Preservation of parchment

Throughout the various periods of history, parchment has been preserved in the best way and at the best technical level that was possible at the time. (Historical Perspectives on Preventive Conservation. Edited by Sarah Staniforth. The Getty Conservation Institute. Los Angeles, 2013).

How much the temperature and relative humidity norms related to preservation conditions have changed during the last nearly three decades is indicated in the following chart.

Literary reference, standard, recommended norm	Temperature, ºC	Relative humidity, %
1. Storage of parchment together with paper materials
Estonian archiving regulation, 1998	15 … 20	30 … 50
National Archive rules, 2010	15 … 20	30 … 50
Archival regulations, 2012. Lisa 2 [PDF]	2 … 18	50 … 60
Finnish archival regulations	18 … 22	45 … 55
ANSI/NISO Z79-199X	Not over + 21	35 … 50
BSI, 1989	13 … 18	55 … 65
Riksarkivet, Norra, 1994	16 … 20	30 … 50
ISO, 1995	14 … 20	45 … 55
Thomson, G. 1978	madal	45 … 65
Baynes-Cope, A.D. 1981	13 … 18	55 … 65
Wilson, W.K., Wessel, C.J. 1984	20 … 21	25 … 30
UNESCO, 2000	13… 20	45 … 55
IFLA-PAC, 2000	<20	30 … 45
2. Norms for storing parchment
The Getty Research Institute		55±5
·        in the case of high preservation risk		40…30±5
Great Britain, Ireland		55…60
BS5454, 2000		45…60
EVS-ISO 11799, 2003	2…18±1	50…60±3
National Library, 2004	17±1	35±5
·        in the case of high preservation risk	7±1	30±5
EK Kanuti recommendations in the conservation protocols
·         1987-2000	18-20	60
·         2000-2007	16-18	50-55
3. IDAP, 2010 recommendations
Parchment in condition 1.	2…18	50…55
Parchment in condition 2.	2…18	50…55
Parchment in condition 3.	2…15	≤45
Parchment in condition 4.	≤5	<45

The decades-old recommendation that parchment be kept in as clean, stable and dark an environment as possible has not lost its validity even today. In order to ensure and assess the cleanliness and stability of the preservation environment, we need rules and regulations.

Photos of the preservation of parchment documents in the Tallinn City Archives.

Among other things, Standard EVS-ISO 11799, 2016 (Document storage requirements for archive and library materials) specifies the recommendations for the preservation of parchment documents.

The standard prescribes the norms for buildings and depositories, along with furnishing, fire safety, security systems, climate, lighting, ventilation, etc. When determining the climate requirements, the parameters should be chosen from among those recommended in the standard that are suitable primarily for the character, value, and condition of the objects being stored. However, the allocated funds also need to be considered.

Choosing the optimal preservation conditions is not an easy assignment. This is especially true if objects made of various materials are preserved in the depositories. In the case of mixed collections, where both paper and parchment are being preserved, compromises must be found regarding the temperature and the relative humidity.

It is generally recognized that lower the temperature and relative humidity will lengthen the life span of objects. However, collateral consequences may have the opposite effect. For example, when reducing the temperature, the possibility that water condensation may develop on the surfaces need to be considered. When the humidity is reduced, many materials may become more brittle. If objects are permanently stored in a depository with both low temperature and low relative humidity, then after they are removed from the depository and before they are used, they need to be properly packaged and acclimated. When deciding on storage conditions, one should keep in mind that when the temperature is lower, the relative humidity of the air must also be lowered (see chart).

When determining the suitable temperature and relative humidity, one should consider that parchment is extremely sensitive to fluctuations in these parameters. The standard specified the maximum fluctuation of temperature in a month as ±1,5 °C and the change in relative humidity as ±3%. The limits for the long-term stability of climatic conditions are lacking.

A higher moisture content in the storage space accelerates the hydrolysis of the parchment’s collagen, which further accelerates the environmental pollution (SO2, NOx, O3 over 5 ϻg/m3 and microbiological pollution and dust). Relative humidity above 60% accelerates the development and spread of mold damage in the depositories.

High temperature and excessive lighting along with UV-radiation above 50 ϻW/Lm accelerates oxidation and the resulting degradation of parchment along with the bleaching of colors.

In addition to the described storage conditions, the properties and preservation of parchment is significantly impact by several other factors.

Storage illumination. To prevent any damage, the strength, duration and spectral distribution of the lighting needs to be controlled. The depository must not be lit more than is necessary for daily work and, in the case of visible lighting, not more than 200 Lx per floor. Windowless depositories are suitable. If there are windows, they must be covered with shades that do not let any daylight through or equipped with UV-filter window glass. If the radiation given off by the lighting exceeds a UV content of 75 ϻW/Lm, it must be covered with UV filters. 75 ϻW/Lm is the maximum allowable value. In the case of UV below wavelength 400 nm, the established norms specify between 20 and 60 mW/m2 or 20 and 75 ϻW/Lm.

Damages and condition of parchment. The impact of the storage conditions will differ depending on the condition of and types of damage suffered by the parchment. The worse the condition of the parchment, the more important it is to prefer  low temperatures and reduced moisture content (see the recommendations on chart IDAP, 2010).

Previous treatmentts of parchment. The preservation of parchment may also be impacted by the treatments used during conservation. Moist processing further accelerates the hydrolysis of collagen. However, drying damp parchment under pressure may significantly change the color and transparency of the parchment. Development of visually discernible glass-like layers as the result of wet processing and flattening under pressure, which are the direct result of a degradation of the parchment. The preservation of parchment that has been processed in this way is cannot be preserved in depositories as well as unprocessed parchment.

Parchment archival boxes for storage. The paper of a archival box that is in direct content with the parchment must confirm to ISO 9706 requirements and be easy to keep dust-free. The boxes should be sealable and support the archival documents without applying any pressure the contents. The protective material must not contain plasticizers and not give off acidic or otherwise harmful gases. Uncovered metal surface must not be in direct contact with the parchment.

The display of parchment at exhibitions. Generally, the same norms apply in exhibition halls as in depositories. Being on display means that the surface of the parchment is illuminated for long periods and often the temperature and the relative humidity in the display cases does not conform to norms. Exhibitions increase the preservation risks for parchment and, therefore, their exhibition time should be kept to a minimum. For observation, visible light of 50 LX is considered to be sufficient. Incandescent lamps, or other sources of light that emanate heat, should not be placed in the vicinity or in the display cases. In addition to the illumination of the light in the exhibitions halls also needs to be controlled. As in the depositories, the established norms must stay within the range of 20 to 60 mW/m2 or 20 to 75 ϻW/Lm, if the radiation wavelength is below 400 nm. With technical solutions for shielding the light and the selection of the correct lighting sources, it is possible to reduce the UV radiation to less than 20 ϻW/Lm. There must be at least 50 cm between the light source and object being on display. In the case of long-term exhibitions, it is recommended that printouts of the digital copies of the parchment documents be used.

Handling the parchment. It is important to agree on the handling of the parchment in the depository (cleaning, placing in the archival boxes and on the shelves, disinfection) as well as the loaning the parchment out to the reading room. As a rule, parchment documents are handled with suitable gloves and in a prepared workplace by people who have been informed in advance of the preservation risks associated with parchment. Minimal use of the originals should be made. Thanks to the large-scale digitization of the parchment collections during the last few years, an opportunity has been created to significantly reduce the use of originals. The digital collection of parchment documents created by digitization, which can be easily found and available online, enable many people to enjoy the parchment legacy, which the public has, until recently, had relatively limited access.