Conservation of leather
This chapter provides a detailed survey of the recommended procedures and method of formula preparation for leather conservation. In addition to the dust and dirt that affects all bookbindings, the damage categories discussed here concern physico-mechanical and chemical damage that is to be found in most of the common kinds of leather bookbindings.
Also dealt with here are a few special damage categories pertaining to certain kinds of leather (alum-tawed, and leather with pigmented finishes), to overfatted leather and parchment, and to biological damage, together with the appropriate treatment procedure and the method of formula preparation.
Cleaning
When an emulsion or dressing is used, the solvent in these preparations may allow dust and dirt to penetrate
even further into the leather or parchment. It is therefore advised to remove any loose dust and dirt from the
bindings prior to treatment with an emulsion or dressing.
Removal of loose dust and dirt
This is best done with a soft brush and in a fixed sequence, for example: head, fore edge, tail, covers, spine. A
vacuum cleaner may be used to remove loose dust by suction, but a soft brush attachment should be used to
avoid (further) damaging the binding. It is also advisable to give the book a final wipe over with tissue paper.
This soft quality paper will mop up any remaining loose dust and dirt, thereby revealing any ingrained
impurities.
Removal of fixed dirt with a surface cleaning agent
A surface cleaning agent can be used to remove fixed dirt. The agent should be whipped to a foam before
using. During treatment the dirt is absorbed by the foam and afterwards foam and dirt can be removed together.
Furthermore, because only the foam is used, the leather suffers minimal humidification. Depending on the
surface structure of the bookbinding and the extent of the damage, the leather surface is cleaned with cotton
buds or a clean flannel cloth.
The main ingredient of the cleaning agent is a neutral (non-ionic) soap made up of molecules with a strong
apolar and a strong polar part.This serves to bind the oily dirt particles, which can then be absorbed by the
agent. The dirt binds with the carboxymethyl cellulose (CMC), which prevents the dirt settling back onto the
book.
Due to the presence of water in the agent this treatment is not suitable for leather with chemical degradation. The
surface cleaning agent works only on the surface and leaves no residue whatsoever after it has been
applied.
Formula surface cleaning agent
996 gram distilled water
2 gram neutral (non-ionic) soap
2 gram carboxymethyl cellulose (CMC), medium viscosity
Mix soap and CMC. Add mixture to water. Shake well. Transfer a small quantity to a low tray. Whip the mixture till
a layer of foam is formed on the surface. Use only this foam.
Deacidification
Deacidifying leather with ammonia is a very effective method for neutralising free sulphuric acid (pH value
below 3.0). During the process ammonium sulphate is formed. The excess of ammonia, which can raise the pH
of the leather considerably, evaporates, leaving the pH once again within the safe pH range: 3 - 5.
This method cannot be used on those types of dye and pigment that are permanently discoloured by ammonia. As it is usually difficult to determine sensitivity to ammonia, one is advised against deacidification of books with prints and maps coloured by hand. In such cases treatment of the bindings can be restricted to buffering.
Brass components, such as clasps and bosses, are corroded by ammonia and may discolour, with the 'yellow' colour becoming darker. Although it is possible to remove this discoloration by polishing, it will also remove the patina of centuries. The discoloration of these kinds of metal components must therefore be weighed against the conservation of the leather.
Deacidification procedure
Deacidification with ammonia is a simple procedure. The bindings should be placed in as small a space as possible. In practice a case that can be tightly sealed is used. A tray or saucer containing a layer of ammonia (15%) is placed on the bottom of the case. The books are then set on a rack halfway up the case. The case should be so constructed that there is no danger of the books coming into contact with ammonia, for instance as a result of the case being knocked over (drawing 8). Because ammonia irritates skin and bronchial tubes, gloves and protective measures for eyes, nose and mouth are necessary. After a quarter of an hour the books can be taken out of the case and put in a fume cupboard, to allow the remaining ammonia to evaporate. After two weeks the books are ready to undergo further treatment.
