Strategy of Exhibiting Archive Materials
 in the Central State Archives in Prague

1. Introduction
	The main task of the archives is to collect documents of permanent value, and to process, protect and provide access to them. Exhibitions are one of the means of providing access to and presenting archive materials (and also archives and archiving as a field of endeavor).
	Archive materials consist of various types of organic and inorganic materials that can be irreparably damaged under unsuitable climatic and light conditions or unsuitable handling during exhibitions. From this point of view, exhibitions always constitute a risk and thus their preparation and provision for them must always be approached with maximal care, responsibility and knowledge of all the potential risks. However, in general, it can be stated that archives can hold short-term exhibitions of the original copies of archive materials, however under conditions of care for the complex safety of the exhibited memorials. If it is not possible to ensure the safety of the exhibited archive materials or a long-term or permanent exhibition is involved, it is essential to exhibit good-quality reproductions and facsimiles of the originals.
	The principles of exhibiting archive materials in the State Central Archives in Prague were drawn up on the basis of a three-year grant1 ”The effect of light and ultra-violet radiation on archive documents, work on which was carried out in 1998 – 2000.  Considerable attention is paid especially to the aspect of lighting of archive materials during exhibitions and new approaches to lighting sensitive archive materials constitute the cornerstone of overall exhibiting strategy. These principles are intended primarily for the institution in which they were formulated, but could constitute an example for other state archives or an inspiration for libraries, museums and galleries.

2. General Principles and Procedures in Exhibiting or Lending Archive Materials for Exhibitions
	Consent to exhibiting archive materials (or lending materials for an exhibition) should always be obtained from the director of the archives. However, this consent is dependent on the consent of the director of the restoration and conservation department. The director of the archive should obtain the request for exhibiting the archive materials at least 6 months before the start of the exhibition. Otherwise, the applicant risks that it will not be possible to lend requested archive materials that will have to be restored before the exhibition.
	On the basis of the application, the director (director of the archive department) designates an employee who will be responsible for professional and organizational aspects of provision for the exhibition or lending. This employee alone, or in cooperation with other professional workers, selects and collects the archive materials requested for the exhibition, which (s)he then submits to the restorers. In addition, as required, (s)he provides for creation or reproductions, arranges for timely establishing of insurance sums, preparation of an ”Agreement on Lending”, submits the archive materials to the commissioner of the exhibition and, after the end of the exhibition, ensures that the archive materials are again returned to the restorers for final assessment of their condition.
	If the archive materials are lent for an exhibition outside of the building of the State Central Archives in Prague, the lender is sent a detailed Questionnaire on lending archive materials for the purpose of exhibitions.
	The restorers at the archives carry out a detailed evaluation of the physical condition of the requested archive materials and, on the basis of knowledge of the physical condition, type of archive materials and evaluated questionnaire, decide whether and which of the requested archive materials will be exhibited or lent for the purposes of the exhibition.
	For selected archive materials, prior to lending, the employees of the restoration department fill out a Protocol on the condition of the archive materials prior to the exhibition. The protocol can be supplemented by a good-quality reproduction (black-and-white or coloured photograph, slide, digital photograph, microfilm) and constitutes part of the Agreement on Lending. Selected archive materials are packaged and submitted to the responsible employee for expediting.
	Archive materials must be insured prior to lending. In determining the insurance sum, consideration is taken of the historical and art-craft value of the archive materials and the cost of restoration in case of damage thereto. The insurance sum is determined by an employee of the State Central Archives and is also stated in the Agreement on Lending. A written document of conclusion of insurance must be sent to the State Central Archive prior to lending the archive materials.
At any time during the exhibition, the employees of the State Central Archives in Prague may control the state of the exhibited archive materials and compliance with the required exhibiting conditions. Similarly, these employees should be present during installation of the loaned archive materials and during liquidation of the exhibition.
Following closing of the exhibition, a responsible employee shall ensure that all the archive materials are returned back to the restoration and conservation department, where the restorers evaluate their condition in the presence of a representative of the borrower and fills out a Protocol on the condition of the archive materials. Then the archive materials may be returned back to the depositary.

