Ligue des Bibliothèques Europeénnes de Recherche, Groupe des Cartothécaires de LIBER
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The conservation of maps
A.D. Baynes Cope
© LIBER and author
Published from: LIBER Bulletin 12(1978)
It sometimes happens that a member of my staff will come to me and ask for a chemical. Being rather senior, I usually ask why the chemical is required and it can happen that after a certain amount of talk I will ask rather firmly "What do you really want to do?" and perhaps I may ask this question more than once in order to discover what is being done, Members of the staff can, indeed must, have any material that is required for the work, together with all the advice on handling it that may be needed but I consider it my duty to see that they are guided into doing what is right, or as the greatest of all Englishmen said "Not what they like but what is good for them".
So it is today. You, by sitting here listening are in reality asking me "How do we conserve maps?" and I shall answer by saying "I don 't know, tell me what you really want to do". The reason for saying this is that I want you to think carefully about what the map needs, because what the map needs is more important than what you may want to do, or be able to do.
What is a map? I doubt if we could give a simple answer, but we might consider it as a two or three dimensional device in which the geometrical relations between certain points are defined as accurately as the maker can devise. We can also say that because of the material on which they are most commonly made, there is a probability that even our best maps are in error by an unknown, but often determinable amount. This is because paper is sensitive to dimatic changes and our own Ordnance Survey, whose dosest rival is here in Denmark, have said that they cannot, for this reason, guarantee the fidelity of a map after it has left the press.
We can also ask cc "When is a map not a map?" and here we are on easier ground. It can be regarded as not being a map when it is a globe, for the processes of conserving a globe are different from those used on a map. It can also be said, in my opinion, that a map is not a map when we agree that we are no longer interested in using it as a map, but are preserving it for its historical associations, or its beauty, or as a record of the opinions current at the time. Once this is accepted, we will probably adopt methods of storage, active conservation and display which are different, perhaps very different, from those which we would use for what we may call working maps.
You will observe that I have used the term 'active conservation'. Dr. A. E. Werner has pointed out that conservation takes two forms, 'active' and 'passive'. In passive conservation all our efforts are directed at creating an environment in which the object exists as safely as possible, but we do nothing to the object itself, whereas in active conservation we actually do things, chemically or quasi-chemically, or physically to the object itself.
If we carryout our passive conservation properly, we expect to reduce the amount of active conservation required and hope that we will never be forced into the unhappy position of having to use drastic methods to save a badly damaged object from virtually certain loss.
Whichever form of conservation we propose to employ, we must consider the materials of which maps are made. Globes of gold and silver, lovely as they are, require the expertise of the silversmith. Our collections will be made of paper, parchment, ink and paint. The inks may be manuscript inks, applied with a pen, such as iron gallotanate, logwood or carbon ink, or printing inks in which pigments and or soluble colours are mixed with a drying oil. Paints are used on those lovely and relatively rare portolan charts.
Parchment and vellum are animal skins from which all the hair has been removed together with the material laying between the collagen fibres. There is one other factor that distinguishes parchment and vellum from ordinary skin, and this must never be forgotten. Parchment and vellum are made flat by brute force and the strain induced will make itself obvious if the conditions of storage are not carefully chosen.
Paper is made from cellulose fibre and paper can be made from cellulose because cellulose has a special relationship with water. Cellulose is a polymer of a sugar called cellobiose which is itself made from two molecules of glucose. The long chain has hydroxyl groups along it and water molecules can form bridges between the hydroxyl groups on adjacent chains and they form semi-chemical links. Cellulose normally contains about 7% of water and this is an important constituent. The effect of the water is to make the cellulose fibre thicker but not longer, with the result that the size of a sheet of paper is governed by its water content and by the extent to which the fibres are aligned or randomly distributed. In practice, machine-made paper bas a very great degree of alignment of the fibres so that when it is wetted, it expands by about 2.5% across the machine direction of the paper. Hand-made paper has a greater randomness of fibre distribution but the shake which the vat-man gives to the sieve as he lifts it from the vat of paper fibres will cause some alignment. As a result, the sheet will expand in both directions when wetted, but more in one direction than the other. Since the chain lines generally run to and away from the vat-man, the direction of greatest expansion is usually along them. Though the difference is small, it is perceptible.
