Parchment is one of the oldest writing supports in history, and was already in use some centuries before the birth of Christ. Parchment generally refers to mammal skin, treated with lime, de-haired, scraped and dried under tension.
This process converts the skin into a material with incredible longevity. Manuscript 55 is a case in point – the parchment is possibly as old as 1500 years and most is in near perfect condition.
As an organic product parchment is very responsive to the environment in which it is kept. A dry environment will cause parchment to shrink, but due to differences between fibres on the outer and inner layer, and to variations in thickness, porosity and fibre alignment, uneven shrinkage is common. This results in curling and cockling patterns, normally towards the hair side of the skin. If the parchment is bound into a book or tensioned in any way, a dry environment can cause uneven distortion and stresses to form. Equally, if parchment is subjected to a humid environment it will relax and gain flexibility. The thinner and more refined a parchment is, the quicker it reacts to different levels of humidity.
Parchment can be quite stable even under relatively high temperature (RH) levels but a combination of extreme humidity and temperature will cause irreversible gelatinisation, transforming the flexible parchment into a stiff and translucent material or even a jelly-like substance.
Excessive humidity alone makes parchment cockle, and in extreme cases it might cause the complete loss of the fibre arrangement imposed by its manufacture, returning it to the natural arrangement found in the original animal hide.
This high reactivity to moisture can be very carefully manipulated during conservation treatments in order to keep control over the movement of parchment during various processes. However, to ensure long-term preservation parchment manuscripts are kept in stable environmental conditions with a constant RH to prevent any changes to the substrate and to the adhesion of any applied inks and pigments.
During both imaging and the conservation of the early Irish manuscripts extreme care is paid to keeping the working area at a constant relative humidity to avoid any distortion to the parchment. An ultrasonic humidifier is in constant use on the conservation work bench beside a digital thermo-hygrometer and two room humidifiers are operated in the digitisation laboratory during imaging to maintain the manuscript at the same levels of RH as is provided in the air conditioned storage area.
Marco di Bella
Senior Book Conservator – Early Irish Manuscripts Project