3. Climatic and Light Conditions for Exhibiting
3.1. Climatic Parameters
	Climatic parameters must be maintained constant in the exhibition areas, i.e. without major fluctuations during the entire duration of the exhibition. The temperature and relative humidity must be measured and recorded during the exhibition. It is recommended that the relative humidity of the air in the exhibition rooms be maintained at a value of 50 % with a maximum deviation of ± 5 %. Similarly, the temperature should be 18 ± 2 oC.
	It is frequently very difficult to maintain a constant relative humidity in exhibition areas, especially when it is raining and when there are large numbers of visitors. Then, it is necessary to limit the number of visitors in the exhibition rooms and to ensure that they leave wet or damp clothing in the cloakrooms.

3.2 Requirements on Air Purity 
	It is important to protect the exhibited articles against the effects of gaseous impurities, especially predominant sulfur dioxide and nitrogen oxides, as well as tropospheric ozone, and this exposure should be minimized, especially in industrial zones and urban agglomerates2.
	Similarly, all material used in the construction of exhibition display cases or for displaying the exhibited items must not release any harmful substances (e.g. formaldehyde, acetic acid).

3.3 Recommended Lighting Intensity and Fraction of Ultra-Violet Light in Lighting Exhibited Items
 	The levels of lighting, i.e. the intensity of lighting, are always necessarily a matter of compromise in archives, libraries, museums and galleries. Light damages most materials of organic origin, but simultaneously a certain light intensity is necessary so that objects consisting of these materials are clearly visible to visitors.
	The proposed exhibition strategy recommends a light intensity of 100 lux for categories 2 and 3 and 75 lux for category 1. A light intensity of 50 lux is recommended for the most sensitive organic materials in category 1 (sensitivity level 1 according to ISO 105). It is naturally expected that any UV radiation from the light source would be filtered out.
	It is important that, in order to successfully view objects at low light intensity, the observer requires several minutes to adapt to the low light level. If the visitor is temporarily exposed to higher light intensity (e.g. when moving from one exhibition room to another through an intensely lighted hallway), the adaptation time is prolonged. Thus, if it is really necessary to employ low light intensities, it is useful if not only the exhibition rooms, but also the hallways and any other nonexhibiting areas through which the visitor passes, have the same intensity of lighting.
	It is recommended that the currently accepted fraction of ultra-violet light of 75 W.lm-1 be reduced to a value of 10 W.lm-1.

4. Recommended Duration of the Exhibition
	Exhibition strategy is based on the fact that all works of art on paper are classified according to their sensitivity to light into one of three categories. These categories are based on British Blue Wool Standard BS 1006 or standard ISO 105 in the following manner:
Category 1: works with sensitivity level ISO 1, 2 and 3
Category 2: works with sensitivity level ISO 4, 5 and 6
Category 3: works with sensitivity level ISO 7, 8 and higher.
	The blue wool system consists of 8 coloured strips of wool that fade with a characteristic velocity under the influence of light. The strip with sensitivity level ISO 1 is least stable and ISO 8 is most stable. A change in the colour of the tested material is compared to the change in the colour of the sample of an ISO level. For the individual blue wool ISO levels, the exposures corresponding to the first just noticeable fade (hereinafter JNF) were determined experimentally. These are given in the following table:
Table 1. Categorization of the sensitivity of objects to light according to the level of the blue wool scale 
ISO level (blue wool)	  1          2          3	     4         5        6	      7         8	
Exposure*causing 1 JNF** (Mlx  h)	    0.4       1.2       3.6	   10        32     100 	    300     900	
Sensitivity category	1
sensitive	2
medium	3
stable	

*  Corresponds to exposure to visible light without UV radiation
** The just noticeable fade was determined colorimetrically using a chromaticity diagram as the colour difference between the original and new colour point following ageing. Exposure to light, that causes a just noticeable colour change for a normal photometric observer, has a value of JNF = 1.

