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4.3 Restoration techniques: materials and procedures
Restoration procedures for parchment are basically the same as for any other archive material but, given the difference in structure of parchment (a protein-based material), some procedures and products are not advisable or, conversely, different processes may be required which are hardly ever used in the case of cellulose materials.
Having said this, it should be pointed out that in this section common treatments which are mentioned in the chapter on paper documents are not described again; the starting point is the insect removal-disinfection stage, the analysis and photography procedures referred to previously being omitted.
4.3.1 Elimination of insects and disinfection
The main consideration to be borne in mind in the treatment of parchment is that, given the characteristics of this material, methyl bromide must never be used as a gaseous treatment as it has been shown that it hardens and denaturizes the skins.
Formaldehyde is not advisable either as it dehydrates the parchments which then harden.
Apart from these two products, any of those mentioned for paper restoration are suitable, used always in the concentrations described earlier. The most commonly used preventive treatment against possible attack by micro-organisms on damp parchment is to place the documents in polyethylene bags with pentachlorophenol and borax (in this way both the development of chemical as well as biological problems is eliminated; it should be remembered that borax is a deacidifier with fungicidal properties).
Another mainly preventive treatement is to keep the documents in contact with paper impregnated with Timol although, in the authors' view, this product is not highly effective.
The best treatment is fumigation, in an autoclave, with ethyl oxide mixed from Freon or carbon dioxide for a period of 2, 4 or 6 hours depending on whether the purpose is to remove insects, disinfect or sterilize. It should be remembered that this procedure is only curative and must be combined subsequently with preventive measures which will stop any further contamination.
4.3.2 Fixing of inks
As in all restoration processes, before any treatment is carried out tests must obviously be done to see if writing will be affected by the method to be used; should this be the case the inks must be protected.
The same methods may be used for fixing inks as those described for paper restoration although the following are the most suitable products for use with parchment:
Soluble nylon, prepared with alcohol, usually gives good results although it has the disadvantage, due to its reversibility, of being incompatible with alcohol (a medium frequently used with parchment). Its advantage compared to other fixatives, is that it forms a protective film which allows the liquid products to penetrate instead of waterproofing the treated area completely.
Cellulose acetate (prepared with, and soluble in acetone) is one of the products most commonly used as an ink fixative. With parchment, it gives very good results and has the advantage that, if applied thickly and once treatment has been carried out, it can be lifted off as a fine film with the use of a solvent. The formation of this film seems to depend not only on the thickness of the acetate but also on the kind of ink. If, when the restorer attempts to pull away the acetate film, there appears to be a danger of removing the pigment layer with it, the fixative should be removed using acetone.
Primal, which is soluble in water but once hardened is reversible in organic solvents - xylene and toluene - is also recommended for treating parchment.
At the present time, the most commonly used product is Paraloid, prepared with and reversible in perchlorethylene, xylene, toluene and especially in saltpetre. This fixative is also available commercially as a spray.
Fixatives are usually applied by means of a fine brush which will cover only the line of writing but, in the case of large areas, the use of a spray is recommended provided a template is used to protect the rest of the document and only the area with writing is impregnated with fixative. Should brush application of a product which is sold commercially as a spray be necessary, a small amount should be sprayed into a recipient. Brush application is safer than spray application.
4.3.3 Mechanical cleaning
Parchment, as a material generally more resistant than paper and with more deeply ingrained dirt, allos of and required direct application of slightlty abrasive methods, the mildew cleaning processes used for paper being discarded.
Instruments generally used for cleaning are erasers of varying degrees of hardness as well as the electric lathe and the glass fibre brush, especially indicated for corners dirtied by the turning over of pages.
When the hyaline layer is cleaned, special care must be taken not to damage or alter this fine surface.
4.3.4 Cleaning using solvents
It has been shown that solvents are not so effective on this material as on a cellulose document, although the use of dichlorethylene, perchlorethylene and chloroform is recommended to eliminate grease stains.
Oxide or iron stains may be treated with a 5% solution of oxalic acid in tepid water although this treatment must be used with caution and neutralization with a deacidifier must be carried out subsequently.
4.3.5 Aqueous cleaning
This is necessary in most cases to facilitate the stretching of the parchment rather than to remove dirt; a water bath with a neutral detergent may be used although this procedure is not very common because the results differ little from those achieved using a water bath alone. There is a risk, furthermore, that, in the long term, some of the residues may be harmful.
