The use of glue in the processing of parchment

Mediaeval bookbinders developed a refined craftmanship in the processing of parchment. Traditionally they used skin glue to glue parchment. For the interested layman this is a great challenge and the results are not always satisfactory. Much easier to handle is dispersion glue, which can be bought under different brand names. On the German market there ist Planatol which is sold by Boesner. My experiences with this bookbinder glue have been very good. The most important ist the degreasing of the surface immediately bevor processing. Acetone does a perfect job. Who opt for milder solutions, then technical alcool is somewhat milder and produces similar results. It is important not to wait too long before further processing. One day is alright. If you wait too long, then the remaining grease from inside the skin diffuses to the surface. Doffocult to work with is parchment, where the hairs have not beet throughoughly removed.

After degreasing, the surface has the be roughened by sand paper. This has to be done carefully, especially when processing fine parchment. All movements have to be done in one direction, otherwise zigzag lines can be visible. Having thin in mind the roughening of the skin should not be neglected. Otherwise, the parchment will not stick. On places without sufficient scratching or on greasy spots, parchment will detach from the surface an – due to expansion –  will form blisters in moinst surrounding. At worst the whole parchment will detach from its surface.

Glue should be applied moderately on both sides. Too much glue will gather and will be visible as elevated spot. Since the parchment will expand due to the moisture of the glue, the works should be done quickly. Otherwise when pressed, the surface will form pleats which might persist. To prevent the parchment from forming blisters, it is possible to use a houshold iron. When touching the blisters with the moderately heated iron, they will shrink and the surface will smoothen. However if the iron is too hot, the parchment gets wrinkled.

While glueing, parchment develops strong pulling forces. Cardboard glued with parchment on one side will bend strongly. To prevent this, parchment can be glued on both sides. Since stronger parchment pulls more than lighter ones, the surface will bend, when the thickness of both parchments is different. Therefore to glue on both sides, parchment of the same strenght has to be used. A difference of 0,05mm is acceptable.

After glueing the cardboard on bothe sides, they should be pressed. After about one minute, the press should be opened again to treat blisters with the iron. One should not wait longer, otherwise the blisters will be too high and cannot be removed by ironing anymore. 

Parchment as living material of the bookbinder

Contrary to leather, linen or paper, parchment is a very dynamic material, which will settle only after a considerable period of time. Even then it will remain active. With this we hint to the properties of the material to react on changes of temperature, humidity and exposure to sunrays. If a newly bound parchment book would be exposed to direct sun, the surface material would shrink to a certain amount and by the pulling forces unfolded the cover would be strongly bent. Equally, if a newly book would be taken out of the press and be kept in a room with elevated humidity – say a kitchen – the surface parchment would absorb humidity and expand. As a result the cover would bent in the other direction. Such deformations cannot only be removed – even by renewed pressing for a lenghty period of time. To avoid such unwanted deformations, according to my experiences the newly bound book should be kept in press – or at least under pressure -  at least for two weeks. A well hardened book will not be too vulnerable to changes of humidity and is only susceptible to direct sunrays in (European) summer. 

The use of different glues

Purists among bookbinders focussing on processing parchment would propably insist on the exclusive use of sturgeon glue . A glue made from the air bladder of a fish called beluga. He might also insist on collecting cuts and leftovers of parchment from former works to cook his own hot-setting glue. This is possible since parchment consists almost totally of collagen- the glueing matter which forms the basis for hot glue. My experiences with these kinds of glue have not always been too successful. Hot glue will settle only within a very definite and tiny span of temperature. If it will be too cold, it will not stick. A further disadvantage according to my experience is the unfavourable property of the glue to become brittle after some time. I collected my best experiences using modern bookbinder glue. In Germany this would be Planatol by Boesner. In Anglosaxon countries equivalents of this material are avaiable. Even with modern glues some things should be kept in mind. After ten minutes Planatol still has the power to glue paper and linen, but no parchment. This is important. The glue will moisten the parchment which will expand and form bubbles. While drying, these bubbles will disappear. Since the parchment on the inner side of the bubble will only come in contact to the material of the book cover after the bubble has disappeared, the glue will be dry and the parchment will not stick on these areas. In order to avoid this unpleasant things, it is possible by counteract the expanding forces of the parchment after being moistened by the glue. This can be done by a pressing iron, whose temperature is adjusted to iron delicate cloth. By heating the bubbles, the parchment will shrink to ist former position. No bubbles will appear. Very important is also roughening the surface of the parchment by sand paper. The parchment should also be de-greased carefully and thoroughly before glueing.

