from anywhere on the surface (grain) of the leather or parchment. The
actual measuring of the pH takes place approximately 1 hours after preparing the suspension, to allow the ions
to migrate into the solution. Depending on the desired accuracy, the measurement can be done once, twice, or
three times, each time with new material from another place, preferably as near as possible to the damaged
area(s).
3/pH meter and standard electrode (extraction)
This method can only be applied if a larger quantity of sample material is available, for instance from the
turn-ins.
Scrape 50 mg leather from the inner side (flesh side) of the bookbinding leather and put it in a 25 ml beaker. Add
1 ml demineralised water, and oscillate the beaker carefully until the fibres are completely moistened. Cover the
beaker and leave for an hour. Transfer the suspension in a test tube and centrifuge it for about 5 minutes at 8000
RPM. Pipette the upper, clear fluid into a clean test tube and measure the acidity with a pH meter to within 0.1 unit
precisely. Wait another 15 minutes and measure the solution again. Repeat the measuring until two
well-matching values are obtained. This method works only with leather or parchment which can be separated
from the boards, for instance at the turn-ins.
4/differential number
The differential number of leather can be easily determined by subtracting the normal pH value of the
measuring solution from the pH of the measuring solution that has been ten times diluted with demineralised
water.
Concentration of ammonium and sulphate ions
Commercial test sets are available for determining both ammonium and sulphate content. They require a 50 mg
sample of leather or parchment. The level of ammonium and sulphate ions in leather and parchment should be
interpreted in relation to the measured pH value. The same extraction fluid used in determining the pH can be
used for the ammonium and sulphate test. If both tests are to be conducted, the ammonium test should be
com-pleted before the sulphate test is started.The test sets Aquaquant 14423 and Microquant 14789 (Merck)
can be used to demonstrate the presence of ammonium and sulphate ions.
They come with step-by-step instructions as to how the test should be carried out.
Caution! Health risk: because of the toxicity of the chemicals used, these tests must be carried out in a
fume cupboard. Always wear gloves!