The most frequently used procedure is a bath using water and alcohol in variable proportions; the alcohol helps to open the pores more quickly, facilitates the penetration of the liquid and also more rapid drying out as it speeds up evaporation.
Mixtures of water, alcohol, acetone and ammonia (the latter has slight bleaching properties) may also be used as cleaning agents. Proportions vary depending on whether the inks are soluble in the different media. All things being equal, it is preferable to use alcohol and avoid the use of acetone which tends to harden the parchment slightly.
The most typical bath involves soaking the parchment for an hour in a solution of water, alcohol and glycerine in the proportions of 15,70 and 15% respectively as the glycerine helps to make the parchment flexible.
4.3.6 Bleaching
As we have already said, parchment does not react well to chemical treatment and even less to chlorine products which, in addition to their lack of real effectiveness, are very harmful, softening and deforming the parchment; even so, mention is made in the bibliography of the use of a 5% solution of chloramine T. rapid washing in water and subsequent neutralization. In our view, the risk never justifies the results, which are poor.
A bleaching product which involves less risk, although the results are by no means brilliant either, is hydrogen peroxide diluted or undiluted.
The use of hypochlorites is out of the question due to the degree of deterioration which they may cause in the structure of the parchment (gelatinization).
4.3.7 Deacidification
Due to the manufacturing process, (pseudo-tanning using lime), it is fairly uncommon to find parchments with such a high acidity level that they require neutralization; however, it has already been seen that this problem may arise due to a poor manufacturing process and, above all, to environmental pollution.
In general, the products used to deacidify paper are also suitable for parchment, although those that are most recommended are calcium hydroxide (as it does not involve the addition of any substances extraneous to the parchment), prepared in water in saturation and barium hydroxide, soluble in methanol, a medium to which parchments respond will (15 9. per litre of methanol in a 20-minute bath.
To ensure greater speed and effectiveness rather than the application of these products in isolation, it is recommended that deacidification be carried out at the same time as aqueous cleaning. To do this, it is sufficient to replace the water or the alcohol in the water, alcohol and glycerine bath mentioned above by calcium or barium hydroxide respectively.
The proportions are:
15% calcium hydroxide, 70% ethanol and 15% glycerine or 15% water,70% barium hydroxide and 15% glycerine.
With either formula two problems are clearly solved at the same time: cleaning and deacidification.
4.3.8 Hygroscopic stabilization
Hygroscopic or hydroscopic stabilization is the treatment par excellence for parchment for this part of the restoration process, apart from rare exceptions, applies only to this type of support.
The term refers to control of either too much or too little humidity. As parchment is a highly hydroscopic material, the degree of humidity to which it has been exposed throughout its existence directly affects its state of conservation, and hydroscopic stabilization is the only way of restoring lost flexibility to parchment.
When reference is made to hydroscopic stabilization, this is generally taken to mean the restoration treatment which restitutes lost humidity, but it is a term which may also be used in regard to treatment for excess humidity. In the latter case the parchment tends to gelatinize and is prone to the action of micro-organisms; this makes dessication treatment necessary.
Apart from simple exposure to air, which is too slow a process for a material such as parchment, the most advisable dessication treatments are as follows:
1. Placing of the document in an ethanol bath. As the alcohol evaporates, it acts as a dehydrating agent drying out the document and at the same time inhibiting possible alteration due to biological factors.
2. Keeping the document in contact with a material which will control the humidity and which is at the same time harmless: calcium carbonate sepiolite, or silica gel are very suitable. These products are sprinkled over the parchment to absorb the humidity and changed regularly until the level of humidity is correct. The most suitable of the products mentioned is silica gel because its colour indicates the degree of dryness: it is blue when it is dry and gradually changes to red as it becomes damp. These products can be re-used once the humidity absorbed has been eliminated by exposure to a heat source.
Between these two systems, which are not unduly costly, humidification-dehumidification chambers may be used. This process, which is much more costly, provides the ideal treatment, because the humidity in the parchment is reduced by controlled dehydration achieved by the gradual evaporation of water due to the progressive increase in temperature.
The results are highly satisfactory because abrupt changes are avoided and there is complete control over the degree of humidity desired.
The ideal state of humidity for parchment depends directly on the environment in which it is to be housed (preferably 5060% RH).
Treatments to replace lost humidity in parchment are more usual than dessication. With the loss of humidity parchments lose flexibility and, furthermore, become wrinkled.