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| Drawing 8 Example of a deacidification case |
Additional remarks
- In principle, deacidification of the leather can also be accomplished by means of the buffer solution described in the next section. For the blue and red damage categories this would mean two subsequent applications of the buffer solution: with the first application the effective compound (imidazole) will function as a deacidification agent neutralising the free acid present in the leather, while the actual buffering effect of imidazole will be reached only with the second application. A single application of the buffer solution with a double imidazole concentration must be advised against, because such a solution would be too alkaline and might therefore lead to damage of the leather.
- Regarding the recommended pH criteria, the treatment procedures for the different damage categories of
leather bookbindings described in chapter 3 are based on the most frequent situation, in which only pH values
of the leather smaller than 3.0 necessitate deacidification, because under these circumstances free acid will be
present in the leather. However, at higher pH values - and especially in the range between 3.0 to 4.0 - a (small)
risk of free acid still being present should be taken into account. The fact that the pH value of leather containing
free acid can still be higher than 3.0, may be caused by the presence of ammonium sulphate, formed during the
leather degradation process; as this salt will cause a pH of c. 5.5 when combined with water, the overall pH
value of the leather may rise and thus, as it were, camouflage the presence of free acid. Ammonium sulphate
concentrations in leather of 0.5 % and higher may already lead to this effect. The presence of free acid above
pH 3.0 and the subsequent need for deacidification treatment can be conclusively indicated by the
determination of the differential number (cf. chapter 7); at a differential number >
0.6 free acid is present in the leather, at values smaller than 0.6 it is not.
Buffering
A stable pH value is of the greatest importance for the conservation of leather. This stability can be achieved
with a buffer (solution).
If the pH value of buffered leather is disturbed, whether from inside or outside, the buffer will ensure that the pH
value remains within acceptable limits.
Since leather (especially chemically damaged leather) should not come into direct contact with water, the buffer
must be introduced by means of an organic solvent. We have opted for imidazole, a nitrogen containing ring
compound soluble in isopropyl alcohol.
Imidazole is a colourless substance that is applied to the leather in soluble form; deposition takes place after
the solvent has evaporated. In addition to its buffering properties, imidazole is also hygroscopic and thus acts
as a moisture retentive. Moreover, with metals such as iron and copper, imidazole forms insoluble salts, which
prevent these metals - if they are present in leather - from acting as catalysts to decay.
How often buffering should be carried out is a matter for further research.
Caution! In general, a buffer may only be applied if the pH value is no higher than 4.0. If used on leather with a pH
value higher than 4, the pH value may increase to 6 - 7. Such leather becomes unstable and discolorations may
occur.
Formula buffer solution
860 gram odourless kerosene
120 gram isopropyl alcohol
20 gram imidazole
As imidazole is not directly soluble in odourless kerosene, it must first be dissolved in isopropyl alcohol that has
been heated to about 30 °C. The imidazole is not fully dissolved until the solution is transparent; after that
the odourless kerosene is added. For product information on odourless kerosene see: Treatment with emulsions or dressings
Caution! Because of the risk of fire an electric hotplate should be used rather than a gas flame. It is
recommended that the buffer solution be stored in brown glass bottles. The solution should be applied to the
leather bookbinding with a soft brush. Brushing the leather surface twice will generally be sufficient.
The editors are aware of the drawbacks associated with the isopropyl alcohol solvent. It is a polar solvent and
may dissolve coloured or non-coloured components of the leather and transport them to the surface. It
evaporates too rapidly to achieve optimum distribution of the imidazole in the leather and is a moisture
expellent. We are constantly trying to adapt and improve formulas with a view to finding a suitable substitute for
isopropyl alcohol.
Conditioning
Conditioning is applied to obtain an optimum moisture content in the leather with a view to making the leather
more supple and elastic. Moreover, humidification renders the internal structure of the leather more accessible
to the penetration of fat, which is not possible if the leather is too dry. For the same reason, one is strongly
advised against treating leather in a dry environment (low RH). In theory, leather contains three balanced forms
of moisture (natural water balance): free water present in the capillaries between the fibre bundles, bound water,
combined to the proteins of the leather fibres as hydrates, and 'associated' water attached to the fibres through
weak physical forces and hydrogen bonds. Bound water is especially important for a good condition of the
leather and the optimal binding of fat (through the apolar hydrophilic part of the fat molecule) to the leather fibres.
