The nature of parchment means that its interaction with the media it supports can be very tenuous. Paper has an absorbent effect on the media, with the level of absorbency being controlled mainly by the size in the paper. As well as providing protection for the paper surface from handling, size provides a suitable writing surface that results in crisp media lines with little or no blurring, but allows sufficient absorption to ensure that the mark is permanent and within the fibres of the paper substrate. Without size you effectively have blotting paper.
Parchment is very different. Here the ink sits on the surface of the substrate with little or no absorption into the parchment itself. This means that the media is much more at risk from mechanical wear and detachment through handling and storage, such as folding or rolling, and during routine conservation treatment procedures such as surface cleaning. Once the gum Arabic binder in the ink starts to degrade areas can detach and flake through the flexing of the parchment causing media loss where the ink has simply cracked off the parchment surface.
Below are two images showing part of a medieval parchment deed. The deed is a lease dated 25th November 1619 of a building and land in Worcester from Joseph Hall D.D. on behalf of the Dean and Chapter to Richard Nash of Quatt in Shropshire. The first image is a general one showing the start of the deed, the second is a close up to highlight the texture of the ink in more detail. This a good example of the problem that arises with ink on parchment and this deed was recently on the workbench for careful cleaning, repair and rehousing by conservators Katerina Powell ACR and Victoria Stevens ACR at Worcester Cathedral Library.
Here you can see that the decorative capital letter in the title line has started to crack with flaking and losses across the whole letter. This is particularly prevalent where the ink is the most heavily applied, which is most obvious where the quill has slowed or stopped to change direction. Various techniques may be used to consolidate such deterioration but in this case housing and careful handling will be the best option for its ongoing care and stability.
Victoria Stevens ACR