There are two long-standing tendencies in the solution of the thorny problem of dehydration. In the one case, an attempt is made to remedy the damage by restoring humidity to the parchment while in the other lubricants (greases) are applied as to tanned hides.
In the first case, the treatment involves keeping the parchment in a humid environment, which is done using a humidification chamber where the parchment is subjected to a relative humidity of 80% at around 15ºC until the correct degree of humidity has been restored. This piece of apparatus which is rather costly, may be replaced by a type of fune cupboard where a high degree of humidity is achieved through the evaporation of water, or simply by using humidifiers.
But keeping the parchment in a humid environment until it is completely stabilized involves the risk of microbiological activity (which may be remedied by use of fungicides), the inks may dissolve or damp and dust stains may appear. It is, moreover, a treatment of limited value in the long term for, once the parchment is returned to its original environment, the problem will recur even although the humidity level is suitable.
Treatment using greasy materials is even less suitable than the previous solution since, as has been seen in the chapter on the causes of alteration, dried-out parchment does not require grease but humidity. Oily substances provide a certain degree of flexibility, do not give rise to problems of attack by micro-organisms or dissolve the inks, but they may make the parchment transparent due to excess grease and greasy to the touch - which leads to increased dust retention; they may produce changes in the colour of the skin and finally oxidation of the oils. What is more, the final results are not satisfactory as parchment scarcely absorbs any grease; the flexibility attained is accordingly limited.
"Oils" are usually applied by rubbing in, generally after the parchment has been soaked in a 50% ethanol and water bath for 24 hours.
The products used have been lanolines, waxes, paraffins, glycerines and oils of different kinds such as castor oil, oxfoot oil (besides giving the document a typically oily appearance, they also leave shiny and transparent areas), and cedar oil. The latter is applied preferably by impregnating the parchment with a cotton swab and then placing it between two plates of glass degreased with talcum powder.
Theoretically, the ideal treatment for a dehydrated parchment would be a combination of both the methods described above ; to improve the degree of humidity of the skin and then to waterproof the surface by covering it with some substance which will inhibit evaporation. However, in practice, a satisfactory symbiosis has not been achieved as the semi-tanned skin lacks the powers of self-regeneration of "living skin" and environmental agents are able to break down the artificial defence.
Other substances which, throughout history, have also been used to restore flexibility to parchment are vitamins, albumen, sperm oil, emulsions and urea. Sperm oil and urea emulsions have sometimes been used in the same treatment.
The results of these processes are not highly successful. In the case of urea, it is known that it produces transparencies while sperm oil emulsions leave white marks which can be removed with benzol. In any case, they have been superseded by the treatment described below:
This involves treating the parchment with polyethylene glycol, a product which has overcome the problem of hydroscopic stabilization very satisfactorily.
Polyethylene glycol is a glycol derivative (polyvalent alcohol) which is obtained industrially by condensing different polymers of ethyl oxide.
It is available commercially in a wide range of molecular weights, from the solid to the very fluid state. That used in hydroscopic stabilization treatments is polyethylene glycol between 200 and 400, which is viscous in appearance but quite fluid.
The following properties make it ideal for conservation purposes: its pH is practically neutral, it is not volatile and has an acceptable degree of penetrability, it does not provoke microbiological action, it has softening and lubricating properties and, above all, it regulates water as its hygrometric capacity enables it to act as a sponge and, depending on the environmental humidity, it absorbs or releases water while maintaining the stability of its internal constants.
Its components (carbon, hydrogen and oxygen) are completely compatible with parchment, as they do not add any extraneous substance.
It may be said that polyethylene glycol acts as a substitute for water, re-establishing the lost hydrogen bridges thanks to its own hydroxyl groups, besides having the advantage of providing a permanent treatment due to the non-volatile nature of the product.
So far no negative effects from the use of polyethylene glycol have been observed, even in artificial ageing tests.
The only precaution to be borne in mind is the solubility of the inks; in some cases, they must be protected using fixatives.
Treatment using polyethylene glycol simply involves impregnating the parchment with the product; the treatment may be carried out in a bath, by rubbing in or by application with a brush depending on the methods that the state of conservation of the parchment will allow.