And it is actually this form of moisture that is brought to a maximum level by means of conditioning.
To allow the moisture to penetrate evenly into the internal fibres of the skin, the bookbindings should be placed
for at least four days in a room with a relative humidity (RH) of 70 - 75 %, at room temperature. A simple
humidifier may be sufficient to raise the humidity in a (smallish) room. If such a room is not available, a sturdy tent
can be erected over the humidifier. It is not advisable to blow steam or spray water into the room if only tap water
is available, because the contaminants in this water, such as salts, will be deposited on the books with all the
detrimental consequences that entails.
Regular circulation of the air is necessary to prevent the growth of fungi. If these are already visible on the books
earmarked for conditioning, they should, of course, be removed first. After the fourth day of the humidifying
process the leather will be in optimum condition to ensure adequate fat deposition. The bindings are treated in
the same conditioned room with an emulsion or dressing. Four hours after treatment with a dressing or emulsion
the relative humidity and temperature may be slowly reduced to normal values of about 50% RH and 18
°C.
Treatment with emulsions or dressings
The previous guidelines (Goddijn, 1987) contained a formula for calculating the quantity of leather dressing to
be applied so as to ensure a correct quantity of fat. Practical experience in recent years has shown that this
calculated quantity is nearly always equivalent to twice brushing the emulsion or dressing over the leather
surface with a flat, pig's bristle brush. The percentage of fat in the emulsion or dressing to be used is determined
by the degree of physico-mechanical or chemical damage:
5% for slight damage;
10% for severe damage.
The basic ingredients of the emulsions and dressings are neatsfoot oil, lanolin and odourless kerosene.
Neatsfoot oil is a lubricator which stands in a long tradition of use by leather manufacturers. Lanolin (wool fat)
has both lubricating and water-retention characteristics. Odourless kerosene, a refined petrol product with a
boiling range of 150 - 210 °C, 'free' of aromates, has proved to be an excellent means of transporting the
fats in the emulsions and dressings into the leather: the kerosene evaporates completely after the fat has been
introduced, but evaporation is so slow that the fats are able to penetrate deeply and to attach themselves
securely to the fibres instead of being transported to the surface again.
Caution! Emulsions and dressings contain odourless kerosene, which is a health hazard. A gas mask
must therefore be worn when working with emulsions and dressings. After treatment the books should be
allowed to 'evaporate' for about a week in a fume cupboard or in a conditioning room equipped with exhaust
facilities.
Emulsions
Emulsions are only suitable for the conservation of leather bookbindings with physico-mechanical damage and
a pH value above 3. The emulsion consists of neatsfoot oil (lubricator), lanolin (lubricator and moisture
retentive), neutral soap (emulsifier), distilled water and odourless kerosene (carriers). The advantage of using
emulsions is that the water (in the emulsion) makes the leather swell, so that the space between the fibres and in
the fibres themselves are accessible to the lubricants.
For good results the emulsion should be applied at a relative humidity of at least 70%. It is therefore important to
begin by conditioning the book for four days. After treatment the books should be allowed to 'evaporate' for
about a week in a fume cupboard or in a conditioning room with exhaust facilities.
Formula for 10% fat emulsion
550 gram odourless kerosene
300 gram distilled water
60 gram neatsfoot oil
50 gram neutral (non-ionic) soap
40 gram lanolin (anhydrous)
Formula for 5% fat emulsion
625 gram odourless kerosene
300 gram distilled water
30 gram neatsfoot oil
25 gram neutral (non-ionic) soap
20 gram lanolin (anhydrous)
Start by mixing the neatsfoot oil, lanolin, neutral soap, and distilled water. The best results are achieved by
mixing the ingredients over warm water (au bain marie). Then add the kerosene.
Caution! Because of the risk of fire, an electric hotplate or heating over warm water (au bain marie)
should be used rather than a gas flame. Store in a glass jar or bottle that can be tightly closed. Always work with
limited quantities in a separate tray and never pour used emulsion back into the bottle. This will keep the fluids
pure. If stored in a refrigerator, fat emulsions can be kept for one year.
