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Acidity is one of the most pernicious agents in regard to paper conservation. Its caustic action breaks down the molecular chain of the cellulose, diminishing its physical properties and even causing it to disintegrate.
By means of the deacidification process, the causes of these alterations (acidity) are removed but the effects produced are not (yellowing and fragility).
Although deacidification is a treatment whose results are not readily apparent, it is of fundamental importance in "curing" a document of one of the most serious causes of alteration.
There is a wide variety of deacidifiers ranging from gaseous products (these are not very effective, or are too complicated and expensive, such as zinc diethyl) to liquids, both aqueous (calcium hydroxide) and non-aqueous (barium hydroxide). The majority of these deacidifers, besides removing acidity, provide the document with an alkaline reserve which strengthens their neutralizing action (Table 2).
Excess alkalinity also causes alterations and the ideal solution is to achieve a neutral or slightly alkaline state depending on the paper (modern paper pH 7, and rag pulp paper pH 8-9). Should deacidification be excessive or produce a whitish film due to decantation of the deacidifier, this problem may be remedied by employing a slightly acid bath (pH 5) or using a few drops of lemon juice or any weak acid.
Bleaching involves removing the colour of a stain or the general yellow appearance of a document but it does not remove the substance which is the root cause; it should not be regarded as an absolute cleaning method but as an aesthetic treatment.
Bleaching is a process aimed exclusively at the external appearance of a work and from the point of view of document conservation it has adverse effects as it leads to serious deterioration of the cellulose.
This degradation is reflected in the loss of consistency of the paper as the majority of bleaching products discolour the stains by oxidation and this action also affects the cellulose, breaking down its molecular chains.
Although the majority of bleaches are highly alkaline (the cause of possible alteration), in the long term they may damage the document due to oxidation and acidity as carboxyl groups are formed in the cellulose. Moreover, in the bleaching bath organic acids are liberated due to the stains and to the decomposition of the bleaching agents themselves. All this speeds up the bleaching process but also produces greater deterioration.
An important problem in the use of chlorine products is that they often continue to work after the process is finished for which reason their action must be stopped with antichlors which also eliminate their harmful residues.
Despite all these risks, bleaching is necessary in the case of those documents whose aesthetic value is of primary importance and which are seriously affected by stains which impair their characteristics. If it is decided to apply this treatment, measures should be taken to attenuate the problems mentioned above.
In the first place, the documents should be deacidified beforehand (preferably using calcium hydroxide) and in this way excessive acidity in the solution will be avoided. Acidity should be checked and alkaline substances incorporated if the pH drops too much.
After bleaching, the document should be rinsed in water and, if a chlorine product has been used, its effects should be neutralized using an antichlor. Generally sodium thiosulphate, sodium metabisulphite or bisulphite are used in an aqueous solution between 2 and 5% but the tendency nowadays is to use a slightly acid aqueous solution (pH 5) with a weak acid (acetic, citric acid...) as it would seem that the residues of the products mentioned above may in time produce stains. After neutralization the document is rinsed once again and the pH is measured so that it may be deacidified again if necessary.
Bleaching is generally done in a bath but it may also be done locally using pastes with colloidal agents applied by means of brushes, blotters, applicators, drops...Some products exist which may be applied in gaseous form (See Table 3).
Light radiation has also been tried but it seems to have harmful effects as it acts by photo-oxidation and presents other disadvantages such as the slowness of the process and its lack of effectiveness on wood pulp paper which it yellows rather than bleaches.
3.4.9 Hygroscopic Stabilization
Hygroscopic stabilization, as a restoration treatment, is a method applied mostly to protein-based materials, but, in exceptional cases, it may be employed as part of the restoration of cellulose-based materials which have become excessively dehydrated and cracked, such as papyrus, amate or certain types of paper (e.g. vegetable paper) which are brittle and creased. In these cases the treatment is carried out using polyethylene glycol as described in the chapter devoted to the restoration of parchment.