In the case of brush application, once the parchment has been impregnated, it should be kept between two sheets of polyethylene or two glass plates until the product has been absorbed (although the latter method is risky as there is a possibility of the parchment adhering to the glass). When rubbing in is used, this is done with the fingertips, using a circular motion. In the bath technique, the product is used in its pure form until it has been absorbed; saturation point is clearly seen because the parchment takes on a transparent appearance which disappears totally when the document is dried.
In the case of treatment by the bath method, tests were initially with a 50% solution of polyethylene glycol in water or aclohol in a bain-marie in thermostatically-controlled recipients at 30ºC. This process was designed to aid penetrability as, when parchments are very thick, dried out and satiny in appearance, treatment may take weeks.
Nowadays, this problem has been solved by inmersing the document in a water, alcohol and glycerine bath (15%, 70%, 15%) for one hour beforehand. This same bath which is used to clean the parchment may also be used for the polyethylene glycol hydroscopic stabilization treatment: the aim is to prepare the pores to absorb the polyethylene glycol. The alcohol opens the pores making the passage of water easier. the water in turn allows the glycerine to penetrate while the glycerine performs the same function for the polyethylene glycol.
This initial bath is not used exclusively for thick parchments and may be employed for any type of parchment to shorten the treatment.
In short, hydroscopic stabilization using polyethylene glycol involves an initial bath in water, alcohol and glycerine for one hour, followed by impregnation with polyethylene glycol (preferably in a bath) to saturation point.
The treatment is completed with a drying-pressing process. In this operation the transparency produced by saturation disappears and is finally eliminated by pressing between blotters.
Obviously this treatment not only restores flexibility to the parchment but also eliminates wrinkles and acts as a preserving agent against possible deterioration due to changes in humidity in the surrounding environment.
4.3.9 Drying and pressing
The most traditional smoothing system is the use of stretchers which pull out the parchment through the action of clamps attached to the edges. This method produces distortion, the edges being slightly irregular because of greater stretching where they are in contact with the clamps. To minimize this effect the clamps must be placed very close together and they should be lined with a soft material, such as felt, to avoid scratches and tears. The advantage of this system, which is still used today, is that the parchment may be moistened locally. This is useful where links are highly soluble and liable to deteriorate.
In this case, water or polyethylene glycol is rubbed into the areas to be moistened and then the parchment is gradually stretched.
Another classic solution is simply pressing the document between blotters after dampening. To avoid the action of micro-organisms, the blotters should be changed frequently, especially if polyethylene glycol has not been used in the treatment. This system is only effective with parchments which are not too wrinkled as deep wrinkles may become folds.
An alternative solution is to place the moistened parchment on a glass plate from which all traces of grease have been removed, using talc, and cover it with a blotter. Stretching is done by suspending weights or lead rods from the edges of the blotter so that the whole is fixed but contraction of the parchment is not impeded during the drying process; otherwise obviously it would tear. This danger is minimized by avoiding excessive weight and too rapid drying; it is sometimes advisable to stretch the document by alternating successive moistening and stretching stages.
One disadvantage of the systems in which blotters are used to press parchments is that they may sometimes absorb the inks which have been softened and weakened after the stabilization process.
The ideal treatment for smoothing out parchments, especially when they are excessively wrinkled, is the following:
Two plates of glass or smooth wooden boards are prepared and degreased using talc.
One surface is sprayed with water.
A sheet of polyethylene is placed upon it and rolled out so that it is completely flat once stuck down due to the moisture.
The parchment - previously treated with polyethylene glycol, the mixture of water, alcohol and glycerine or other products depending on the circumstances - is placed on the polyethylene.
The damp parchment is covered completely with another sheet of polyethylene.
This sheet is made to adhere by using the roller which eliminates air bubbles and flattens the parchment. With the pressure of the rollers, the parchment should lie completely flat and smooth between the polyethylene sheets. To make pressing and the elimination of air bubbles easier the roller should start from the centre and work outwards. In this way any excess product will be removed at the sides.
A second support, preferably glass, is placed over the "sandwich" support, polyethylene, parchment and polyethylene. The weight of the glass prevents wrinkles from forming and, as it is transparent, the state of the inks and the parchment in general can be monitored at all times.
After approximately 24 hours, the glass is lifted off, the upper layer of polyethylene is replaced by a blotter, and the glass or wooden board is replaced.
After a further 24 hours the other polyethylene sheet should be replaced by a blotter; the document, between the two blotters, is placed in a press until it is completely dry. Until the humidity has been completely absorbed these blotters should be replaced at regular intervals in order to avoid fungi and alteration of the graphic elements.
