The repair and mending of parchment has taken place for thousands of years. Methods from the earliest hand stitching of tears to today's use of modern equipment to mend and fill parchment show the importance that has been placed on its preservation and conservation.
Flattening and crease removal 
Repairing curling and crease lines in parchment is generally achieved by raising its moisture content, though some modern techniques use little or no water. Wetting parchment can cause damage, though it is sometimes unavoidable.
If the entire piece of parchment needs to be flattened, one technique is to place the parchment into a controlled humidity chamber with a relative humidity of 95% or more. According to Bety Haines', one important factor to monitor is to make sure the vapor has penetrated to the core uniformly, and at its own rate, and not just the surface of the parchment. Once this has happened, the use of tensioning with clips will allow creases to be eased.
If just a few creases need to be eased, 80% to 90% isopropyl alcohol may be applied to the creases directly. Heavier skins may need the higher water content. The Alcohol-Water mixture is azeotropic, meaning both components will evaporate simultaneously. The solution has a lower surface tension than water, which causes the fibers to suffer less shrinkage. The solution can be applied with a cotton swab and then gently pulled flat by hand.
The easiest and safest method to flatten parchment is to humidify and then dry and flatten it on the vacuum suction table rather than using the clips and tensioning frame. Suction table flattening is widely employed in the United States.
A fully humidified parchment is placed on a suction table at low suction and then covered with polyester film strips of varying sizes. Once the suction is turned up, the parchment is gently stretched and mainpulated until flat and then held in place with the polyester strips. After the parchment is satisfactorily flattened overall, the polyester strips are removed and replaced with a blotter or Gore-Tex sheet for further drying and flattening. After treatment on the suction table, the parchment is placed in a weighted blotter stack to further dry and flatten.
Another method of localized treatment involves the use of an ultrasonic humidifier to add humidification in a controlled area. During the process, tension is gradually placed on the edges of the parchment to release the creases and then it is dried under pressure between polyester web and thick wool felts.
Any necessary cleaning is done before using any techniques that raise the parchment's moisture content. Humidification can cause the dirt to become ingrained in the parchment, potentially making stains irreversible.
Before cleaning, it is imperative that the conservator assess the condition of the media on the surface of the parchment. If insecure media such as pen and ink is noted, consolidation is required before surface cleaning. Consolidation is undertaken locally by introducing various dilute adhesives like isinglass, gelatin or parchment size through brush application or by the use of an ultrasonic humidifcation appratus. The quality of parchment size varies widely so the use of laboratory grade gelatin is preferable. Subsequent humidification treatment of the parchment is also thought to reestablish adhesive bonds between the media and the parchment.
Surface cleaning of parchment is typically completed using white vinyl erasers (solid and grated) and confined to areas where no media is present.
Mending tears 
Early mending techniques were often as basic as hand-stitching the tear together. Modern techniques for repairing splits and tears use adhesive-coated tissues and animal membranes.
The adhesive chosen for parchment mending is important so as not to further damage the item being repaired. To minimize damage, repairs are performed using adhesives with the least amount of moisture required for the specific task.
Parchment is mostly collagen, and collagen becomes gelatin when boiled in water. A gelatin solution (although pure, without lime) can be used for parchment repair. Gelatin is somewhat hydrophilic and has similar aging characteristics to parchment. The solution used is typically 12% powdered gelatin added to 88% water and warmed to 80 degrees Celsius while stirring continuously. Anthony Cains recommends for every 100 mL of solution, adding three drops of 2% aqueous sorbitol as a humectant, and two drops of an aqueous solution of 1% acetic acid to aid the effectiveness of the adhesive. Earlier techniques used honey as a humectant.
Before use, the solution is heated to 100 degrees C for 5–10 minutes which to give it a higher degree of tackiness. Prior to application to the parchment, the solution must be cooled but still liquid. The adhesive can turn the parchment transparent if too warm, so the adhesive is applied to the repair material and not directly to the parchment.
When it is acceptable for a minimal amount of water to come in contact with the parchment, it is possible to use an alternative technique that uses Isinglass. Isinglass is a film known for its adhesive strength made by soaking and then cooking a dried Russian sturgeon bladder at low temperature. In the technique, as described by Tatyana Petukhove, a paper conservator at Cornell University Library, the isinglass is reactivated with ethanol and water. Then honey or a few drops of glycerin are added before applying the material to the parchment.
Adhesives used to repair parchment in the United States includes isinglass, laboratory grade gelatin, wheat starch paste, aquazol, and in some instances, PVA. Mixtures of the wheat starch paste and other listed adhesives are also recommended.