	The table summarizes some important basic information. It indicates the approximate exposure to light in Mlx.h necessary for achieving JNF = 1 for each ISO level. Although the materials of works on paper cover the entire range of ISO levels from 1 to 8, it was decided to employ only three broader categories. In order for it to be possible to determine the recommended exhibiting time for the individual categories, it was necessary to concentrate on one of the ISO levels in each category and select it as a standard. For categories 2 and 3, the lowest ISO sensitivity level was chosen - ISO 4 sensitivity level for category 2 and ISO 7 sensitivity level for category 3. For the most sensitive group (category 1), it was not possible to select ISO 1 sensitivity level as the standard because it contains materials with the highest sensitivity to light. Selection of the most sensitive material as a standard would mean very limited acceptable exposure for the rest of the category. Thus, it was decided to select ISO 2 level as a general representative for this category.
	In practice, it is not easy for conservators to differentiate materials with ISO levels of 1 or less. These materials often changed long before they were included in the collections and changed at any time over the centuries even under very good exhibiting conditions. The conservator frequently encounters difficulties in estimating an exact classification. It was thus found necessary to consider all works that are suspected of containing materials with ISO levels of 1 or less as an exception in the strategy and in these cases it is necessary to prepare recommendations for exhibiting on the basis of individual decisions.
	The actual exhibiting strategy is summarized in Tab. 2 and is based on determining the approximate amount of light dosage in lux-hours that could cause a just noticeable fade of JNF = 1 for the most sensitive work in each category (this number of lux-hours is given in the second column). On the basis of these values, the time of the year for exhibiting is proposed for each category (see the third column of the table). If the work of art is exhibited according to these recommendations, it can be assumed that work in a certain category will fade noticeably in the number of years given in column four. It should be noted that these are approximate data. The rate of fading and especially the dependence of this rate on time over the long time of the existence of the object with an unknown history of lighting are very difficult to predict exactly.

Tab. 2. Recommended time of exhibiting objects in the individual categories
	Exposure causing just noticeable fading
(Mlx·h)	Recommended exhibiting time
(weeks/years) or total annual exposure (lx·h p.a.)	Time necessary to cause just noticeable fading
(years)	
Category 1
(sensitive – ISO 1, 2, 3)	1.2 Mlx·h
(ISO 2)	4 weeks
or 12000 lx·h	100	
Category 2
(medium sensitive – ISO 4, 5, 6)	10 Mlx·h
(ISO 4)	10 weeks or 
42000 lx·h	250	
Category 3
(stable – ISO 7, 8 and higher)	300 Mlx·h
(ISO 7)	20 weeks or
 84000 lx·h	3500	

	In calculation of the recommended exhibiting time, an exhibiting week is considered to consist of 42 hours and a lighting intensity of 100 lux for categories 2 and 3 and 75 lux for category 1. It is assumed that any ultra-violet light from the light source would be filtered off. If the exhibition lasts for a longer period of time, it is recommended that the exhibiting time be adjusted as follows: 4 weeks per year or 8 weeks over 2 years or 12 weeks over 3 years, etc. However, it is recommended that the exhibiting time should not exceed 20 weeks.

5. Classification of Materials in Individual Categories3
	Classification of materials according to their sensitivity to light in individual categories on the basis of British Standard BS 1006 or Standard ISO 105 is given in Table 3 – 5. It should be pointed out that these tables will be regularly extended to include new materials.

Tab. 3. Materials of works of art on paper classified in category 1
Definition of works:
All works with colours, binders and supports with light stability classified according to the ISO blue wool standard as 3 or less. The following materials and techniques have been classified in category 1:
•	Coloured pencils, some sensitive dyes, unknown and cheap palettes
•	Water colours, some sensitive dyes, unknown and cheap palettes
•	Gouache, some sensitive dyes, unknown and cheap palettes
•	Coloured printing inks (including oil-based), some sensitive dyes and unknown palettes, i.e. lithographs, silk-screen printing, etc.
•	Multi-coloured tempera paintings with undefined pigments (illustrated manuscripts, oriental gouache or tempera on paper and silk)
•	Most toned paper, i.e. grey-blue, green-grey
•	Coloured photographs of unknown quality, including older coloured photographs
•	Polaroid pictures
•	Most historical natural dyes on textiles (with the exception of indigo and madder on wool)
•	Drawings made with a lettering pen
•	Bistre, sepia (sensitivity of unknown iron-based inks)
•	Complex black inks
•	Unknown yellow and red in Japanese block prints
•	Unknown yellow and red in European manuscripts
•	Feathers, low-quality commercial material used in collages	
Specific pigments:
All pigments with ISO sensitivity 1, 2 and 3 are classified as category 1, including the following:
•	Gumigutta
•	Complex black
•	Madder and indigo on cotton
•	Indigo in water colours
•	Thin tints and washes of intermediate pigments, i.e. most carmine lake pigments (quercitron, carmine in water colour washes on white paper)
•	Safflower
•	Curcuma
•	Commelina communis	

	Some of the most sensitive materials in archives consist in archive materials with records and stamps from the 2nd half of the 19th and the 20th centuries, which were written using inks usually based on aryl methane dyes. For these archive materials, the lowest light levels recommended for materials in category 1 are too high and thus they should practically not be exhibited.

