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No. 67 Skin |
Parchment is the dehaired, untanned skin of various animals such as calves, sheep and goats dried under tension on frames. A terminological distinction is sometimes made between vellum, made from calfskins, and parchment, from all other skins; but the words are often used interchangeably. Although parchment has largely been replaced by paper – to say nothing of the computer screen! – it still finds many uses today. As a writing material it is employed for all kinds of 'quality' documents, from wedding invitations to diplomas, from retirement scrolls to limited-edition and artist's books. The copies of acts of the British Parliament signed by the queen are on vellum, and a special white vellum called Kelmscott is used for painted botanical drawings kept at Kew Gardens and libraries in Oxford. This was first produced in 1891 by William Morris for the Kelmscott Press, the high whiteness being obtained with a coating of parchment glue and chalk. Nonliterary uses include the membranes of percussion instruments such as tambourines and drums, and translucent forms of parchment are used to make lampshades, although for this purpose the skin is also tanned with alum. A brief history of parchment manufacture The Ancient Egyptians employed three writing materials: rolls of tanned leather were almost exclusively reserved for architectural plans, papyrus was used for cultic documents, while for a long time, private letters and calculations were solely inscribed on 'ostraka,' sherds of pottery or limestone. The Greeks were the first to use papyrus for more everyday purposes, but it remained a relatively expensive material. The oldest surviving written texts on parchment have come down to us in the form of scrolls. The script was written mostly left to right in adjacent columns. For deeds in the Middle Ages, however, the text is commonly vertical, i.e. from the top to the bottom of the scroll in the direction that it unwinds. The development of the codex, i.e. text bound in book form, from the 4th century onward gradually replaced the scrolls, until these were used almost only for deeds and similar official documents. The introduction of parchment codices, whose structure and binding resembles that of today's books, brought many advantages, not least that bound pages are simpler to handle than scrolls. It is much easier to find a section of text flipping through the pages of a codex than by unwinding a scroll that might be up to 25 m long. Furthermore, writing on both sides of a scroll is awkward and difficult, while both sides of the page can be used in a codex. Nevertheless, scrolls with writing on both sides are known and are called opisthographs (from the Greek opistho = behind and graph = written). The production of codices benefitted greatly from the availability of parchment. Parchment is smooth, both sides can be used for writing because it is opaque, and it doesn't split when folded. Papyrus, in contrast, is much more brittle, and this tendency to break renders it almost totally unsuitable for a codex. According to the reports of various Classical authors, parchment was 'invented' by Eumenes II of Pergamon (197–159 BC). To perpetuate his memory and fame, he wanted to establish a library in Pergamon that would rival the most famous library of the time, that in Alexandria. This required the copying of enormous numbers of scrolls and, thus, the acquisition of enormous quantities of papyrus. However, not only did the Ptolemaic ruler in Egypt have a monopoly on papyrus production, the material was in short supply as a result of wars, and so Eumenes had to search for alternatives. Contracts written on parchment from circa 258 BC are known to us from Dura-Europos, in the border region between what are today Syria and Iraq, and Eumenes seems to have returned to this earlier technology and reintroduced the production of parchment scrolls. These thin sheep's skins were also given as gifts to the Romans who called them membrana pergamena (Latin for 'skins from Pergamon'). Sadly, none of the scrolls from the libraries of either Alexandria or Pergamon have survived the vicissitudes of time. The origins of a saying still current in German, ”das geht auf keine Kuhhaut“ [literally, 'that won't fit on any cowhide,' in other words, 'it's absolutely beyond belief'], lie in the use of parchment from the skin of a cow or calf as a writing material. This saying is illustrated, and dramatically, in a 14th-century decoration on the north wall to the left of the pulpit in the Church of St. Georg in Oberzell on the island of Reichenau (Lake Constance) (fig. 1): four devils hold out a cowhide, three pulling at it, one biting it, with, above, the figures of two women in conversation and a hanging church lamp. Another devil has written the following text on the skin:
The allusion to gossiping women is directed toward the church, not the community. Being displayed on the wall near the pulpit, its role is to exhort the preacher to keep the sermon short.
The biochemical structure of parchment The essential components of parchment from animal skins are the fibers of the connective tissue. This extracellular matrix in the skin is composed of bundles of long-chained fibrils of the scleroproteins collagen and elastin. In living skin, the intercellular spaces are filled with fluid or plasma. The basic unit of collagen is tropocollagen (MW 300 kD), a right-handed helix of three polypeptide chains. A single tropocollagen molecule is a microfibril 280 nm long and 1.4 nm wide. Packed together by hydrogen bonds, salt bridges and covalent intermolecular bonding, these microfibrils form fibrils with a diameter of circa 250–500 nm. Adjacent intertwining molecules in the fibrils are displaced relative to one another by a quarter of their length, giving rise to a periodicity of about 70 nm visible as dark bands in the electron microscope (fig. 2).