Leather dressings
The leather dressing is based on an organic solvent and is intended for chemically damaged leather.
Application of the leather dressing brings the fat content of the leather up to standard. The lanolin in the dressing
also helps to restore the moisture-absorbent quality of degraded leather.
The leather dressing must be applied in a conditio-ned room (after the leather has been conditioned for four
days at 70% RH).
After treatment the books should be allowed to 'evaporate' for about a week in a fume cupboard or a
conditioning room with exhaust facilities.
Formula for 10% fat leather dressing
900 gram odourless kerosene
60 gram neatsfoot oil
40 gram lanolin (anhydrous)
Formula for 5% fat leather dressing
950 gram odourless kerosene
30 gram neatsfoot oil
20 gram lanolin (anhydrous)
Caution! The lanolin is dissolved by heating it in a portion of the odourless kerose-ne. Because of risk
of fire, an electric hotplate or heating over warm water (au bain marie) should be used rather than a gas flame.
The neatsfoot oil can then be added to the warm solution with the rest of the kerosene without any problem. The
leather dressing will keep 'indefinitely'.
Lanolin dressing
The only movable parts of the bookbinding are the hinging points (joints) and, to a lesser extent, the spine. This
means that the leather in these places requires extra care. Deposition of fat into the fibres is certainly necessary.
This holds equally for the spine which is especially susceptible to degradation. The joints and spine of the
bookbinding can therefore be treated with a special dressing containing a high amount of lanolin.
This lanolin dressing can also be used in places affected by physical damage: abraded grain, scratches,
etc.
After treatment the books should be allowed to 'evaporate' for about 1 week in a fume cupboard or in a
conditioning room with exhaust facilities.
Formula for lanolin dressing
900 gram odourless kerosene
100 gram lanolin (anhydrous)
Dissolve the lanolin in a portion of the kerosene which has been heated to a maximum of 40 °C. Then add
the rest of the kerosene. Because of the risk of fire an electric hotplate should be used, or heating over warm
water (au bain marie). Apply the dressing to the damaged parts only, using a soft brush. The lanolin dressing
will keep 'indefinitely'.
Impregnation
Impregnation should be seen as a last effort to preserve bookbinding leather. It is irreversible and therefore
only used for bookbinding leather that would otherwise have to be considered as 'lost'. It is used mainly for
leather with chemical damage such as red rot and loosened parts of the grain due to aggressive marbling.
Impreg-nation does not improve or maintain the overall quality of the leather; its sole purpose is to anchor loose
fragments (flakes).
As impregnating agent one may use a partially cured polyurethane (a polymer dissolved in an organic solvent
with reactive isocyanide groups). The efficacy of the polyurethane is determined by the reaction of the polymer
with the functional groups of leather fibres. The fine network of lateral links so produced indirectly reconnects the
leather fibres and in so doing improves the cohesive quality of the leather. Because the impregnating agent
extracts fat from the leather, the fibres must be lubricated before impregnation is started. Failure to lubricate
results in a sharp reduction of tensile strength.
For effective impregnation the reactive isocyanide groups must react with the hydroxyl groups in the leather
fibres. However, if the moisture level in the leather is too low, the free isocyanide groups react with the moisture
in the air instead of with the hydroxyl groups in the leather, resulting in too few bindings with the fibres.
Impregnation should therefore take place after lubrication and in a room where the bookbinding is first
conditioned for about 48 hours at a RH of 70-75 %. It is difficult to say how much impregnating agent is
necessary to build a sufficiently strong network. Practical experience has shown that a good result is obtained
by brushing the leather (locally) twice with the undiluted polyurethane product SU41/H51. If the undiluted fluid
detaches fragments of grain from the fibre network layer, the impregnating agent can be diluted with odourless
kerosene. Apply the agent with a pig's bristle brush and wear a gas mask and protective gloves. Keep the
storage jar closed as much as possible to prevent hardening. Any leftovers should be deposited in the
chemical waste bin. Shiny patches can be wiped off with a dry cloth one day after impregnation.