Paper has a memory and each sheet of hand made paper remembers exactly both its maximum (wet), and minimum (dry), size. If it is wetted and dried under constraint, it will stay large; but if it is re-wetted and dried 'free', it will on drying come back to its proper minimum size.
It is now possible to make paper with fibres other than cellulose and very strong paper for maps is now made with a high percentage of nylon fibre. Nylon is permeable to water, as any fisherman knows, but not to the same extent as cellulose.
We have also to consider the purely chemical behaviour of the fibres and there are two reactions that are important. Cellulose is attacked by acids, which split the long polymer chain into shorter lengths and thus weaken the fibres. The reaction is what chemists call a 'zero order' reaction, as the acid is not used up. The acid may come from two sources, from aluminium sulphate used in the manufacture of paper, though modern map paper has little added acid, and acid from polluted air, especially in industrial cities. Another important reaction is the chemical action of light, which degrades cellulose and inks. Though this applies chiefly to inks during display, it is a problem that the conservator can meet with cellulose.
The inks used in map making vary enormously. The chart made by William Dampier was drawn by hand using an iron gallotannate ink, which was, presumably, black when drawn, but it is now brownish. Such an ink, if not carefully made, is acid and can attack the paper severely, often enough to cut through it.
The most common printing ink is made from carbon, a perfect black pigment which is chemically inert and soluble only in boiling sulphuric acid or molten iron, materials unlikely to be used on maps. If, however, we want a coloured map, whether useday informative or pretty colour, then other materials are used. Pigments may be of mineral or plant origin, and when these are used as decoration they have been used as what we would now call watercolours, that is, they would have been dissolved or suspended in a water-soluble gum. Later, synthetic dyes could be used but many of these, like the natural materials, could fade and be also readily soluble in the liquids used in conservation. What is also important is that colours may be soluble in adhesives even if they are not soluble in the solvents which dissolve those adhesives.
We have one other factor to consider - biological enemies. I fear that man must rate highly in this connection, but what I am really referring to are insects, and micro organisms. Insects will eat maps because they are food, or because they get in the way of their travels. Micro organisms will break down cellulose or parchment in order that they themselves may multiply, but fortunately for us, they have preferred dimatic conditions for living, though it must be understood, very dearly, that only if conditions vary too far from the desirable will growth stop completely and even then death may not occur. Further, microbes of different kinds will each have their own favoured conditions and it seems likely that 'foxing' is caused by an organism which will grow only in what are usually regarded as rather dry conditions, probably about 75%-80% RH, dry that is for the usual run of black and green moulds which prefer about 95% RH.
Since chemical reactions proceed faster with an increase in temperature and moulds prefer very humid conditions, it is easy to see that documents should be kept cool and fairly dry, but excessive dryness is bad, as the dehydration of cellulose makes it brittle and parchment and vellum suffer badly if the relative humidity of their environment falls much below 50%. We usually assume that the correct conditions for document storage are a temperature of 18° to 20° centigrade with a Relative Humidity (RH) at a steady point in the range of 55% to 65%. This is quite easy to say, yet at first sight, not so easy to achieve. However, in practice, provided that excessive heating is avoided and that there is good circulation of air, things can generally be arranged so that documents do not suffer badly. One factor that is very much in our favour is the way in which cellulose and parchment adjust their equilibrium moisture contents to the humidity of the atmosphere by exchange of moisture so that, given an even temperature, these substances can buffer the relative humidity of their local atmosphere.
We can now consider a particular example of conservation, the care of the portulan charts at the National Maritime Museum in England. A portulan chart is an illuminated manuscript on vellum and it has all the weaknesses of such objects, for the base is a highly strained material reacting by dimensional, often three-dimensional change, to changes in temperature and humidity with pigments contained in a natural organic medium laid on it, with no certainty that the rate of movement of the paint medium will match that of the vellum. It must be stored, studied and exhibited.