TABLE 3 : BLEACHING AGENTS
|Sodium Hypochlorite||L||2-10%||15m max.||O||Very effective, especially recommended for stains left by micro-organisms. N|
|Calcium Hypochlorite||S||0.5%||O||Poorer bleaching power than sodium (yellowish shade).|
|O||For local treatment paste is used. N.|
|Chloramine T and Chloramine B||S||5%||15 m max.||O||Slower than hypochlorites. Residues difficult to eliminate. N.|
|Sodium Chlorite||S||O||Has little degrading effect (does not form oxycellulose). Toxic and explosive (use extractor hood). Basis of following products:|
|Chlorine Dioxide||L||10m max.||O||Sodium Chlorite 2% +25 ml. formaldehyde/1 or sodium chlorite 1% + sulphuric acid pH3.|
|Chlorine Dioxide||G||5m||O||In special chamber. Only bleaches damp paper. Not very stable. Affects organic and metallo-acid inks.|
|Chlorous Acid||L||O||Slow. Sodium Chlorite 5% + acetic acid pH 3.6.|
|Potassium Permanganate||S||0.5-1%||O||Degrading effect. Its purple colour impedes control of degree of bleaching. N. with metabisulphite 5% or oxalic acid acid 3%.|
|Sodium Boron Hydride||S||lg/100g||R||Recommended for lignin papers. Inflammable (extractor hood). Has little degrading effects.|
|Hydrogen Peroxide||L||Variable||O||Degrades cellulose with metal particles. Acidifies. Wash and deacidify after treatment. Not very effective.|
|Sodium Perborate||S||1-2%||15m||O||Not very effective. Better results if dried in sun. Very slow and weak.|
|Sun Bleaching||O||Natural sunlight or UV light. Only for paper with no lignin. Better results if paper immersed in alkaline medium. Prolonged exposure.|
O - oxidant
R - reducing agent
g - grammes
m - minutes
max - maximum
N - neutralize
Hygroscopic stabilization to reduce excess humidity is included in the section on drying-pressing.
As a result of washing or other types of immersion, or because of the natural reduction of the properties of the support, it may be necessary to re-glue or re-size cellulose materials in order to offset the loss of the adhesive which consolidated their structure.
Glueing is done either in baths or by impregnation using a brush; the degree of intensity will depend on the characteristics and condition of the document. The products used for the purpose of consolidation are the same adhesives which are used in restoration as fixatives or to mend tears but in the case of glueing they are considerably diluted.
The most highly recommended are semi-synthetic cellulose derivatives such as methyl cellulose or hydroxypropyl cellulose when aqueous treatment is not possible as they are much more stable than adhesives of natural origin (See Table 4).
3.4.11 Drying and pressing
Any treatment involving moistening produces a series of defects because the fibres increase in volume and this leads to deformation and considerable warping. This is all correct by a process of pressing.
So that the document will recover its former size, rapid-drying techniques and the application of heavy pressure should be avoided at all costs as this will only lead to greater deformation. The elimination of moisture should be done slowly so that the fibres can adjust freely.
The procedure that is most recommended is airing followed by light pressing at room temperature or at a moderate temperature under wooden boards or a light press.
When pressing a document, it must be borne in mind that the texture of the document should not be altered by pressure. The solution is to use a support which will be either wrinkled or smooth according to the texture in question (Reemay, polyester or polyethylene sheets, paraffin paper...). Should there be marks such as the impression of an engraving, these must be protected by overlaying a cardboard emplate or by placing blotting paper on both sides of the document.