Printing parchment with inkjet and laser printers

It is possible to print on parchment using an inkjet printer, although some points have to be considered. The parchment should be carefully degreased on both sides. It should lie flat without ripples and waves. When the parchment has been stored as roll it should be bent and rolled in the opposite direction to flatten it. Parchment, however, can be printed on when it is slightly bent. Parchment of 0.2-0.3mm strenghts is most easily printed on. Stronger parchment might create difficulties with the feeder. It might not feed in. Difficulties might also be created by defects and unevennesses which might be on the surface – especially seams are difficult to print on. The ink might smear on these spots.

In general, since parchment is not as absorbent as paper precautions should be taken not to touch the freshly printed parchment. Sometimes it is useful to guide it manually when it comes out of the printer so that no part of the printer will touch the surface. After drying however, the print is as durable as any paper-print.

To print parchment using a laser-printer is a different story. Generally speaking, it is possible to print on 0.2-0.3mm parchment and to obtain the desired pattern. The pigments however are not fixed and can easily be obliterated. I made some trials using air spray as fixation. The results are quite o.k. but now the surface  was shiny and glossy so the original character of the parchment surface was lost. The same was true using hard wax – some of them come as oil. If a shiny surface is not a problem, then both substances can be used.

Screen printing however is not a problem. The result is much better than using inkjet printer. Even small defects and seams can be printed on. The picture will be clear except on small deep ripples who will usually appear, when the parchment has been heated on some spots or when it has been kept in moist places. 

Veinlets and antique parchment

 „Antique“ parchment offers interesting insights in otherwise little explored properties of this interesting material. When an animal has died a natural way, veinlets will be visible on the lower side of the animal. The blood stays inside the skin and will not be drained as it would be the case by „normally“ slaughtered animals. It will be visible as dark lines who form intesting and unique patterns. During the parchment making process the chemical bound iron of the parchment reacts with the quick lime used during parchment production the create permanent colours.

Since mediaeval times, parchmenters knew how to prepare transparent parchment. They prepared very thin parchments from young animals. Then the skin was soaked in a solution from old white egg respectively gum arabic and fish glue or glue prepared from parchment skins. After this procedure, the wet skins were spread out and the remaining moisture rubbed into the surface. Now the skin became transparent. According to anonther recipe, the scratched skin was rubbed with lineseed oil. It is important not to stretch the skin. This is also visible in a 18th century picture, were normal parchment was put into a solution of potassium and then dried pressed between two wooden boards. Transparent parchment was used in monastic scriptoriums during the illumination process. It was also used as primitive magnifying glasses or as window glass.

Parchment from stillborn animals was prepared in mediaeval times from sheep- and goatskins. Although they were extremely fine, the skin structure and strenght was fully developed. The even set the standard of high parchment quality. Later calf skin was preferred, since goat skins were quite small.

Parchment for gold leaf makers was prepared from the appendix of calfs. It is thin, strong and can be stretched without tearing. Gold leaf makers used it to separate the different gold layers before starting the hammering process. 

the hygroscopic character of parchment

During the preparation of parchment different mechanical and chemical processes take place. Notably the inner structure of the skins is completely changed. After scratching and washing it in quick lime solution, it almost completely consists of collagene – a natural glue, which is not soluble in cold water. If cooked, however, the collagene would melt into a liquid glue. During the parchment making, the collagene tissue will be mechanically changed. The previously chaotically arranged molecule chains now arrange into a parallele structure. The change is accompanied with a new look: the surface changed from transparent to opaque. The changed skin structure comes with new properties. Parchment is very hygroscopic and the surface is not stable. It widens and shrinks according to humidity and exposure to direct sunlight. Therewith the surface becomes wavy. Untreated parchment has a remaining moisture of 10%. With a humidity of 70-80% it increases to 25%. This process takes two to five days. When immersed in water, the parchment would become soft and similar to untreated skin. When stored with a humidity below 40%, the parchment would dry out. This process takes several months or even years. The parchment would become stiff and the surface would eventually crack. The ink or colour would equally crack at these spots. To prevent a loss of moisture, the surfaces is treated with hygroscopic substances like gum arabic, honey (!), skin glue or egg white.