Caution! Colourless, undyed leather becomes slightly darker as a result of treatment with the
impregnating agent.
Formula for pre-impregnation dressing
880 gram odourless kerosene
80 gram lanolin (anhydrous)
40 gram neatsfoot oil
The dressing should be applied before impregnation.
Treatment of specific kinds of leather
Alum-tawed leather
After tanning, this kind of leather has more fat between the fibres than other tanned leathers. It is therefore
unnecessary, and in most cases even detri1mental, to add to the fat content. Apart from removal of dirt,
bookbindings of alum-tawed leather should not be submitted to any further treatment.
The (dry) treatment of dyed alum-tawed leather consists of the removal of any dust and dirt with a soft brush and
dirt-absorbing paper (tissue paper).
The (dry) treatment of undyed alum-tawed leather consists of the removal of dust and dirt with a soft brush and
dirt-absorbing paper (tissue paper) and the removal of fixed dirt with a cleaning agent developed specifically
for alum-tawed leather.
Formula for alum-tawed leather cleaning agent
950 gram alcohol
50 gram ammonia (15%).
Caution! Health risk: this agent is a powerful irritant of eyes, bronchial tubes and skin. Treatment should
therefore be carried out in a fume cupboard and (neoprene) gloves should be worn. The agent should be
applied with a dry cloth that is changed regularly.
Leather with water-based finishes
Finishes based on binding media such as casein, albumen and gelatin are not very resistant to contact with
ammonia, alkaline solutions, or water with neutral (non-ionic) soap. They have a small wet-rub resistance.
To distinguish leather with a water-based finish, from leather with a solvent- or dispersion-based finish, a drop of
leather dressing is carefully applied to the leather with a pipette. If the dressing (quickly) penetrates into the
leather, it is a water-based finish. This leather can be cleaned without resorting to fluids and treated with the
leather dressing. Leather dressing has to be chosen because acid may be present in the leather and acids can
cause discoloration if an emulsion is used. Deacidification and buffering is not recommended for this type of
leather because it is not very resistant to alkaline substances.
Leather with solvent- and dispersion-based finishes
Because solvent- and dispersion-based finishing coatings are highly dirt-repellant, organic dirt is easily
removed. Leather finished in this way has a high wet-rub resistance. It is difficult for fluids to permeate the leather
because of the closed surface of the finishing coating.
Treatment consists of several stages:
- Removal of loose dust and dirt.
- If necessary, further cleaning with a surface cleaning agent.
- As the plasticiser may have migrated from the layer of paint into the leather, it is useful to treat the layer of paint with sulphonated castor oil. This renders the layer of paint elastic, even after the disappearance of the plasticiser (one often finds cracks in old finishing coating, due to the aforementioned migration of the plasticiser). To keep the layer of paint in good condition, a little castor oil on a damp, white flannel cloth will suffice to treat the whole binding.
- If there is also physico-mechanical damage on the spine and/or joint, local treatment with leather dressing is necessary. This is best done with a soft paint brush. To ensure an effective fat deposition in the leather, the book should be conditioned beforehand.
Treatment of leather and parchment overtreated with fat
If the leather has become too dark in colour and the leather or parchment feels (somewhat) sticky, it has been
overtreated with fat. The surplus fat can be removed by a surface defatting agent made up of refined petrol, a
neutral, non-ionic soap, and distilled water. Refined petrol is highly volatile and very good at dissolving fat. As
with the cleaning agent, soap is added for easy removal of surface dirt. Water is added to dissolve the soap.
Because of the gelatinous nature of the solution and the volatility of the solvent (refined petrol) the defatting
agent works only on the surface. The water does not penetrate into the material and only surface fat is
dissolved. The amount of water and soap is very small, so that the agent does not leave a residue and does
not cause discoloration. It can be used to remove wax, shoe polish, smoke stains, organic dirt, and surplus fat
(from earlier treatments) from the leather surface. The fairly gelatinous fluid is applied to the leather with a flannel
cloth with a circular motion. The agent can be applied to bookbindings with a surface layer consisting of paint,
varnish or some other surface coating. However, some care should be exercised in applying the agent.