Books whose dampness is due to flood damage present serious drying problems. One of the best solutions to prevent the problems of a prolonged waiting period is lyophilization. Should this system not be available then
TABLE 4: CONSOLIDATING AGENTS, ADHESIVES AND FIXATIVES
|Gelatine||Protein-based. 30 g/l water in water bath at 60°C. S in water but can be hardened using formol (insoluble) at 1/16th its weight. May be attacked by micro- organisms. Crystallizes in time.|
|Flour Paste||Cellulose-based. Prepared in hot water. S. in water. May be attacked by micro-organisms. May crystallize in time.|
|Starch||Cellulose-based. S. in water. May be attacked by micro organisms. May crystallize in time.|
|Methyl Cellulose||Semi-synthetic cellulose. S. in water and some chlorinated hydrocarbons and alcohols. Stable against micro-organisms. R. Culminal L52 and Tylose MH 300.|
|Sodium Carboxy-methyl Cellulose||Semi-synthetic cellulose. S. in water and mixtures of water, alcohol or acetone. Stable against micro-organisms. R. Tylose CMC.|
|Hydroxypropyl Cellulose||Semi-synthetic cellulose. S. in water, alcohol or methyl chloride. R.Klucel G.|
|Soluble Nylon||Polyamide. S. in methanol and hot methanol. When dry, forms flexible, breathing film. Activated by heat. R. Calaton.|
|Polyamide||Polyamide sheet. No fibre direction. Melts at 80°C. Reversible in hot alcohol. Permeable. Yellows with age. R. Bifix F95.|
|Paraloid||Acrylate. S. in perchlorethylene, aromatic hydrocarbons (toluene, xylene), acetone. Melts at 80-90°C. R. B72.|
|Plexigum||Similar to Paraloid. Melts at 60-70°C. R. MB 319|
|Primal||Acrylate in aqueous suspension. S. in same media as Paraloid. Melts at 90°C. R. AC 234 and AC 61.|
|Plextol||Similar to Primal. R. D 542.|
|Polyvinyl Acetate||Thermoplastic synthetic polymer, in aqueous emulsion. S. in alcohol. R. Mowolith DMO 25 and A 34 K 3.|
|Polyvinyl Alcohol||Thermoplastic synthetic polymer. S. in water.|
|Cellulose Acetate||Cellulose ester in sheets. S. in acetone. Melts at 150°C. In ageing tests releases acetic acid.|
|Polyethylene (stabilized)||Laminar appearance. Reversible in perchlorethylene, xylene or toluene at 70-80°C on electric plate without flame. Melts at 100°C. Waterproofs documents against external agents|
R. - Recommended
S. - Soluble in
N.B. Most of these adhesives can act as consolidating agents when considerably diluted and as fixatives in a much thicker state. The proportion will depend on the application and the type of support being dealt with. the document may be frozen in an ordinary refrigerator. It must be pointed out that leather bindings do not stand up well to either treatment.
A simple solution to prevent attack by micro-organisms is to air the open books at a controlled temperature (not more than 25°C); when they are practically dry, they should be lightly pressed.
Drying may be speeded up using hydroscopic products which absorb damp (silica gel, sepiolite...) or by dampening the documents with a volatile substance which will hasten evaporation (alcohol bath).
One system which gives good results with materials which are difficult to treat, cannot be dampened or are resistent to pressing, is the suction table. The document is made to adhere to a smooth porous surface thanks to the air-absorbing action of the table and, with the aid of volatile solvents, vapour or simply an air current, it may be dried and smoothed out.
3.4.12 Repairing cuts and tears and restoration of the support
Repairing cuts and tears involves restoring the damaged parts to their former place in a cohesive and resistant fashion. Tears are stuck down using an adhesive such as methyl cellulose with a little polyvinyl acetate (See Table 4). For this purpose the edging is utilized and if there is none the damaged area is reinforced using transparent tissue paper or thicker paper depending on the document.
A perfect fit is achieved using a light-table (a glass table lit on the underside). The transparency of the table makes it possible to see if the edges of the cut overlap or are too wide apart.
In the case of a tear in a thick support, reinforcement using tissue paper is usually not enough as the crack tends to open. In this case the best course of action is to cut both edges down until, when placed together, they form a V-shape and a wedge reinforcement may be inserted.