One of the skills of mediaeval scribes, was to prepared the parchment before writing according to the needs of the parchment. When dry and hard, they were stored in a moist place to become writable. The best conditions to store parchment is around 40% humidity and with a temperature around 20 degree celsius. These experiences form the work of the curators in the museums. They store calf parchment in a constant temperature of 20 degrees and a humidity of 25-35%. When stored below 11%, the parchment would become brittle. When stored above 40% it becomes susceptible to fungus grow and a gelaninization process. When stored in moist places, it starts to rot. Parchment however is susceptible to excessive heat. Although it can survive temperatures of 100 degree celsius, it starts to shrik and wrinkle irreversibely when heated above 130 degree. Parchment however is much more resistant to aggressive inks and colours than paper. Even acidic liquids can hardly influence the skin. Parchment has kept from the manufacturing process a natural alcaline reservoir, which quickly neutralizes acids. 

industrial parchment processing

From antiquity to modern times, the chemical and mechanical principles of the production process have not changed. Even the introduction of new technologies could not drastically alter this situation. Skins are bought as „fresh skins“ (salted) or „dried skins“ . The article is assorted according to weight and up to 400 pieces per washing barrel. After thoughoughly washing and soaking, the skins are treated with a mild alkaline solution of sulphuric sodium and quick lime in order to make the skin receptible and to loosen the hair. After one to two days the skin looses hairs and the hair side is clean. After washing the skins are taken out of the barrel and mechanically cleaned on the flesh side and planed down to the lower skin tissues. Now the skins are given back to the barrel, rinsed thoughoughly, chemically neutralized and asssorted according to ist later use. The thickness is reduced to a standard size by rolling. Very often, the parchment is split to its ultimate thickness. Since splitting weakens the skin structure, this parchment is very susceptible to tearing. It nevertheless is used in bookbinding, although the pore structure of the skin is lost. In the 1950s, tndustrially produced parchment was widely used in the production of lampshades, to cover furniture and in interior design. It could not compete with the manufactured parchment, since skins from industrial production often had greasy spots. 

The health of the animals and its relation to the quality of the parchment

While looking at the prepared parchment much can be said about the living conditions and the health of the animals. Injuries can lead to scars which will be seen on the prepared skin. Bites by lice and bugs can also lead to scars when the animal reacts by scratching or biting the itching spot. Also visible on the skin are bad general living conditions of the animal. To prepare expensive and chosen parchment books, in mediaeval times the parchment was carefully scutinized. The Codex Sinaiticus from the 4th century – a parchment book which was found in a monastery in Sinai peninsula in Egypt – scars can only be found in the margins of the pares. It is also possible that the animals designated for this book experienced special care. The quality of the parchment depended on the quality of the skin. The parchment maker carefully had to scrutinize the fresh skins for injuries. Scars and insect bites have been equally visible like the colour of the hair. Skewbald skins will turn to skewbald parchment, while black hair will lead to blackish hides. Even the age of the animals will be reflected. The use of saddles and thrust bridges can equally be seen. It will lead to patches of callused skin. Considering this it is clear that parchment made of skins of younger animals tends to be of better quality. Even blood stains on the skin are visible, if not immediately removed. 

The properties of parchment made by different animals and its specific use

Velum is very thin and strong. Since it often comes without a pronounced surface texture, tt is the traditional material for manuscripts and for book binding. Goat parchment is slightly cheaper that velum. The quality is somewhat lesser and the pore structure os more pronounced. Colour and strengh differ from skin to skin according to the lifestyle of the animal. In comparition pig parchment can barely be used for book binding. It ratther served for drums and kettledrums and to make colander (sieve) to prepare gunpowder. Pig and cow parchment was used for lampshades, wall coverings and table coatings. Sheepsskin in comparition to goat has a weaker texture and is susceptible to brittles. Since these skins often contain much subcutane fat, it is difficult to work with. Sheep skins are generally bigger than goat, so they are used for furniture works and interieur designs. Since many centuries, goatskins is prefered by bookbinders. It is highly dense and difficult to tear. It can be incredible thin and flexible. colour come as natural, white and chream. Untreated it is used for bookbinding, furniture works and interieur applications. Treated on one or both sides it is prefered by calligraphers. Goat skin can also easily be dyed.

The prepard parchments differ in strengh. Skins from the race of the African Oasis Goat can measure as little as 0.15mm in thickness. Races with thicker skins are prefered by the music industry. West african drums (jembe) are covered with skins of 3mm. Bigger drums are covered with skins of 0.5-0.6mm strengh.