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A system for the storage and display of manuscript charts on vellum

Christopher Terrell
National Maritime Museum, Greenwich

© LIBER and author
Published from: LIBER Bulletin 22(1982)

The National Maritime Museum at Greenwich has a collection of some 45 early manuscript sea charts drawn on vellum, often called 'portolan' charts. The special nature of these items -their large size, their rarity, their historical value, and the visual excitement which they generate- led the Museum some twelve years ago to give special consideration to their long- term conservation. The staff contained no experts on illuminated manuscripts drawn on skin, so it was decided to look for outside advice, and a committee was formed composed of the curators most concerned with the problem together with those in the United Kingdom known to have most experience in this field.

This committee, which became known as the 'Portolan Chart Committee', first met in May 1973 with the brief of advising the National Maritime Museum on the best methods of conserving, storing and displaying their collection of vellum charts. It is not proposed in this paper to describe the deliberations of this committee in detail; these ranged freely over all the problems associated with the treatment of illuminated manuscripts on skin, but instead to concentrate on the most tangible result of this series of discussions, and that most likely to interest map curators and archivists, which was a storage system for the charts.

Vellum in itself is an extremely tough material - which of course is why it was used for making sea-going charts in the first place. The main problems associated with its use as a base for illustration arise from its origins. It is made from the skins of animals, usually calves, sheep or goats, which after cleaning has been moistened and stretched to a flat shape. This stretching process results in uneven stress in the fibres of the skin which has an innate tendency to return to its original animal shape on drying, producing the familiar 'cockling' effect observable on so many of the charts. Materials of varying consistency were used to draw the charts, generally described as either ink or paint. A definition of the difference, if one is needed, is that ink is a solution normally applied with a pen, while paint consists of solid pigments dissolved in an adhesive medium. Ink is subject to photochemical fading from the effect of light, as it is on paper, and this has to be reduced as far as possible. Paint on the other hand has special problems of its own when applied to vellum. Local surface movement of the skin caused by changes in temperature and humidity can cause a breakdown in the adhesion of paint layers. This in turn, often accentuated by abrasion from a variety of causes, can result in sections of paint actually falling off and important detail being lost.

The aim of any storage system must be to provide protection for the chart from all these hazards, including actual physical attack from chemicals, mould or insects, in all three of its states of existence - in store, under examination, or on display.

Vellum-charts in institutions all over the globe can be found being kept in three principal ways; either rolled, folded, or flat. The rolled state is particularly convenient and economical of space and is how their original makers intended them to be kept when not in use, certainly on shipboard. This method has disadvantages in the long term, however, in that the continual flexing of the chart in the rolling and unrolling process may dislodge paint layers while the general handling needed coupled with the necessity of using weights to keep the chart flat when unrolled is likely to cause damage from abrasion. The folded state is less common, and even less desirable, as stress at the folds is almost certain to result in damage in the long term while quite brutal treatment is often needed to get the chart flat enough for study. Finally there is the flat state, difficult for the very large specimens but probably the most commonly found, either in folders or in specially designed cases or an too often framed behind glass and hung on a wall. One further state, a variation on the folded, is worth mentioning and this is where the chart is glued onto two or four hinged and folded wood boards. This form, particular designed to give protection to the chart under rough usage at sea, was common throughout the 17th century .And it does indeed give a good deal of protection to the chart when folded; certainly from excessive light, partially from dust, and, because the binges hold the chart surfaces rigidly against each other, to a considerable degree from the effects of abrasion. The charts were not dissected first but glued to the boards complete, resulting in a tight fold at the junction of the boards. It says much for the toughness of skin to find many of these folds still intact after many centuries of use, though they have naturally shed the ink and paint along the crease. Warping of the wood boards sometimes has the result of putting the fold under tension when open; and these factors tend to turn them into dissected charts in time.

So it was decided that flat storage was undoubtedly best for the long-term interests of the chart, with its surface free from contact which could cause abrasion, protected from physical attack and insulated as far as possible from rapid changes in temperature and humidity. To achieve this a case was designed in which the chart could rest horizontally contained in its own micro-environment and in which it could be examined, displayed or stored without ever having to be handled or removed ((Fig. 1: Portolan case).

