Conservation and restoration of parchment
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
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.Sometimes visual examination is not sufficient to distinguish parchment from certain types of highly calendered papers. Misidentification of these materials can lead to inappropriate preservation methods or conservation treatments. Analytical testing, which involves removing a small piece of parchment, can be done by or under the supervision of a professional conservator-restorer to ensure positive identification. One type of analytical testing involves examining the parchment specimen under a light microscope or scanning electron microscopy. A simple flame test can also be done on the parchment specimen; true parchment will emit the meaty smell of burned protein, while other look-alike materials will smell like paper or wood.
Agents of deterioration
- Relative humidity changes
- Water and excessive moisture
- Oxygen
- Mold
- Heat:
- Light exposure
- Flaking and friability
- Poor storage
Storage and preventive care
Proper storage environments can help ward off structural, chemical and environmental changes which affects the long-term preservation of parchment. Storage factors must take into account the factors of the particular parchment object, including its condition, age, storage history and plans for use. Illuminated manuscripts and composite parchment objects, which may include seals and ribbons, may have additional storage needs. A consistent storage environment is crucial for the long-term stability of parchment, which is especially vulnerable to changes in humidity, temperature and other environmental factors.Microenvironments are less expensive ways to provide consistent storage environments for parchment if the external storage conditions are not ideal. Moisture sensitive parchments can be stored in a Plexiglas sandwich by inserting the matted parchment between two sheets of acrylic and taping off all sides. Parchment can also be stored in envelopes constructed out of polyester sheets.
For the long-term preservation of organic material like parchment, the ideal temperature range is with a relative humidity level of 30–50%. The ideal storage and display environment is oxygen-free, as oxygen prevalence has been shown to react with collagen over time, leading to increased brittleness of parchment. Oxygen-free storage and display cases are filled with inert gas as well as a chemical substance that reacts as an absorber if any stray oxygen leaks into the encasement.
Ethical concerns of conservation
The American Institute for Conservation's Code of Ethics and Guidelines for Practice set forth the conduct for conservation and restoration treatments of parchment objects. The hazards of specific treatments need to be weighed against the benefits, as many traditional liquid-based conservation techniques can pose risks to parchment.Conservation treatments
Removal of previous restorations
Previous repairs and conservation treatments are removed if it is determined that the parchment is in good condition and that the repair is not of historic importance to the parchment. Dried out patches are removed via cleaning and humidification techniques. Parchment objects that have been attached to mounts are removed with a sharpened lifting tool and a small amount of moisture. Previous laminations, like Goldbeater's skin and Mipofolie, can sometimes be removed with solvents and mechanical techniques, though this removal technique can risk additional damage to the original parchment.Cleaning
Surface cleaning of parchment has been developed after methods used for cleaning paper, with a few key differences related to the structural and chemical properties of parchment. For the most part, chemical treatments are ineffective and some, like hypochlorites and methyl bromide, can denaturalize parchment. Prior to cleaning, any flaking or vulnerable media is consolidated to ensure it does not detach from the parchment substrate. Consolidation of parchment media is undertaken by applying various dilute adhesives like isinglass, gelatin or parchment size through brush application or by the use of an ultrasonic humidification apparatus.If their presence is determined upon examination, insects and pests are eliminated and the parchment disinfected. An appropriate round of fumigation in an autoclave is used to remove pests and sterilize the object. Dry and wet cleaning techniques can both be used effectively on parchment objects. Mechanical cleaning using erasers and other abrasive tools is used to remove mildew and dirt. Surface cleaning of parchment is typically completed using white vinyl erasers and confined to areas where no media is present. Water baths, which sometimes use neutral detergents or alcohol as additives, can be used to clean as well as prepare the parchment for stretching as part of the conservation process. Another wet cleaning method, which is especially useful for removing mold and mildew, uses a cotton swab covered in fluid, usually denatured alcohol, ethanol or saliva. Laser cleaning may provide a non-contact cleaning process, though conservation labs throughout Europe still mostly use traditional wet and dry cleaning methods for parchment.