A committee was set up to consider the problem of the care and conservation of these charts and the recommendation was this. That we should do no active conservation as yet but that we should devise a container in which the chart would live permanently, for storage, display and study. Only when the charts had settled down for some years in ideal conditions would we begin to consider active conservation.
The case consists of a wooden base covered with a thick layer of acid-free mounting board, over which is placed a non-reactive doth. The chart is held on this base by plastic dips. The lid of the case is an acrylic resin moulding which is sealed to the board and has small holes which contain tiny cotton wool filters. The insulating properties of the board and the acrylic resin help to prevent sudden changes of temperature. The cellulose and parchment buffer the relative humidity to a proper level. Together, the case and the chart produce conditions which change only very slowly, allowing both the skin and the paint medium to move in unison. The cases can be stored in rackes in a room in which only temperature need be controlled. They can be laid on a table for the chart to be studied or placed on a frame in a gallery for exhibition, safe from harm by hand or climate. I like to think of this as a conservation system and the idea was in fact derived from a description of the British fighter plane, the 'Lightning' as being not just a simple fighter but a complete 'weapon system'.
It seems a natural step now to proceed from sea charts which are no longer usable for navigation to land maps which are no longer usable for pedestrians, horsemen and motorists. The lovely early maps which shewed towns only by conventional signs, and which omitted roads and most rivers, are not now instruments of navigation and in many ways they may be regarded as having retired from service to, we hope, an honourable and comfortable existence. Though they would have been prepared by printing, the colouring would have been applied as watercolour, in an aqueous medium, with the result that after some four centuries, the pigment is not strongly held in place. In short, the basis of the conservation of such maps is that they should, save for one matter, be treated as watercolours and be handled as fine art. The one matter which separates them from ordinary watercolours is that they bear a scale, which was put there for use. It is therefore an essential preliminary to any treatment of a map that it be measured as exactly as possible, and these measurements, of all four sides and both diagonals, must be recorded and kept with the map. It would not be going too far to say that, if during conservation treatment the map will be completely wetted with water, it would be helpful to know the maximum wet size of the map and if possible the minimum dry size. Measurement takes only a few minutes but the information can be invaluable.
Since we cannot know the moisture content of the paper when the map was printed, we cannot determine the exact size of the original plate, but a fair approximation to it can be made if we can match the length of the scale against a known system of measurement. We can assume that the maximum stretch of the paper will not exceed 2.5%, and will probably be 2.2% or less.
If we find a map which differs from its reputed peers by more than that amount we have grounds for suspicion. It is not practicable to quote in fun Lindley's and Crossley's comments on the variations in size in the map of Surrey which they published in 1793, but it is a valuable contribution to the study of maps. The first volume of International Critical Tables contains a very useful list of obsolete systems of measurement.
We come now to the working map, which like Saturday's child, has to work hard for its living. In view of the quite comprehensive advice given in British Standard 5454 - Recommendations for the Storage and Exhibition of Documents and BS.4971, Pt. I, Repair and Allied Processes for the Conservation of Documents, I don't propose to give a great deal of detailed advice now because you can read these at leisure. I must admit to bias in their favour because I was on the two committees which produced them. I hope that you will the better understand the importance of science in document conservation when I tell you that the only two people who sat on both committees were chemists.
What I can do today, is to encourage you to think about why you want to conserve the maps. Some maps you will conserve because you have a legal obligation so to do. They are archival in nature, for as Sir Hilary Jenkinson has said "They were drawn up in the course of a transaction of which themselves formed a part" and few English County Record Offices are without Tithe maps of large size. Some maps will be kept in national cartographic establishments because they will be the basis or source of information for other maps. Some maps will be kept on police station walls so that car thefts can be recorded. Some maps, and these I pity, will be used in field work, perhaps by students, perhaps by field archaeologists or civil engineers. Some will live in record offices as historical records of what was where, when, or as records of variations in place names with age. It is reasonable to expect that each will require different treatment and if that is so, then the conservation department will require equipment that will enable it to cope with the work.