In contrast to the relative rigidity of collagen, elastin stretches and recoils. It is assembled from tropoelastin monomers, and hydrophobic domains alternate with intramolecular cross-linking (at lysine residues) domains which confer the elasticity on the randomly coiled molecules. The production of parchment The preparation of parchment for writing showed very little alteration between antiquity and the Middle Ages, and indeed remained fairly unchanged right into modern times (fig. 3). Though we know little about how early parchment was produced at Dura- Europos or Pergamon, many medieval recipes have come down to us. Only with the introduction of industrialized technology and modern chemistry has the process changed significantly.
The first step involves immersing the fresh animal skin for 2–6 weeks in a 5–10% solution of slaked lime (a process called liming). The different layers of the skin swell at different rates and gradually begin to break up (fig. 4). The epidermis reacts most quickly. Because the hairs have their roots here, after the immersion process, the hairs along with the roots are easily removed by draping the skin over a beam and shaving with a dull blade (fig. 5). The skin is then reversed and the remains of fat, muscle and loose flesh are likewise removed from the flesh side. After washing, the transparent skin is stretched and dried on a frame. These physical and chemical processes orient the fibers in sheets and open up the inner structure of the collagen so that air penetrates between the layers causing the parchment to become opaque, and thus suitable for writing or decoration on both sides.
To enhance the properties of the surface for writing, both sides of the dried parchment are carefully polished so that they are neither too rough nor too smooth. Polishing (also known as pouncing) is done while the skin is still stretched on the frame, either with a crescent-moon-shaped (semilunar) knife (Latin lunelarium) (fig. 6), with pumice or with a specially prepared sanding bread. For the latter, bread dough is mixed with glass splinters, formed into small rolls and baked. The parchment surface can be treated far more sensitively with such sanding bread than with pumice or a knife. With inattentive use of the knife, the skin can be damaged quite quickly; pumices are not completely homogenous and contain hard stone-like nodules which leave scratches on the parchment surface. Cuts and tears that occur before the stretching procedure are usually sewn before the skin is put into the frame, so that they don't stretch or expand during drying. They can be cut out after drying, but often they are left in the parchment and can be seen today in the old manuscripts (fig. 7).
From antiquity onward, the Jewish practice was to use a fermenting flour or bran paste to prepare the fresh skin, creating a very fine high-quality product. The parchment of the Dead Sea Scrolls was apparently produced this way. The enzymes that built up during fermentation facilitated the removal of the hair. The rotting paste mass was spread directly on the skins which were piled one on top of the other and left for several hours or days. Because heat also builds up during the process of decay, the decomposition can run out of control, eating holes into the skin. The enzymes attack not only the epidermis but can also penetrate to deeper layers, and may ruin the entire skin. These drawbacks have led to the increasing replacement of fermentation by liming in the Jewish manufacture of parchment. Nevertheless, special laws of cleanliness still have to be observed in preparing parchment for Jewish writings. In the modern manufacture of parchment, sodium sulfide and enzymes are used for dehairing. The result is a product differing significantly in durability and quality from historic parchment, which can cause problems if modern material is used in parchment restoration. Goldbeater's skin One rather special parchment product is goldbeater's skin, made from the appendix of calves (fig. 8a). After a short lime bath, the outer skin layer of the 40- to 80-cm-long appendix is stripped off and stretched. The result is a very thin (0.05–0.01 mm), elastic and long-lasting skin. The fibrous structure is clearly revealed in the scanning electron microscope. These thin skins were used to beat out gold leaf, hence the name. Small sheets of gold were laid between the pages of a book made of goldbeater's skin. The book was beaten with a hammer until the gold leaves had spread out to the size of the book's pages. The gold sheets were cut into four pieces which were put back into the book which was beaten again. The process was repeated until the gold leaf was a mere 0.001 mm thick (fig. 8b). The goldbeater's skins were so elastic that even under very heavy beating they did not tear, and were so thin that up to 120 sheets of gold could be beaten at one time.
Parchment manuscripts For over 1000 years, parchment was used not only for de luxe manuscripts but also for everyday documents and texts. Because of its unique, durable structure, surviving manuscripts even from the 4th century AD can give the impression that they were written only yesterday. Particularly valuable parchment manuscripts were produced with imperial purple coloring. However, the purple dye produced by the mollusc species Murex brandaris L., M. trunculus L. and M. erinaceae, and used for dyeing cloth for example, was not used. Instead, the dyes employed for painting were extracted from plant materials such as berries, lichens, roots and resins (fig. 9). The mollusc purple extract, 6,6'-dibromo-indigo, could not be used on parchment because, as in indigo dyeing, the development of the color proceeds via a redox reaction. If parchment gets very wet it shrinks and has to be dried by restretching on a frame, and acidic chemicals, like those that must be used in the reduction process, keratinize and destroy the collagenous material. Instead, plant dyes such as litmus from the Rocella lichen, madder from Rubia tinctorum L. or lac dye from shellac seem to have been used as the purple coloring on old manuscripts. Berries – bilberries, elderberries and privet berries – also appear to have been a source of color. Analysis of these means of coloring is one of the active areas of research at the Laboratory for the Nondestructive Analysis of Artworks at the University of Applied Sciences, Cologne.
