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6. Seals

6.1 Definition and composition
6.2 Deterioration factors and preventive measures
6.3 Restoration techniques: materials and procedures


6.1 Definition and composition

Seals are elements which are attached to a document as a mark of its authenticity or as the personal identification of the author; they may be considered as a substitute for the signature. There are some seals which, besides this function, serve to close documents.

The origin of the use of seals is very remote, and in the Bible itself there are references to their use, both as a sign of the validity of a document and as a means of keeping its contents secret.

The oldest seals are stamp seals. This is when a matrix is stamped directly on a soft substance on which the impression or cast is left.

These seals are usually placed on the reverse side of the document and may be applied, directly or indirectly.

Directly applied seals are seals where a matrix is stamped directly on a soft substance producing an impression. These were already in use in the Roman Empire and their use extended throughout Western Europe into the Middle Ages.

Indirectly applied seals are made when the soft substance lies between the document and a piece of paper on which the impression is left. This type of seal came into use later.

The earliest stamp seals were simply signet rings used as a matrix. As time passed they increased in size and the matrices were no longer rings or pendants but objects quite separate from the person whom they represented.

This increase in size, together with the fact that frequently one document required several seals, led to the development of the appended seal which ousted the stamp seal in the course of the twelfth century.

Appended seals consist of a piece of wax or metal with an impression on one or both sides; they are fastened to the document by means of a cord or ribbon and their function is to authenticate the text.

Appended metal seals appear to be Byzantine in origin and they were used by the Pontifical Chancellery from the sixth century at least. Their appearance is attributed to the need to use a substance less fragile than wax and the impossibility of attaching this material (metal) directly to the document.

This type of seal, especially those made of precious metals (gold and silver), were used for documents of great importance or belonging to eminent persons or institutions. Towards the end of the eleventh century they began to be made of more consistent waxes until they replaced the stamp seal.

In the thirteenth century the use of seals spread to social classes of a lower rank; this meant that smaller seals with an impression on only one side became common. At this time too the typical round seal gave way to other shapes such as trefoil, star-shaped, polygonal, double ogive, and so on, while the natural wax colour was replaced by others (red, green, yellow, vermilion) due to the introduction of metallic oxides.

When parchment went out of favour as a result of the spread of paper, the stamp seal was reintroduced as the new writing surface, which was less consistent than parchment, and could not withstand the weight of appended seals.

So, during the first half of the fourteenth century, these seals reappeared and appended seals were reserved only for documents of great importance. At the end of the fourteenth and during the fifteenth century, the use of seals began to decline as they were replaced by the individual signature. Their use became further reduced until today they have practically disappeared.

It should be borne in mind that seals could be made of either metal or wax. The most commonly used metal was lead, followed by copper and its derivatives (bronze and brass). As has already been mentioned, gold and silver were used for seals of major importance although examples in precious metals are somewhat rare.

Wax seals could be made of natural beeswax alone or mixed with other ingredients which hardened it (resins, chalk, sealing-wax, etc.). Wax hardened with clay and caranday is known as sealing-wax, a harder but more brittle material which was used frequently for stamp seals.

Today, the type of seal most commonly used, as it is harmless to the document, is a dry seal which is simply a mark embossed on the support.

6.2 Deterioration factors and preventive measures

Wax and its derivatives are more stable than the cellulose and protein-based materials discussed earlier.

Climatic factors are of minimal importance except high temperatures which may soften and melt the materials (wax begins to soften at 40C; sealing-wax is more resistant).

Attack by biological factors is also less common, although it is possible as some insects may consume the wax, especially if it is not mixed with other elements. Sealing-wax is not prone to this kind of alteration.

The main cause of deterioration is physical-mechanical, i.e. due to use or the conditions in the place where it is stored, either the document or the seal becoming damaged (breakage, scratching...).

This problem is remedied by correct storage of the document, the seals being protected with a material which will guard them against knocks (polyethylene with air bubbles) or being kept in boxes or containers made of cardboard or other material so that the document is prevented from moving about and sustaining damage. Documents with appended seals should be stored in such a way that the seal fits into a specially designed cavity.

Reproductions, made with the use of practically unbreakable material such as synthetic resins reinforced with glass fibre and latex or silicone moulds, provide a remedy against the risks of indiscriminate use.

Metal seals present quite different problems as their deterioration is usually chemically produced. The main problem is oxidation and corrosion of the metal due to the seal being in contact with the humidity and oxygen in the atmosphere. The danger is increased if sulphurous gases and carbon dioxide are present.

In the specific case of lead it is normal to find it covered with an oxide film which may protect it against chemical degradation. However, when this is formed in an impure atmosphere where there is excess humidity, it may produce basic lead carbonate. This is milky in appearance (whitish grey) and will corrode the piece, turning it into a porous material which, as it increases in volume, will blur the details and outlines of the design.

The formation of metallic incrustations is also common when the seal comes into contact with saline compounds. These are generally harmless but they detract from the external appearance of the seal.