Let's take a simple example. A very large old map is to be repaired and remounted. It is perhaps nine square metres in area. It will demand sole occupancy of a very large table for some days or weeks. It is common practice in the U.K. now for such maps to be handled not on a table but on a wall and candidates for the 'Society of Archivists Certificate of Proficiency in Document Conservation' must have satisfactorily completed a course of training in map repair both on a table and on a wall. The advantage of having a large table free for use at all times is immense, especially when space is limited.
For the ordinary Ordnance Survey map, on good paper, the prime consideration is passive conservation but even these may need protection and this means that active conservation techniques must be used. The traditional method of mounting maps on paper, or an paper and linen has three disadvantages. There is a considerable increase in thickness, some distortion is to be expected and linen provides an abrasive surface which may scour its own map if that be rolled or other maps in these are stacked flat under and over it. It is well worthwhile considering modern lamination techniques as a means of protection, but very serious thought must be given to the materials used. The greatest advantage is the speed of work, for conventional mounting is slow work. A good man may mount forty or fifty maps in a week by conventional methods and be able to laminate a similar number in a morning, with less increase in their thickness.
This advantage can be more than counterbalanced by long term damage from unsuitable lamination materials, particularly adhesives. An adhesive can behave in several undesirable ways. It can be a poor adhesive and lose its grip in a short while. It can discolour because it is unstable and breaks down to give coloured compounds. It can be excessively acid or alkaline. It can be a good solvent and dissolve a colour used in printing. It can 'bleed' and make a mess which sticks to other objects, or penetrates paper to make it transparent. If we realise that the coloured inks used in printing maps may be soluble and acid-base indicators, we can see several possibilities for trouble, if not disaster and these can be increased if the application of the adhesive requires moisture or great heat. It is a principle of conservation that no process should be used which cannot be undone without harm to the object.
About 10 years ago I began to wonder what happened to maps when they were laminated. I knew that Mme. Flieder in Paris was working on what happened to paper when it was laminated, but maps are not just paper. So I obtained examples of maps having all the colours then used by the Ordnance Survey and Geological Survey and had them laminated by all the processes then in use. Each map was measured first - all four sides and both diagonals, then laminated, then remeasured. Each map was then stored in a drawer in the Laboratory for five years, then re-measured. They will be re-measured this year.
The lesson that are to be learned can be seen from the examples which I have brought to show you. They can be summarized as follows:
We are now forced with a rather harsh truth, that conservation in more likely than not to be governed by expediency rather than principle. We come back to the question that I asked in the beginning -"What do you really want to do?". If you want to support a map, using a lamination technique this is easily possible. If you want to protect a map so that it can endure harsh treatment in field use, this can be done but the price paid will almost certain1y be the bleeding of colours. If you want to apply a tough, slippery protective surface to a map so that it can withstand friction and abrasion in say, a busy cartographic establishment this can be done, but the map may be unpleasantly shiny. In no case can you reverse the lamination process without loss of colour from the map. In each case you can be certain that you will not distort the map.
- If the adhesive is a f1uid, or when the adhesive is fluid, it will dissolve soluble colours.
- If the adhesive has a degree of acidity or alkalinity which is different from the 'natural' pH of the ink there is a likelihood that the ink will change colour.
- If the map is stretched or shrunk by the conditions used in lamination, it will retain its abnormal size for at least five years.
- If the adhesive is a good solvent and dissolves a colour, the colour can be removed on delamination with a solvent even if the colour is not soluble in the solvent employed.
When I used to lecture to conservation students, I would read out the 'Ellis-Jenkinson' rules for the conservation of archival documents and then tell them that I was there to teach them how to break those rules. There is no branch of document conservation of which that is more true than the conservation of maps. I have talked to you for an hour. If you really wanted to become masters of map conservation, I should need to talk to you for a week and to have you undertake a long course of practical work. When you had mastered both theory and practice, you would still be faced with the need to make decisions which were based on expediency and be able to offer only one explanation for those decisions "I was there and you were not". While I am here with you I will be very happy to advise you and you can always write to me. But I can be no more than an adviser because the decision as to which process of conservation will be used is a curatorial one.
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