To restore the support (filling in gaps or lost areas) manual or mechanized procedures may be used.
The manual systems involve inserting a piece of material similar to that of the document. The insert can be made to fit by using edging obtained with the aid of the scalpel, perforation with a pin, tearing from a flaw or by directly sticking a fragment of paper along the edges of the gap, later eliminating the excess material. Small holes may be filled in with paper pulp.
The mechanized systems are based on the manufacturing process used for hand-made paper. In general terms it involves placing the document on a grid which acts in the same way as the old page shapers; water containing fibre - dispersed in a quantity equivalent to the volume of the material to be replaced - is then passed over it.
When the water is forced through, the fibres are retained by a filter placed between the document and the grid, thus stopping up the holes through which the water passes (precisely those areas where there are gaps) .
This system may also be used to join gaps as fibres will always remain between both borders but, should there be considerable edging, it will have to be stuck beforehand or else it will remain loose. When there is no edging, the borders are slightly defibred to help the fibres stick more easily.
Once the fibres have been deposited on the areas where holes are present, sufficient pressure is applied to flatten them. Once dry, they will adhere to the paper through hydrogen bonds (chemical bonding), or mechanical adhesion if a glueing substance (synthetic resin, gelatine, methyl cellulose) was included in the dispersed fibres.
Adhesion by glueing may affect the functioning of the machinery and involves, furthermore, the addition of a material which is extraneous to the document) chemical bonding by hydrogen bridges is usually sufficient and it is normally quite unnecessary to add glueing substances to the pulp.
To achieve a good chemical bond the only requirement is that the fibres should be sufficiently refined and shredded (2530º SR and 2-3 mm.) and the press used, preferably a hydraulic press, should be capable of applying pressure (5 Kg./cm2) sufficient for the hydrogen bonds to be formed.
There are variations in the mechanical systems depending on the mechanism which passes the water through the document. The simplest is the free-fall system, the water descending under its own weight. This method may give rise to suction which, because of the slowness of the process, may leave fibres decanted on the document covering areas which are not to be repaired.
This problem may be avoided using an apparatus which produces suction by means of a pump (Vinyector system, Spanish) or employing the Venturi absorption system (Recurator, Israeli).
With the Vinyector system the result of the suction produced by the pump is that the fibres are decanted only in those places where there are holes or gaps. Moreover, the water may be re-used.
The Vinyector system may also be used for different types of restoration treatment. Once the document is placed on the grid, the whole restoration procedure may be carried out (disinfection, cleaning, deacidification, bleaching, neutralization...) simply by adding different recipients containing the products required for each phase.
The fibres used in mechanical restoration should be similar to those of the original document although often, for ease of operation or due to proved effectiveness, a standard mixture of wood pulp fibres in sulphate, bleached or raw (25 9.), cotton (3 9.) and linen (2 9.) may be used. The use of a particular amount of sulphate, bleached or raw, or better still, of cotton dyed to a darkish shade, gives the bone shade typical of paper (generally 5 9. of raw or darkish cotton and 20 9. of bleached are mixed). Linen provides firmness and cotton sponginess. Patches for supports which are a different colour from this bone shade are obtained by dyeing cotton fibres with the colour in a water suspension. For this purpose red (magenta), blue (cyan) and yellow dyes are used which, in conjuction with white, make combinations which will produce the desired colour (quadricolour).
Lamination is the process whereby one or both sides of the document are reinforced with a material which wild give it the resistance necessary for improved handling and conservation.
This covering must be as thin and transparent as possible, except in the case of those works which have no writing on the reverse side and can be strengthened on that side with the use of more consistent opaque reinforcing material.