Although it may appear simple enough much discussion and thought went into the design of every part of the case. The base is of 19 mm blockboard, edged with hardwood and sealed with two coats of William P U Lacquer No.555. The object is to seal in any harmful substances that might exist in the wood or adhesives comprising the blockboard, and this particular varnish is a polyurethane type which cures by the catalyst process forming a very tough film which is unlikely to craze or crack. After several trials this particular brand of varnish was found to answer the purpose very well, but there may well be other brands just as suitable. The cover is made of Plexiglass 201, a methyl methacrylate polymer manufactured in West Germany which has the important property of being impermeable to ultraviolet (U/V) radiation. Early designs of the cover were made of flat pieces edge-glued together, but later it was found possible to mould a number of covers by investing in a steel former . This gave an altogether stronger, more secure and more attractive result, but the cost of the former meant that we were restricted to one size of case. So we settled on a convenient size that would take eighty percent of our charts while the eight oversize charts are accommodated in larger, edge-glued versions of the same design.

Inside the case the aim is to provide the chart with its own stable climate. First there is a thin sheet of Melinex laid direct on the base board as one further safeguard against the entry of undesirable substances from the wood in the event of failure of the varnish seal. Above this there are two sheets of 800 gm acid-tree rag mounting board. The purpose of this is to provide a quantity of bulk cellulose material inside the case to act as a humidity 'sink' and to iron out any violent changes in relative humidity that occur outside. This effect bas been tested in practice and does work. The more bulk the better, but the thickness of this layer has to be balanced against the clearance needed above the chart for any bad cockling not to come in contact with the inside of the cover. Obviously if the case was to be kept in a climate of very unstable relative humidity (RH) then a thicker layer of mounting board would be desirable. All these layers are then contained by a cover of brushed nylon fabric, tested for chemical inertness, and secured to the back of the case with non-ferrous staples. The colour of the fabric, providing as it does a visual background to the chart, can be varied to taste; but it is important that a fresh sample of any roll of material purchased at any one time should be tested as manufacturers can often change their specification for apparently exactly similar fabrics with the same name.

Finally the chart lies on top all these layers held gently by sprung perspex clips which, while gripping firmly enough to hold the chart vertically if necessary, allow enough movement to prevent any stress building up in the vellum. One further item that needs to be mentioned are the ventilation holes in the cover. These are fitted with cigarette-type filters to allow air in but excluding damaging dust particles. They were included in the design originally when it was thought that there was a possibility, in extreme conditions, of a build-up of dangerously humid air in the corners of the case. But this assumed a much tighter fit of the cover on the backboard than is the case in fact, and it is now considered that sufficient ventilation can take place around the edge of the cover, with the dust filtered by the nylon material, to make these holes unnecessary.

The charts in their cases are stored in specially built racking with a working surface on top where they may be examined. One drawback is that they require two people to get them in and out, but few researchers are so infirm that they are not able to assist if necessary .The system has now been in operation for over three years and is working well. No researcher has yet asked to get at the actual surface of the chart but such a request would be considered if convincing reasons were produced for so doing. Two charts have been on display in the galleries for a little longer, inside their cases but otherwise unprotected. This is not proving entirely satisfactory as the acrylic cover scratches easily and they have suffered from contact with the backs of school clipboards and over-energetic polishing with scratchy cloths. Our plans for future gallery display will put the chart case inside a glass display case and will also include even more light control - much lower overall light levels with local enhanced lighting controlled by push button time switches.

The case has proved its worth in the instance of loans, especially for exhibitions abroad. The chart travels in its case ready for display and protected against many potential disasters; the case may get damaged but that can always be replaced. In addition there are often problems of maintaining security in the general last-minute mayhem which often attends the setting up of a large exhibition, and the case considerably reduces the vulnerability of the chart to both theft and accidental damage.

In conclusion I realise that what I have been demonstrating is expensive, both in materials and space, and that compared with some institutions our problem is relatively minor. But I make no apologies for this. These charts are extremely rare and valuable historic documents and it is important that the detail they show should not be lost. They are already four, sometimes five centuries old so it does not seem unreasonable to take somewhat extravagant steps now to preserve them intact for a few centuries more.

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