‘Adhesives for Parchment Treatment’
A training day at the National Records of Scotland Conservation Services Branch with Antoinette Curtis and Yuki Russell
Author’s note: I am very grateful to Antoinette Curtis for generously offering to review the lines that follow. It is now a few weeks since Antoinette retired (congratulations!) and I feel so glad to have had the chance to benefit not only from the latest fruits of her smart research, but also from the example of the craftsman’s attitude with which that research has been pursued. I owe the same degree of gratitude to Head of Conservation Linda Ramsay and colleagues at the National Records of Scotland.
Following on from the success of the ARA training day on adhesives held at General Register House of the National Records of Scotland (NRS) on 5 November 2014, Antoinette Curtis and Yuki Russell, from the Norfolk Record Office (NRO), were invited to visit the Conservation Services Branch of the NRS to deliver a workshop on the principles, materials and techniques currently applied in the treatment of parchment at the NRO.
The workshop, as rich in theoretical information as in practical tips, took place on 18 March 2015 and included presentations and demonstrations on the properties and use of gelatine, isinglass, ZorbixSM and alum-tawed caecum, as well as an update on the Apocalypto Project.
The NRO conservators’ preference for gelatine and isinglass for parchment treatment is based on:
The principle of ‘like for like’, i.e. the compatibility between materials of similar proteinaceous nature
Stability over time as demonstrated by historical evidence and specialist research
Control over the mobility of the adhesives
The adhesives’ availability
Antoinette and Yuki had presented the poster Which gelatine to use? The choices for conservators at the Adhesives and Consolidants for Conservation symposium at the Canadian Conservation Institute in Ottawa in 2011. Yuki started off her talk by reminding us of the key properties of gelatine for conservators: bloom and type, and their influence on pH, viscosity and strength.
At the NRO, gelatine is used for:
Consolidation: 1% type A, relatively low bloom (around 100), as a warm solution.
Repair: 3-5% type A, medium-high bloom (around 200), in cold, mousse state, obtained by sieving through a piece of Terylene. Advantages of the mousse texture are the control over the mobility of the adhesive, the increase in the volume through sieving and the potentially quicker drying because of the air contained in the mousse. A possible disadvantage could be the reduction of the strength of the adhesive due to the weakening of the three-dimensional molecular structure of the solution by the sieving action.
Poulticing: This less-common application of gelatine was the focus of Yuki’s presentation and subsequent demonstration. As revealed by tests undertaken at the NRO, a higher concentration (8-10%) is required here to avoid an excessive release of moisture. The bloom does not seem to be relevant to this aspect, but rather for the consistency of the gel; the lower the bloom, the softer the gel. The poultice is locally applied as a rigid gel, sandwiched between pieces of spider tissue (underneath, in contact with the object) and polyester film (on top), under light weight, for approximately 15 minutes. A Petri dish is a handy container for limited quantities of this preparation, with discs of siliconized paper at the bottom with a little tab for easy removal, and spider tissue on top of the solution. The transparency of the glass allows a precise cutting of shaped pieces of gelatine for local application.
Isinglass for remoistenable tissue
In its purest form, isinglass is made from the membrane of the sturgeon air/swim bladder. Compared to gelatine, isinglass presents a neutral pH and no bloom value. Some of its advantages are slow gelling time, flexibility and fast hydration for application and removal. Disadvantages include an undesirable sheen with light repair material and sensitivity to RH fluctuations.
New international regulations[i] restricted the fishing of wild sturgeon from 1998, and the most commonly available isinglass nowadays (e.g. Salianski Kremer) is a by-product of German sturgeon farming in caviar production. A key difference between the two types is the higher fat content of the latter, which visually results in a yellower colour.
With Antoinette’s and the NRO’s collaboration, Edward Hems, from the John Innes Centre, Norwich, has successfully conducted several tests using a method known as sonication to try to remove part of the lipids in farmed isinglass with the aim of reducing the yellow hue in its colour. The description and results of these experiments have recently been published.[ii]
The pieces of dried isinglass mixed with an appropriate amount of de-ionized water are heated to 42oC to produce the adhesive solution. Antoinette led a complete preparation of isinglass pre-coated tissue, for which she uses a polyester film sheet lined with a matte laminate, whose texture is to be transferred to the coated paper, thus helping to reduce the above-mentioned sheen that frequently results from this type of preparation. The piece of paper is laid onto this base and the adhesive, in warm liquid state, is carefully and evenly brushed through with a sizing hake.