Tab. 4. Materials of works of art on paper classified in category 2
Definition of works:
All works with colours, binders and supports with light stability classified according to the ISO blue wool standard as 4, 5 or 6. The following materials and techniques have been classified in category 2:
•	Wood pulp paper another low-quality paper and paper supports
•	Coloured slides that are known to have the Kodachrome, Ektachrome, Fujichrome, etc. trademark
•	Cibachromes
•	New coloured photographs (approximately to 1980)	
Specific pigments:
All pigments with ISO sensitivity 4, 5 and 6 are classified as category 2, including the following:
•	Some traditional dyes on textile
•	Vermillion (tends to darken rather than fade)
•	India yellow
•	Basic brilliant red: carmine, madder and alizarine lake	

Tab. 5. Materials of works of art on paper classified in category 3
Definition of works:
All works with colours, binders and supports with light stability classified according to the ISO standard blue wool as 7, 8 or more. The following materials and techniques have been classified in category 3:
•	Good quality rag paper
•	Carbon-based inks
•	Graphite, charcoal
•	Drawing with a silver or metal point (coatings for paper for the metal point technique are probably acceptable if they are white and not coloured)
•	Earth pigments, ochre, umbra, etc.
•	Natural chalk, blood red, brown, black and white (conté crayons)
•	Black-and-white silver/gelatine photographs
•	Gold or selenium toned or otherwise permanently treated photographs
•	Plastics, polyethylene, synthetic resins	
Specific pigments:
All pigments with ISO sensitivity 7, 8 or more are classified as category 3, including the following:
•	Highest quality modern colours, including water colours, colours for gouache, coloured pencils, etc.
•	Modern cadmium red
•	Ultramarine, most blues
•	Aureoline (cobalt yellow)
•	Indigo and dye madder for wool	

7. ”Light” history of the exhibited archive materials
	As the effect of light has an accumulative character and especially attractive archive materials are exhibited more often, it is necessary to know the amount of light energy that the material has absorbed in the past. This information should be employed in making decisions on future exhibiting.
	For these purposes, a form has been proposed, in which is recorded basic information on the exhibited archive materials, the length of the exhibition, the intensity and quality of the light, the temperature and the relative humidity of the air in the exhibition halls.
The recorded information is also employed to monitor post-radiation effects following the exhibition (changes in the colours of the archive materials and records following exposure to light during the exhibition). These effects have not been well studied.

8. The manner of exhibiting some kinds of archive materials and their preparation 
	The surface of the archive materials must not be disturbed under any circumstances. Consequently, nothing may be glued, pinned or otherwise mechanically attached to the support. Any unevenness on the surface may not be forcefully evened out.
	Books shall be installed in display cases in a horizontal or slightly slanted position. Supports may consist of glass stands, foam blocks or special stands made of polymethyl methacrylate (plexiglass). Books shall be closed or only slightly opened, so that the binding and the spine are not damaged. Sheets that are exhibited so that both sides are visible may be held by a lead weight sewn into cloth or strips of inert polyethylene terephthalate foil (e.g. Melinex, Mylar, Tenolan, etc.).
	Individual pages (e.g. documents, graphic sheets, plans and maps of small dimensions) shall be exhibited in the horizontal or vertical position. The support must consist of inert material (glass, nonacidic, woodpulp-free cardboard or carton of archive quality). Archive materials of large dimensions (maps, plans, posters) may be exhibited in a vertical position held between pieces of glass or in a horizontal position attached to an inert support by strips of polyethylene terephthalate foil.
	Documents with seals may be exhibited in a horizontal position or on a slanted stand. Parchment sheets should be exhibited in a vertical position only in protective polyethylene terephthalate foil with a special lightening casing for the seal. This type of archive materials should be exhibited in a vertical position only exceptionally.
	Sphragistic material (seals, metal seals for Papal bulls) – if not exhibited together with the documents – shall be exhibited in a horizontal position on an inert support. Metalseal matrices do not require any special exhibiting regime.
	In exhibiting photographs, it is necessary to comply with the lower limits of the required climatic and light parameters. It is necessary to cover photographs with effective UV filters. It is not recommended that coloured photographic material be exhibited.