To prevent or attenuate all these factors, the seal may be covered with a protective material (microcrystalline wax) or the environment may be controlled by isolating the seal in a glass case with inert gases (an excessively costly method) or the air may be filtered, thus preventing atmospheric pollution.

In general, lead is very sensitive to organic acids and is also affected by the tannins present in poorly dressed wood or oakwood. For this reason, seals of this nature should not be kept in wooden containers unless covered with a coat of varnish.

6.3 Restoration techniques: materials and procedures

6.3.1 Wax seals

Following identification, analysis, diagnosis and the photographing of the piece, the next step is to clean it.

Cleaning wax seals is usually done mechanically, with the use of brushes and also vacuuming and blowing methods. In extreme cases the scalpel may be used to eliminate elements which have become incrusted. Aqueous cleaning may also be carried out, preferably locally, applying a mixture of water and some detergent substance (Teepol, Lissapol) with a cotton wool swab.

There are several methods of consolidating the pieces (joining fragments and repairing gaps).

Any method of joining fragments by melting the edges must be rejected as it involves the degradation of the original and possible disfigurement of the design near the crack.

Another method is to apply hot wax either alone or mixed with substances such as colophony which act as a joining element, consolidating and covering the small gaps in the crack. But this system may make it difficult to distinguish the original areas from those added later and, moreover, slight halos may form in the area where the join has taken place due to the colour of the seal being absorbed by the other products that have been added.

Both problems may be remedied if, instead of using beeswax or any other natural wax, microcrystalline waxes (semi-synthetic substances derived from petroleum) are used. They may be distinguished from natural wax by their composition and they do not cause the "halo" effect. Their microcrystalline structure also gives them a high degree of plasticity. They are fairly stable products and are not liable to attack by insects.

Microcrystalline wax is applied by placing a small amount of melted wax on the edge of the cracks and pressing both pieces together until a perfect join is achieved.

In the case of thick or heavy pieces, this method is not effective as it does not provide the necessary resistance. To avoid this problem, V-shaped metallic pieces may be inserted which will hold the fragments together. This is done by making perforations using a hot needle and inserting the pieces which have been heated beforehand.

The disadvantage of this method is that, if the seal is knocked it tends to break along the joint with consequent exfoliation. This type of accident is quite common and has disastrous effects as the seal suffers serious damage.

The alternative to this procedure is to use a thermostatically-controlled stylus, one millimetre in diameter and twenty millimetres long, at 100-150C. With this instrument slightly bevelled grooves are made where the hot microcrystalline wax is deposited.

When fragments are missing, inserts must be made to consolidate the piece. In the past these inserts were made using coloured gypsum on which the design was subsequently engraved. Today microcrystalline wax is preferred and the design is not reproduced for ethical reasons. To reconstruct the lost areas the first step is to obtain a wafer of wax of the same diameter as the seal but slightly thinner. This is made by pouring molten wax on to a base on which a strip of card forming the outline of the seal has been stuck. Another, rather more complicated method is to prepare a shape using plasticine or other malleable material over which silicone is poured. A mould is thus obtained which can then be filled with wax.

Once the wax wafer has been made, the imprint of the original is obtained by placing the original seal on the wax mould. This area can be separated from the rest using the thermostatically-controlled stylus or a jigsaw.

The join between the insert and the original seal is ensured using the thermostatically-controlled stylus which is introduced obliquely, being applied on both sides, from the insert to the original. The inserts are placed at a middle height so that, when the piece is seen in section, a slight difference in level is visible both on the right side and the reverse.

This difference in thickness is proof of the fact that the repaired part is not original. The insert is finished once the joins have been gone over with a burin, and the surfaces have been evened off mechanically (fine emery paper, glass fibre...) or with the use of organic solvents (naphtha, xylene...) which smooth the microcrystalline wax (soluble in these substances).

In no event should the design be reconstituted, even when there is a reliable model available for the aim is simply to consolidate the seal. The aesthetic effect may be enhanced by dyeing the microcrystalline wax to tone with the whole.

The most suitable method for dyeing the wax is to add oil when the wax is melted, bearing in mind that the aim is to make it tone in and not to match the original exactly. To test the shade, a drop of wax is allowed to cool as the colour changes depending on the state of fusion.

Oils, due to their stability and the fact that they are not harmful, are the most suitable dyes. Results using other materials such as natural or aniline pigments are less satisfactory, the former because they tend to decant and the latter because they fade in sunlight. The ribbon

Both wax and metal seals are appended to the document by means of a ribbon or cord. Obviously, if the ribbon is cut, the document is deprived of its seal and therefore loses proof of its authenticity.

If the document or the seal requires immersion treatment which would harm the ribbon or cause it to stain the document, it should be covered with a waterproof material, such as natural or microcrystalline wax which may be removed by heat or solvents (xylene, naphtha, carbon tetrachloride...) once the treatment is finished.

When the ribbon is fragile or in poor condition it may be reinforced using nylon thread which is practically imperceptible and very resistant.