Lamination is a curative method which should not be applied indiscriminately as it always modifies the surface and increases the thickness of the document. It may detract from the clarity of the writing and aggravate alterations of an internal nature if they are not eradicated. Lamination is only to be recommended when the support is so weakened or fragile that its reduced consistency does not ensure safe conservation. Lamination consequently becomes necessary and even inevitable for documents affected by acidity, oxidation of the inks or disintegration caused by insect pests.
It is a serious error to laminate before remedying or eradicating all the factors or elements which affect or might affect the support to be restored. If this is not done, the effect of these aggressive agents may be aggravated and cause serious damage to the work; in addition there is the need to delaminate which puts the integrity of the document at considerable risk.
Lamination may be done manually or mechanically. The classic manual method consists of the following operations:
1. Once the document has been cleaned, smoothed, fixed and neutralized etc. it is placed face downwards on a flexible, impermeable support such as Teflon or polyethylene. It is moistened using a spray to mould it perfectly to the surface; in this way subsequent dilation which might produce deformations or wrinkles is avoided.
2. The same operation is carried out with the paper, crepaline, cloth or whatever is chosen as the reinforcing material.
3. Once the document and reinforcing material are laid perfectly flat on their support surfaces and excess moisture has been removed using a blotter, the chosen adhesive is applied with a soft brush, starting from the centre and working outwards to prevent folds and wrinkles.
4. The support on which the reinforcing material is placed is turned over and laid on top of the document in such a way that, should the fibres run in any marked direction, the fibres of the one will cries-cross with the other, thus preventing tension and cockling.
The manner in which the reinforcing material is placed on the document is very important if lamination is to be carried out correctly. The document must remain spread out and perfectly still on its own surface on the work table. The following methods may be used:
(a) One edge of the reinforcing material should be aligned
with the corresponding edge of the document; the rest of the
reinforcing material is slowly lowered onto the document until a
complete join is achieved.
(b) The reinforcing material may be held in a V over the centre of the document, contact being made first of all at the centre. The edges are then placed together and thus the document is covered completely.
5. Having covered the document perfectly, slight pressure is applied with a roller, starting at the centre and working outwards to eliminate excess adhesive and remove bubbles and blisters which might have formed during the operation.
6. On completion of this initial pressing operation, it is wise to remove the reinforcing material from its support (which will now be on top). A few radial cuts in the protruding edge of the reinforcing material prevent deformation during the drying process. If no remains of adhesive are apparent, a piece of blotting paper may be placed on top to speed up the final drying-out process. If there should be adhesive, it is preferable to use a non-stick, non-woven (Reemay type) support which 'breathes' so as to facilitate drying.
7. The final pressing and drying process is done by placing the laminated document between blotters which are changed from time to time, either in a press or under wooden boards.
A rapid solution which is only possible when the reinforcing material is not very thick (tissue, crepoline) is to place it directly on top of the document and apply the adhesive which should be fluid enough to pass through the reinforcing material and secure it to the document.
Another possibility is to use paper reinforcements prepared with a covering of thermoplastic adhesive or adhesive which reacts with solvent. The most commonly used adhesives are Primal, Paraloid or similar products which may be softened by exposure to a heated plate and/or application of the corresponding solvent with a brush or cotton swab once the reinforcing material has been placed on top of the document. In both cases, sufficient pressure to join the two materials being used in the lamination process is necessary.
Mechanized lamination systems are based on the application of a certain degree of heat capable of melting a thermoplastic adhesive (e.g. polyethylene) thus joining the reinforcing material to the document. Various types of apparatus are available on the market; in general terms, they consist of two thermostatically-controlled electric plates between which the document to be laminated is placed, two rollers acting as extracting and pressing elements.
When a document is to be laminated on both sides, a board (preferably of asbestos fibre on account of its resistance) is placed on the side-table of the laminating machine. This acts as a general support for the whole operation. A Teflon sheet which acts as an insulator is placed on top and then the tissue or other reinforcing material is placed over this followed by the thermoplastic adhesive film and, finally, the document itself which is covered with another sheet of adhesive, tissue, Teflon and cardboard.