The times given for the drying process are merely a rough guide and will depend on the characteristics and thickness of the parchment.
4.3.10 Repair of the support, cuts and tears
The oldest method of repairing tears in parchment was by stitching, preferably using herring-bone stitch and twine, gut or in more modern times, nylon. However, this method is clearly not suitable as it involves perforating the original support and, despite the remarkable nature of some of the sewing, it is always an anti-aesthetic solution.
Another traditional method of joining pieces and repairing cuts and tears was by impregnating the edges to be joined with acetic acid. This acted as an adhesive by gelatinizing the parchment which stuck when dry. This method is not highly recommended either because the join is not very effective and inevitable deterioration is caused to the document.
The ideal method of joining parchment is with the use of adhesives but not all the adhesives used to repair cellulose are suitable for parchment.
Of the adhesives of natural origin, animal glues were originally used to join parchments. However, their use is not recommended as with time they darken, crystallize and lose their adhesive properties. The same may be said of cellulose adhesives (fluor paste) with the added disadvantage that they provide scarcely any adhesive power.
Semi-synthetic cellulose adhesives do not present the same problems as animal glues, but their poor adhesive quality rules out their use.
Synthetic adhesives applied at high temperatures must be ruled out completely as parchment cannot withstand temperatures above 40ºC.
Although applied with the use of solvents, other types of synthetic adhesive such as Primal, Paraloid, cellulose acetate etc. are not recommended for restoring parchments as their adhesive power is not sufficient.
Contact glues have sufficient adhesive properties but they are not acceptable as they dirty the surface.
The ideal adhesive is polyvinyl acetate, although results depend on the brand and the state of purity. Nevertheless, it is this type of adhesive to which preference should be given in the restoration of parchment as its effectiveness has been fully proved. It should be remembered that these polyvinyl glues are reversible only in ethanol and although in certain cases they are not totally reversible, reversibility is improved by adding a small quantity of semi-synthetic cellulose adhesive, preferably methyl cellulose.
As opposed to paper, the adhesive must never be dried with the aid of thermostatically-controlled spatulas; it is better not to use manual spatulas to any great extent either and to leave the document to dry with only small weights laid on it.
When there is an edging wide enough to permit the tear to be repaired by adhesion alone, the edges should simply be scraped lightly to smooth the inside surface. In the case of cuts or very narrow edging, reinforcing material will have to be used.
The reinforcement traditionally used to join cracks was animal gut (ox or pig) which was conserved in alcohol. It was placed over the crack like tissue. An improvement on this method is to use vellum or the hyaline layer itself.
The hyaline layer of a parchment is the somewhat grainy, harder part which is sometimes slightly yellow. To separate it from the fleshy layer it is sufficient to tear off a piece of new parchment and, very carefully, using tweezers, to pull off strips as long and as wide as possible (the tear should be similar to the one made in paper when a very wide flap is to be left).
This hyaline layer is extremely fine and transparent and, when set in place by an expert hand, is virtually invisible to the naked eye. It is laid on the right side of the parchment (over the hyaline layer of the original) and if it is necessary to reinforce the reverse side, tissue, crepoline or, better still, scrapings from the fleshy layer may be used as reinforcing material.
Cuts in thick parchment where there is no flap need to be reinforced. Reinforcing may be done, as in the case of cardboard, using a wedge-shaped insert, provided that any writing is confined to one side of the parchment.
There are two formulas for patching or '"grafting"; the classic bevelled patch using a scalpel, or the "cap-type" insert which is typical of parchment repair.
The disadvantage of bevelled patches is that, as it is necessary to bevel not only the patch but also the original support the document suffers deterioration; accordingly, this method is not recommended when there is writing on both sides.
With the "cap-type" patch the original does not have to be touched as all the operations are carried out on the insert itself.
With the fleshy layer uppermost the original is placed on top of the patch and the perimeter of the hole is marked; this mark is best done with a fine sharp object (scalpel point) thus avoiding the dirt of a pencil.
The patch is cut out leaving a concentric border of about 3 to 5 mm. around the outside of the mark.
The fleshy layer of the border is removed completely, leaving the hyaline layer with minimum thickness and maximum transparency. The area between the flap and the patch itself should not be bevelled but should be as close as possible to a right angle.