6.3.2 Directly applied seals

These are normally sealing-wax seals. The treatment for this material is similar to wax but cracks may be joined using adhesives, preferably Primal mixed with hydroxypropylcellulose or epoxy or cyanocrilate resins.

If there are pieces missing, the normal procedure is not to carry out repairs; as the seal is fixed directly to the paper, the fact that a piece is missing does not endanger its resistance. Should inserts be made, the same indications should be followed as for wax seals.

6.3.3 Indirectly applied seals and dry seals

Indirectly applied seals have two component parts: sealing-wax and paper. When both parts are loose they may be joined using any of the adhesives used to restore paper (e.g. polyvinyl acetate).

Should the sealing-wax be broken, it should be stuck together using the products mentioned in the case of directly applied seals, care being taken not to stain the paper. If the sealing-wax within the seal has turned to powder or is excessively broken up, the best solution is to remove this decomposed matter and stick the seal to the paper once again. In this case the restorer should respect the stain made by the previous substance (sealing-wax or wax) which should be kept as proof of its existence.

In the case of both indirectly applied seals and dry seals, in the event of gaps and dirt, the paper should be treated in the same way as any document made of the same material (mechanical, aqueous cleaning, patches...). The only difference is that care must be taken when the document is pressed for, if the seal is not protected in advance, its design would be lost.

With seals of this type, as well as with directly applied seals, pressing should be avoided unless it is absolutely essential. In this event the seal should be protected with a cotton or foam cushion and a hole the same size as the seal should be made in the blotter or support which is used for pressing so that the seal will not be crushed. If the volume of the seal is considerable, several layers of blotting-paper or a thicker material (cardboard) should be used until the height of the seal is reached and its protection completely ensured.

6.3.4 Metal seals

It has already been seen that the main problem with metal seals is oxidation. There are several classic ways of tackling this problem. However, none of them is suitable in the restoration of seals appended to a document as they require the object to be immersed in a medium which would damage the ribbon.

In the case of metal seals not appended to documents, restoration methods are used which are similar to those used for any metal object (electrochemical, eletrolytic and acid treatments...).

In the electrochemical method, the seal is placed in an iron container and covered with zinc. It is then boiled in 1020% sodium hydroxide. This treatment can also be done locally despite the ribbon by covering the seal with zinc and applying 90% sulphuric acid. The electrochemical method destroys the patina on the seal and eliminates the deteriorated parts instead of turning them into harmless material; for this reason it is not recommended for seals which may lose the sharpness of the design.

The electrolytic method involves immersing the seal in 5% sodium hydroxide, a connecting wire acting as a cathode. The anode consists of iron plates, also placed inside the container, connected to the electric current. As the current passes through, electrolytic reduction takes place converting part of the deteriorated material into "healthy metal" once again.

The seal must be washed in distilled water after being treated with either electrochemical or electrolytic methods, and finally consolidated.

Another method, which is less highly recommended, is the use of acids, such as formic acid for silver seals, citric acid for copper seals, etc.

To treat lead, which is the most commonly found metal, there are more specific methods, such as immersion in nitric acid and subsequent neutralization using alcohol (although with this treatment the lead takes on a milky appearance), the Caley method, and ion-exchanging resins.

The Caley method consists of immersing the seal in a bath of hydrochloric acid in the proportion of 100 ml. of acid to one litre of distilled water. The seal is drained and rinsed in distilled water and put into another bath of hot ammonia acetate (100 9. per litre of distilled water) for one or two hours. It is washed in distilled water, drained and washed again. Then it is dried by airing or immersion in alcohol and consolidated by being plunged into paraffin at 100C.

Ion-exchanging resins are used by covering the seals with granules of resin in a container with distilled water; generally the resins have to be changed several times until the incrustations disappear. This method was developed by the British Museum and represents an improvement over other methods because it does not involve the use of chemical products or washing.

To treat lead seals attached to documents, the Servicio de Libros y Documentos (Spain) uses an effective electrolytic method.

This involves using a platinum needle as the anode and sulphuric acid as the electrolyte. The seal acts as the cathode and is connected to the electric current by means of clamps. Electrolytic reduction takes place by placing a drop of sulphuric acid on the seal and applying the platinum needle which is connected to the current. In this way, the parts that have suffered deterioration are transformed once again into metallic lead and the seal recovers its former appearance.

To prevent deterioration due to the sulphuric acid, the seal is rinsed several times in distilled water until the absence of acid can be verified. Finally it is brushed to eliminate residues and is then consolidated.

Metallic material is consolidated by immersion in hot microcrystalline wax, synthetic resins or adhesives, preferably thermoplastic. Ideally the seal should be completely impregnated by using a vacuum. The apparatus required for this operation is extremely costly but the Venturi system may be used instead. The force of the jet of water produces absorption of the air in a hermetically sealed container and thus facilitates impregnation with the consolidating material without the formation of bubbles.

Consolidation by immersion is not suitable in the case of seals attached to the document which must be impregnated using a brush. A 10% solution of Paraloid in nitro or perchlorethylene is suitable for this as it coats the seal with a transparent non-shiny film.

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