The conservation and restoration of parchment constitutes the care and treatment of parchment materials which have cultural and historical significance. Typically undertaken by professional book and document conservators, this process can include preventive measures which protect against future deterioration as well as specific treatments to alleviate changes already caused by agents of deterioration.
Parchment manufacturing and properties
Parchment is the skin of an animal, usually sheep, calf or goat, which has been dehaired, processed with a lime solution and stretched under tension. The dried material is a thin membrane which is most commonly used as a writing surface, but can also be used to make other items like bookbindings and drumheads. Throughout Europe, parchment was the primary writing substrate from its development in the 2nd century BCE through the Middle Ages, though it is used through the present day for various official documents. Typically parchment made from calfskin is called vellum, though the term can also be used to refer to very fine quality parchment made from the skins of other animals. For the purposes of conservation and restoration, the term parchment is used in reference to vellum objects, as the terms have been used interchangeably throughout time to refer to objects with the same conservation concerns.
Parchment has unique structural qualities which differentiate it from paper-based materials. Typically, parchment is resistant to mechanical damage like tears or creasing, though it is easily susceptible to damage from mold and high temperatures. Parchment is also highly hygroscopic in nature, meaning that changes in relative humidity can cause irreversible variations to its structural makeup. These specific qualities dictate the conservation and restoration treatments required for parchment.
Physical and chemical properties
Most skins used for parchment are 1–3 mm in thickness before processing. Animal skin used for parchment all has the same basic structure, with slight variations due to the species, age and diet of the specific animal. Skin is composed of innumerable fibrils made up of the protein collagen, which are held in bundles that interweave in a three dimensional manner through the skin. The fibrous material is composed of many long chain molecules of collagen, which can react with certain environmental factors. One universal property of collagen is that it exhibits sudden shrinkage when heated in water, starting at . Prolonged exposure to alkali, like in parchment liming process, changes the amino acids, consequently dropping shrinkage temperatures as low as .
Manufacturing
Once it is removed from the animal, the skin is temporarily preserved, either by drying or liberal application of salt, until it can be processed. The skin is then immersed in water for 48 hours, which cleans and rehydrates it. This step also removes the non-collagenous materials, like hyaluronic acid, dermaten sulphate and plasma proteins. The skin is then soaked in a lime or alkali solution, known as the liming process. In the 19th century, chemicals were added to speed up the liming process, which resulted in weaker parchments. These added compounds sometimes reacted to produce gypsum, giving the parchment a characteristic gray hue. The skin is then stretched in suspension on a frame, constricting it as it dries. This ensures even contraction across the entire parchment which ensures that it will remain flat when dried. After being prepared, parchment is sometimes coated so that it is more receptive to pigment and ink. Historical coatings, which include chalk, egg whites and matte paint, must be taken into consideration as a meaningful part of the preservation and conservation plan.
The manufacturing process, which removes the skin's natural fats and oils, means that parchment is more reactive to moisture and relative humidity than other skin-based material. After being stretched, parchment has an inherent desire to revert to its original animal shape, especially if left unrestrained or exposed to repeated changes in relative humidity.
Identification
Parchment is usually positively identified by sight, sometimes with the assistance of a hand lens or microscope. Visible hair follicle pattern, veining, scars, bruises and sometimes fat deposits all help confirm the animal origin of the material. Additional light sources including ultraviolet lights, can make these properties more easily identifiable.