The conservation of the Manor of Aylsham Lancaster court roll, 1509–46 (NRS 13434), supported by the National Manuscripts Conservation Trust and Aylsham’s local community, undertaken between September 2012 and March 2014, was a challenging opportunity for testing new combinations of existing materials and techniques, among them isinglass pre-coated Gossamer (Berlin) tissue and ZorbixSM.
The most innovative aspect of the process is the use of ZorbixSM to reactivate the adhesive. In 2011, Nicholas Yeager (Artifex Equipment Inc.) announced the commercial launch of ZorbixSM as “a reusable, super-absorbent sheet for quickly drying water damaged books and documents”, claiming that it could absorb “50 times its weight in water, making it 25 times as effective as blotter paper”. However, ZorbixSM has also showed itself to be ideal for controlled humidification, due to the gentle moisture release from its inner starch reservoir.
The repair process devised at the NRO is as follows:
The document, supported by a sheet of Bondina, is laid on a suction table with a shaped piece of pre-coated tissue placed over the area requiring repair.
Polyester film discs are used to mask the areas of the document not requiring hydration.
A sheet of hydrated ZorbixSM is laid on top, which, in turn, is covered with a sheet of polyester or polythene.
The vacuum is turned on at low pressure for 3–5 minutes.
The object is put to dry between Bondina and wool felts under weight overnight. The opposite side is treated the following day, after which the document is left to consolidate between Bondina, felts and boards for up to several months.
This technique ensures a repair with a limited and controlled introduction of moisture, uniform adhesion and little or no distortion.
(Update, 16 February 2016: For a description of this method in Antoinette’s own words, follow this link.)
Alum-tawed caecum[iii] (alum-tawed goldbeater’s skin or, simply, caecum, as it is usually referred to) from cow has become a much appreciated alternative to goldbeater’s skin as a sympathetic repair material for parchment due, primarily, to a greater physical stability that makes it easier to handle. It is available in three thicknesses (one, two and three layers) and a versatile, wide range of colours. On its own, or combined with Japanese papers, caecum is used for infilling losses and tear repair.
For a remoistenable preparation, two layers of 4% gelatine adhesive are recommended. When used in combination with gelatine mousse, the edge of the caecum piece can be pared with a scalpel on a piece of board (to minimize the risk of slipping), or by carefully sanding it with fine grain emery paper. The caecum is then humidified (for small/medium-sized repair pieces, Antoinette uses a sort of humidification ‘wallet’, as seen in the photograph below) before the adhesive is applied. Shaped pieces of polyester film can help as a carrier to precisely position the repair onto the required area of the membrane.
The Apocalypto Project
This project is a collaboration between the Institute of Dentistry at Queen Mary University of London, which is a leading centre for very high contrast X-ray microtomography imaging, and experts in computer vision systems in the Computer Science department at Cardiff University. Together, they have been developing a scanning and scan-processing technique that has already produced promising results for virtually revealing the textual content of damaged (including burnt) parchment documents otherwise impossible to unroll and/or read. The technique is based on the possibility of imaging small differences in X-ray absorption between inks with a metallic component and their support, i.e. paper or parchment, as long as this has not been heavily limed during its production.
In-depth information about Apocalypto can be found on the project website: http://www.apocalypto.org.uk, and the following article on the work with the NRO’s Bressingham Roll: Mills, David et al. (2014) Apocalypto. Revealing the Bressingham Roll. IADA Journal of Paper Conservation Vol. 15, No. 3, pp. 14-19.
Salvador Alcántara Peláez is a graduate in History of Art, and has an MA in Museum Studies. He completed an FdA in Book Conservation at Camberwell College of Arts, London, which was followed by an Icon/HLF internship at the Wellcome Library. Previously a conservator at the National Records of Scotland, Edinburgh, he now works as a conservator for The British Library/Qatar Foundation Partnership digitization project.
[i] CITES regulation, EG (338/97).
[ii] Hems, Edward and Curtis, Antoinette (2015) Decolourising isinglass derived from aqua-farmed sturgeon by sonication. Journal of the Institute of Conservation Vol. 38, Issue 2, pp. 188-199.
[iii] “Cecum, also spelled caecum, pouch or large tubelike structure in the lower abdominal cavity that receives undigested food material from the small intestine and is considered the first region of the large intestine.” (Cecum. 2015. Encyclopædia Britannica Online. Retrieved 12 November, 2015, from www.britannica.com/science/cecum). The outer layer of the cow caecum is the part usually processed to produce this conservation material.