The conservation of parchment The biochemical structure of parchment influences its response to the environment, to aging and during restoration. The collagen fibers are heat sensitive. Above 70°C, they begin to shrink irreversibly and denaturation sets in. The fibers also respond to changes in humidity by continuous shrinking and stretching. This is known as the climatic reflex because it is an automatic response which can only be prevented by keeping the parchment in a constant environment. This is one of the most important basic conditions for the preservation of painted parchment manuscripts. The pigment layers in Byzantine illustrated books, in particular, have a tendency to peel off if heat and humidity keep changing because the parchment surface was treated with size or gum arabic prior to being painted. While the ground material shrinks and stretches, the paint layers cannot; a tension develops between them and the paint layers steadily peel from their base (fig. 10). Originally, the parchment was sometimes coated with a medium to improve the adhesion of the paints. In the long-term, though, this can also have disastrous consequences, threatening the total destruction of the entire manuscript. To preserve such documents, they have to be completely protected from all climatic variations, whether in storage, in the reading room, in exhibitions or during transport.
High humidity or water are hostile to all parchments: they encourage molds or cause the parchment to become transparent and to decompose. In high heat, parchment shrinks to an unredeemable lump. Thus, one of the major tasks of archives and libraries is to protect parchment documents and manuscripts from heat and water – a major undertaking in the event of floods and fire! The threats posed by water and heat also have to be taken into account during any restoration procedure. To ensure that any piece of parchment will not wrinkle after it has been glued or repaired, it is important to know exactly how the parchment will respond to the adhesive. Research has shown that isinglass, a gelatin obtained from the air bladders of the sturgeon (Acipenser sturi L.), is one of the most suitable adhesives for parchment. The sturgeon uses the air bladder to regulate its depth position in the water: filled with air, the fish rises to the surface; emptied, the fish descends. The bladder, therefore, needs to be highly elastic, and is in fact comprised predominantly of collagen fibers with a high proportion of elastin. While an animal skin will contain anything from 2 to 5% elastin depending on age, the sturgeon's air bladder can contain up to 20% elastin. Hence the bladder's special elasticity which can be put to good use if extracted and prepared correctly. The glue is used today not only to repair parchment but also in furniture restoration, because no other modern artificial resin can achieve such elasticity at the adhesion site. Preparing the glue, the bladder must not be heated above 42°C. It is soaked overnight in water and the following morning, slowly melted at 40°C. The resultant glue is fluid only when warm and gelatinizes at room temperature. Addition of a small amount of tragacanth, the water-soluble gum resin from Astragalus tragacantha L., improves the distribution of the proteins in the glue, forestalling its premature gelatinization such that it can also be used at room temperature. Another problem encountered in parchment restoration are folds and wrinkles, which may be centuries old in some documents. Creases can be eased out with the careful use of moisture, but if they have existed for a long time, the structural changes will be imprinted in the inner collagen structure, and will tend to reassert themselves again over time. In such cases, the sheets of parchment must be weighted down for more than 2 months so that the new form is imprinted in the fibers and persists. Collagen also has a 'memory' for environmental air pollutants. Traces of the bombing of Dresden with phosphorus bombs during World War II can be detected in the parchments kept in the Sächsische Landesbibliothek in Dresden, along with evidence for the sulfur dioxide emitted from the burning of lignite in factories and houses in East Germany in the ensuing 40 years. Parchment is a material of outstanding stability that can survive for many centuries; archived correctly it can theoretically be preserved, in an unchanging state, indefinitely. Nevertheless, it has certain innate properties requiring of the restorer special knowledge, skills and 'a feeling for the skin' (fig. 11).
 Robert Fuchs obtained his PhD in chemistry at the University of Tübingen, with Egyptology as a subsidiary subject. From 1984 to 1989 he was scientific coordinator of the ‘Forschungsstelle für Technik mittelalterlicher Buchmalerei’ at the University of Göttingen. Since 1989, he has held a chair for the Restoration and Conservation of Archives, Graphics and Book Illumination at the University of Applied Sciences, Cologne. He is also Head of the Laboratory for the Nondestructive Analysis of Works of Art in the North-Rhine-Westfalia Research Center for Restoration and Cultural Heritage, and since 2003 has been President of the Group of Archeometry in the German Society of Chemists.
Prof. Dr. Robert Fuchs Fachhochschule Köln Fachbereich Restaurierung u. Konservierung v. Schriftgut, Graphik u. Buchmalerei Ubierring 40 D–50678 Köln Germany Homepage: www.re.fh-koeln.de Email: fuchs@re.fh-koeln.de |
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