Theoretically the organic solvent could dissolve such substances as binding agents and dyes. It is therefore
advisable to test the agent using a cotton bud on a small, inconspicuous area - for instance on the turn-in of the
binding - in order to find out whether there are any undesirable side effects.
If a layer of dirt and/or fat is difficult to remove, a larger quantity of the agent may be applied with a soft brush
and then left for an hour to allow it to be absorbed. The effect the agent is having on the leather or the surface
coating should be checked regularly during this hour. Before it is completely dry, it should be carefully wiped
with a soft, smooth flannel cloth. If necessary this treatment should be repeated several times. After defatting the
book still needs to be conditioned to ensure a better distribution of the remaining fat in the leather.
Formula for surface defatting agent
890 gram refined petrol
100 gram distilled water
10 gram neutral (non-ionic) soap
Mix the substances together in a glass jar that can be tightly closed and shake until the mixture is fairly
gelatinous. The surface defatting agent will keep 'indefinitely'. The refined petrol product used in this formula
has a boiling range of 100 - 140 °C.
Caution! The ingredients used in the formula have been chosen to combine optimum effect of the agent
with minimum detrimental consequences for the user's health. Nevertheless a number of preventive measures
should be taken, especially for long or regular use of organic solvents. Refined petrol is a dangerous
substance, not only because it is inflammable, but also because it represents a health risk when inhaled. It is
therefore necessary to work in a fume cupboard with this solution and to wear protective gloves during work to
avoid defatting the skin. The books that have been treated must be left in the fume cupboard until the refined
petrol has evaporated.
Treatment of biological damage
Fungi
Sound climatological conditions and hygiene are a primary prerequisite for preventing fungal growth. Regular
inspection is also crucial for timely detection of possible problems.
The first step in treating a case of fungal attack is to inspect the location of the affected book. Are other books in
the vicinity (or in the same series) infected? Isolate the objects that are affected, treat them and inspect the room.
If humidity is too high, the cause needs to be traced. If the storage conditions are good and one is faced with
individual cases of fungal attack, it is most likely to be an old infection that is no longer active. Isolate the
infected books by sealing them in plastic bags. If possible remove them to a separate area where they can be
disinfected and cleaned before returning them to the collection.
If practically possible, the least drastic treatment is mechanical removal of the fungi using a fine paint brush, a
brush, and an exhaust system. Care should be taken that the fungal remains being removed are not blown back
into the room. If necessary racks and bookcases can be cleaned with a disinfectant such as a quaternary
ammonium compound. Books that have been cleaned can be put back once they are dry and the room meets
standards for storage conditions. In the case of large-scale infection and 'fungal explosions' it may be
necessary to resort to mass disinfection methods, such as irradiation with gamma rays. But this should only be
done after a careful consideration of the advantages and disadvantages and after seeking advice from experts
in this particular field.
Insects
Good insect control starts with preventive measures: preventing the pests from entering the building (closing
openings, joints, and cracks, placing insect screens in front of windows, checking all incoming objects), sound
hygiene and regular inspection.
The first step in insect control is to check which species is involved. The next step is the same as for fungi:
localise the source of the attack and check whether it is an individual case or whether more books are affected.
Isolate the objects that are affected, treat them and inspect the room.
For the treatment of material affected by insects there are a number of non-toxic methods, of which freezing is the
simplest one for books and archives. But this cannot be done if objects are painted, or consist of several
attached layers, each with a different stretching and shrinking behaviour.
Standard treatment takes 48 hours at a temperature of -20 °C and can be repeated after some time, if necessary. Other non-toxic methods are fumigating with carbon dioxide or nitrogen (carried out commercially in some countries). In principle high temperature treatment is also an option, but little is known as yet about the effects of heat treatment on different materials. Toxic insect control methods have a number of drawbacks: fumigating with methylbromide is not suitable for protein-containing material such as leather and parchment; in principle fumigating with phosphine is possible, but inadvisable if metal parts (bosses, clasps) are present because it is a corrosive gas.