Infilling of Losses 
Repairing ragged edged holes or missing areas is more extensive. This process is called infilling and uses a pulp mixture to fill in the missing areas.
In a treatment used in 1985 by Per Laursen, a Dutch conservator, "the parchment ...is laid on a paper suction table and a dry powder, made from unprocessed animal hide, is applied to the area of loss with a spray apparatus. The excess hide powder is brushed away from the surrounding area and the fill is lightly sprayed with ethanol and smoothed in place through a piece of polyethylene. A casein-based adhesive....is applied with a brush to the dry powder fill, which is left to dry for about 10 minutes."
The parchment is then dried between polyester web with blotters under pressure for 12 hours. This particular technique has some issues. One problem is the fill is not always even in thickness and. Another problem is that large areas have trouble adhering and need to be supported on both sides with goldbeater's skin.
An alternative technique was developed in Hungary. It uses a pulp made from untanned animal hide ground up with Japanese paper (for color) and a sulfite-processed paper pulp. Parchment size, made from wine vinegar, hydroxyethyl methylcellulose, ethanol, isopropyl alcohol, and a fungicide is added to the pulp. The parchment is placed on a suction table with a light box underneath and the pulp applied with an eye dropper. The fill is applied to both sides of the parchment if needed.
Another fill method applies the pulp to a piece of silk cloth. The prepared pulp is later lifted off and pressed gently into place on the parchment. This method introduces less moisture to the parchment.
Japanese paper inserts are also used by conservators to fill losses in parchment. Acrylic-toned Japanese papers can be treated with various adhesives such as B-72, Aquazol, or PVA to mimic the translucency of the surrounding parchment.
Problems with past conservation methods 
Parchment is very durable and many pieces have lasted hundreds of years, often with little help. Sometimes the "help" from conservation methods in the past has caused more damage than good.
Despite the damage water can cause to parchment, water has often been used to flatten parchment to make its information more accessible and to facilitate further repairs, such as mending with starch paste.
Starch paste can also cause problems. Starch paste is a low-tack glucose-based carbohydrate. In repairs using starch paste, the paste moistens the parchment, but typically, just the area where the paste is applied becomes moistened. In this case, as the paste dries, there is a humidity imbalance in the parchment that results in a distortion around the paste. This distortion irreversibly affects the skin structure.
Instead of using starch paste, it would be better to use something protein based and hydrophilic. According to Woods, "starch cannot be considered compatible with parchment. My understanding of compatibility is the use of like with like...The principle is that if genuine, damage-free reversibility is not possible, it may be better to use the same material as that of which the conserved item is constituted, since its aging characteristics are known to be sympathetic."
Starch-based adhesives repair parchment are used by many conservators in the United States and Canada.
Parchment preservation ethics 
There are many ethical issues when working with parchment manuscripts. One of these issues is whether or not to add any toning to pulp to color it, or to use colored Japanese papers for mending. The concern is that the repair should be easily recognizable as a repair and not try to look like the original parchment. Some conservators feel though that some color is desirable, as it reduces the brightness of the new materials.
- Woods Chris, The Conservation of Parchment, chapter 20, pp. 200–224 in Conservation of Leather & Related Materials, Kite, M. and Thomson, R. (eds.), Butterworth-Heinmann, Oxford 2006 ISBN 113641522X.
- Woods, Chris (1995). "Conservation Treatments for Parchment Documents". Journal of the Society of Archivists 16 (2): 221–239. doi:10.1080/00379819509511780.
- Cains, A. (1983). "Repair Treatments for Vellum Manuscripts". The Paper Conservator 7: 16–17.
- Abt, Jeffrey; Fusco, Margaret. (1989) A Byzantine Scholar's Letter on the Preparation of Manuscript Vellum, Journal of the American Institute for Conservation, Vol. 28, No. 2, pp. 61–66.
- Quandt, Abigail B. (1996) Recent Developments in the Conservation of Parchment Manuscripts, The American Institute for Conservation Book and Parer Group Annual, http://aic.stanford.edu/sg/bpg/annual/v15/bp15-14.html
- Wachter, Otto. (1962) The Restoration of the "Vienna Dioscorides", Studies in Conservation, Vol. 7, No. 1, pp. 22–26.
- Hansen, Eric F., Lee, Steve N., Sobel, Harry. (1992) The Effects of Relative Humidity on Some Physical Properties of Modern Vellum: Implications for the Optimum Relative Humidity for the Display and Storage of Parchment. Journal of the American Institute for Conservation, Vol. 31, No. 3, pp. 325–342.
- Reed, Ronald. (1975) The Nature and Making of Parchment. United Kingdom, Leeds; Lemete Press.