The insert is fitted into the original so that the fine hyaline border overlaps. Areas with transparencies indicate that the overlapping exceeds the limits of the insert. In this case it is better to repeat the operation, although the problem may be remedied by taking some scrapings from the fleshy layer which adhere to the hole once the piece has been stuck down.
Once a perfect fit has been achieved, the pieces may now be joined by applying a small quantity of polyvinyl acetate to the edge of the hyaline layer. When it has dried slightly the two parts are joined together, a weight is put in place, and the necessary waiting time must be observed.
If the insert should overlay the original, the edge of the original should never be pared down.
Inserting other materials (such as paper) in parchment is quite out of the question because materials whose composition is different have different reactions and deformation would soon become apparent.
One type of insert still being perfected is the kind which imitates, in a certain measure, mechanical restoration (not yet applicable to protein materials).
The aim of this process is to fill in small holes using powder obtained by grinding down small pieces of parchment. This is recommended in the case of parchment attacked by pests. The method is as follows:
the holes are covered by laying tissue on the fleshy layer;
the small holes are impregnated with adhesive;
the holes are sprinkled with parchment powder;
they are then covered on the hyaline layer with ox-gut.
The adhesive used in this process is gelatine (20 9.) with water (500 cc.) ethanol (500 cc.) and acetic acid (250 cc.).
A variation which, in the authors' view, improves the treatment, is to use polyvinyl acetate as an adhesive and to eliminate the outer layers added to the insert. In this way, the ideal adhesive is employed and tension between different media is avoided.
4.3.11 Dyeing inserts
Frequently the pieces to be restored have acquired, with time or due to the tanning process itself, a characteristic yellowish hue. Pieces of modern parchment used to make inserts do not normally have this colour and are whiter in shade.
Inserting a piece of a different colour may create an anti-aesthetic effect, the importance of which will depend on the type of document.
A colour similar to that of the original parchment can be achieved by dyeing the inserts, although it must be borne in mind that restoration criteria make it essential to have a clearly distinguishable difference in hue so that no suspicion of falsification may arise.
There are several methods of dyeing.
The oldest method and the one preferred by book-binders who wanted to disguise patches on old bindings uses potassium permanganate in a variable solution depending on the intensity desired. Potassium permanganate is dissolved in water and spread over the parchment with a cotton swab. If a deeper shade is required a second coat may be applied or the proportion of the product increased. If a lighter shade is required, the dye may be "rubbed out" using the corresponding neutralizer: sodium metabisulphite in the appropriate solution.
The problem with this method is that permanganate, besides being a dye, is an oxidizing agent and in the long run may harm the document. If it is neutralized the colour disappears. It is, then, not recommended for dyeing parchment used in restoration.
The method most commonly used today is simply to apply oils with a cotton swab and, to improve the adherence of the colour, a mixture of oil with a siccative is recommended.
From the point of view of conservation, acrylic colours have the advantage over oils, which are liable to oxidation, although with inserts this problem is minimal. Acrylics are naturally stable colours and with polyvinyl acetate as a binding agent they adhere easily to the parchment. The main disadvantage is their rapid drying property - when a certain surface area is involved, it is difficult to control the homogeneity of the stain.
Another method - more cumbersome and less effective - is to dye the parchment using infusions (tea). Watercolour pencils may also be used, diluted in xylene. The insert is coloured according to the intensity desired, the colour then being shaded off using a cotton swab impregnated in xylene. As this is a slowly evaporating solvent, it enables the desired nuance to be obtained.
To repair writing on parchment see 2.3.
4.3.12 Lamination
Due to the thickness and resistance of parchment, examples which are so fragile and deteriorated as to require lamination are rarely found.
If such should be the case, it must be remembered that mechanical lamination by applying heat should never be used on protein-based materials.
The most efficient laminating method for parchment is manual lamination with crepoline adhered with polyvinyl acetate. Crepoline is a very fine synthetic material (like tulle) which is totally transparent and sufficiently consistent.
The adhesive is applied to either the parchment or the cloth or to both materials. The parchment is impregnated with adhesive and the crepoline is placed on top of the parchment and is impregnated until the adhesive penetrates through to the parchment (the latter method is more advantageous).
Other types of adhesive are not recommended since, for, as pointed out in the section on cuts and tears, they are weak, crystallize or leave stains. Neither is it advisable to use tissue, paper or cellulose materials - normally very weak - as reinforcing materials. Cloth, for its part, is not transparent enough and natural silk, which was used quite regularly in the past, is also ruled out on account of its short life.