This "sandwich" is placed between the hotplates where it should remain long enough for the adhesive to melt (usually between 25 and 30 seconds). The rollers begin to function immediately and extract the document at the same time as they exert pressure on it, thus obtaining perfect union of the document and the reinforcing material.
Should only one side have to be laminated, no adhesive or reinforcing material should be placed on the side which is not to be treated.
When the document has holes or gaps, 'grafts' may be placed on these areas in the following way: the restorer should proceed as though only the right side were to be laminated, placing the pieces of paper chosen as inserts on the reverse side so that all the holes or gaps are covered. Then the Teflon and board should be put in place and lamination should proceed. The initial result will be that each paper insert will now be stuck to the document in precisely those areas to be covered due to the adhesive penetrating through the holes. The excess paper which is not stuck down should be trimmed with a scalpel. Once this operation has been carried out the reverse side may be laminated and the document will now be laminated on both sides with the patches filling in the gaps.
In any case, the laminated documents should have a narrow edging of from 2 to 5 mm. all around to prevent exfoliation of the document; at the same time, it will be better protected against possible contamination by micro-organisms which might affect it by penetrating the cut edge of the page.
A simplified variant of heat and pressure lamination is provided by thermostatically-controlled presses where several documents may be placed, in their respective "sandwiches", one on top of the other inside the press. The advantage of being able to process various documents at the same time may be offset by the irregular fusion of the adhesive due to the differences in temperature and pressure according to the position of the documents within the press.
In contrast to these more traditional mechanical laminating-machines, there is another type which has a conveyor belt on which the "sandwich" described above is placed but without the outer board layers. As it passes through, hot air produces a rise in temperature: finally rubber rollers carry out the pressing operation.
In certain cases when the possibility of aqueous treatment exists and the documents, especially prints and drawings, have to be patched and consolidated, it may be possible to achieve lamination-type reinforcing using the mechanical process for paper restoration (described in 3.4.12).
All that is required is to release more pulp than is necessary to plug the gaps. In this case the excess pulp will cover the reverse side of the document in its entirety, the thickness depending on the proportion of pulp used. The document must be placed reverse-side upwards and there should be no writing on this side as the finish will be opaque.
Finally, it is worth mentioning the suction table as an appropriate tool for carrying out manual lamination, especially in the case of very deteriorated or fragmented documents. These can be smoothed and fixed to the table through its suction action while the adhesive and reinforcing materials are being applied.
This system has also proved useful for laminating processes using thermoplastic adhesives which are melted locally with hot air applied by means of thermostatocally-controlled spatulas.
An alternative to lamination is sealing in a "sleeve"; this is a system for protecting highly fragile documents and is the best way of protecting them during transport and exhibitions. It is also a method of conserving flat works permanently (prints, drawings, etc...) The document is placed between two transparent sheets which, once sealed on all four sides, act as a 'sleeve' which affords the document greater consistency and, moreover, isolates it from external aggression. The document remains unattached inside the 'sleeve' , the interior dimensions of which should correspond to those of the document to prevent it from moving and becoming damaged.
The simplest way of carrying out this operation is as follows: a sheet of the chosen material is spread out, preferably polyethylene terephthalate (Mylar, Melinex) or some similar product, the thickness of which should be in accordance with the dimension of the document to be sealed. The work is placed on top of the sheet and surrounded with double-sided adhesive tape, leaving a space of around three to five millimetres. Another sheet is placed on top so as to stick to the adhesive tape. A roller is passed over the 'sleeve' to extract the air trapped inside: a small space is left in one corner (a tiny hole is sufficient) for this purpose.
Finally the outer edge is cut or guillotined.
Besides this manual procedure, there are special worktables in which a thermofusion or ultrasonic system is installed. This melts the materials, and ensures a hermetic seal with a better finish.
Obviously, as with the lamination procedure, before processing any document it must be free from any internal factors that might cause deterioration.
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