Conservation of parchment
In the past parchment bookbindings were usually omitted from bookbinding conservation. Research into the
correct treatment of parchment bookbindings was still at an early stage and so - quite rightly - no treatment for
parchment was recommended. (P.A. Goddijn (et al.) Richtlijnen voor het conserveren van leer en perkament
van boekbanden. 's Gravenhage, Koninklijke Bibliotheek/Centraal Laboratorium 1987, p. 10.)
Besides, for a long time it was thought that parchment did not need conservation treatment because its built-in
buffer of calcium carbonate protected it from pollution (Plenderleith, 1946). We now know this to be untrue.
Moreover, parchment can also be affected by chemicals added during manufacture or by dust and dirt from the
environment.
Cleaning
Although aesthetic considerations have sometimes led people to decide cleaning parchment bookbindings,
this is not recommended because of the detrimental consequences involved in cleaning parchment. There are
disadvantages attached to both dry and wet cleaning of parchment.
Dry cleaning, for instance, with (powdered) eraser, removes the porous finishing layer (soap, oil, egg white) that
may have been applied by the bookbinder. It also loosens the fibres of the grain, resulting in a rough, dirt-prone
surface.
Wet cleaning of parchment presents the same disadvantages in intensified form, particularly with respect to
damage to the fibre structure. There is also a real possibility that the fibre structure will open up at the surface
with the result that part of the dirt becomes permanently entrapped after drying. Furthermore, it takes a lot of time
and effort to clean a parchment bookbinding uniformly and without leaving stains.
Texts written in ink on the spine (title, author, etc.) present an additional cleaning problem. Since it is not unlikely
that such texts will be damaged by cleaning, they must be left untreated, resulting in a dirty 'lettering piece'.
Summarising, we can say that the cleaning of parchment is best restricted to removing loose dust and dirt as
described for leather. If it is decided for certain reasons to remove fixed dirt, the surface cleaning agent used for
leather may be applied.
Deacidifcation, buffering, conditioning
Although the sulphuric acid formed in parchment reacts with the calcium carbonate resulting from the
manufacturing process to form calcium sulphate (gypsum), part of the sulphuric acid may remain behind in free
form. Deacidification of parchment with ammonia is a very effective method of neutralising this free sulphuric
acid. Ammonium sulphate is formed during this process.
The pH value of 'sound' parchment fluctuates between 6 and 7. If a pH value of 5 or lower is measured in
parchment, deacidification and buffering is necessary; if the pH value is between 5 and 6 buffering will, in
general, suffice.
In some cases of parchment with chemical damage due to acid, the pH will not be unduly low (below 5,0) and
deacidification will therefore, in general, not be necess-ary. This is because the (originally low) pH can rise over
the course of time in response to a gradual increase in the ammonium sulphate level in the parchment.
The procedure for deacidification, buffering and conditioning parchment is the same as for leather (see chapter 5).
Treatment with parchment dressing
Parchment is a dried, stretched skin. If the water balance is disturbed, degradation of the material occurs,
resulting in a decreased capacity to absorb moisture from the air. The fat component of the parchment dressing,
developed with this problem in mind, consists entirely of lanolin. Lanolin is hygroscopic and acts as lubricator
between the fibres. Odourless kerosene is used as the carrier in parchment dressing.
Adding 2% fat to the kerosene produces a dressing which, while it is not heavy enough to overfat the parchment,
nevertheless contains sufficient fat to improve lubrication and water balance. The parchment dressing is only
effective if it is applied to pre-conditioned parchment in a room with 70% RH.
Depending on the absorbency - which in turn depends on how the parchment has been finished by the
bookbinder - once or twice brushing with the parchment dressing, using a flat brush, should be sufficient.
Formula for parchment dressing
980 gram odourless kerosene
20 gram lanolin (anhydrous)
The lanolin should be dissolved in a portion of the kerosene, heated to a maximum of 40 °C. Then the rest
of the kerosene is added. Use an electric hotplate or heating over warm water (au bain marie) rather than a gas
flame because of fire risk. The parchment dressing will keep 'indefinitely'.