Seals

Since the Middle Ages, seals have been appearing in many different forms. They are made of diverse materials with variable properties. Hung on, impressed on or pushed through letters and documents, as a closure and credential, they show a variety of motives: buildings, persons with or without an attribute like crown or scepter, a sign which was executed by hand or a rider on a jumping horse, for example. [1]
The damages that occur on seals are varied. First of all, the main types of seals will be named. The following is a description of the problems involved in the difficult task of preserving this cultural asset.

Seal types

The wax seal is probably the most well-known and commonly encountered seal. Also metal seals were quite common. Furthermore, paper, wafer and varnish seals are among
the seal types.

Signet

A signet is made of metal and means a seal containing a knob to use by hand, which allows the pressing (sealing) of a seal in various materials (wax, varnish, etc.). The older type of this stamp is the signet ring. [2/3]

Wax seals

Pure beeswax is too soft for processing into a seal, so additives have been appended to increase its consistency, for example fats, resins, chalk, flour and linseed oil. By kneading the wax, a vessel was modeled, which was filled with a layer o fine wax. This could be formed under pressure with the signet. [1]
The different colors of seal wax indicate the seal type. Black wax is still used today for funeral letters. Green was reserved for monasteries. White was an indication of Free Imperial Cities and red wax was used by emperors and kings only. [2]

Metal seals

These seals could be made from lead, silver or gold, depending on social ranks and wealth. Lead was most commonly used to seal. Kings, princes, emperors and sometimes popes used gold instead. Silver was used very rarely. Metal seals are also called bullas. They are made by pressing and engraving two metal plates, the sealing cord as a bond between seal and document. [4]

Paper seals

The imprint of the seal is visible in a paper or parchment that covers the underlying substance of varnish, wax or wafer. This substance gives a plastic shape to the seal. The covering paper mostly looks rhombic or stellar. It is fixed on the ground while pressing the heated signet on it. [1]

Wafer seals

Very thin round pieces of wheat flour, containing binders and colorants, are called wafers. The staining could be done with cinnabar, indigo or lampblack. The covering was a little paper which should probably prevent sticking together when folding the document. Wafer seals have been proven to exist since the 16th century. [1]

Varnish seals

This material is more heat-resistant, compared to wax. Since the 16th century, it is in use to seal. [2]
Varnish was mainly used to seal letters or in the shape of small drops to fix the corners of a covering paper. The original ingredients are shellac and rosin mixed with turpentine or turpentine oil. Varnish could also be colored. [1]

Damage to seals

The restoration of seals places high demands on experts because of the different materials using while production. For this reason restorers should definitely be consulted before measures for conservation and restoration are made directly on objects!

Voids and cracks

Probably the most common damage to seals is caused by their natural aging process. These are typically flaws and cracks or dissolution from base. The latter can be accelerated by a degradation of the carrier material. Wafer, varnish or wax seals mostly show these types of damage. Regarding to wax seals various recipes for production of wax using for restoration are recommended by the archives directorate of Baden-Württemberg, during their further education for restorers in 2002. [1]

Dents, scratches, heat

In matters of voids and cracks, metal seals are insensitive compared to varnish, wax and wafer seals. Oxidation sometimes leads to cracks in metal. In most cases, however, careless handling and wrong packaging cause surface deformations and scratches. Gold bullas are particularly sensitive to pressure because it is a very soft material.
In case of fire, seals formed of lead are especially vulnerable as they melt very quickly. [1]
To prevent damage during handling, a suitable protective packaging made of nonaging and acid-free cardboard can be used.

Dirt and mold

Mold is caused by improper storage of cultural assets. Dust, for example, provides a good culture medium for fungal spores that are in the air. In particular moisture ensures rapid growth of cell plexuses. An acidic environment also promotes spread, for example at a pH between 4 and 6. In addition, it is a visual impairment of the objects. Also the temperature can be a factor. Critical values are in range of 15 °C to 30 °C. These factors do not necessarily lead to damage in individual cases. But if they are ignored over a long period and appear in combination, they can have a devastating effect. [5]
Metal seals are the least susceptible to dirt damage. On silver and lead, dirt deposits can cause corrosion. [1]
The document itself, which is of paper or parchment, can be cleaned of superficial dust with a latex sponge. While purging the seal, be very careful with it! Fragile varnish seals can lose individual fragments. Particulates can be removed with brushes made of goat hair or with a museum vacuum cleaner. [6]

Scuffing

A damage which concerns wafer seals, above all. Protein and starch are included in wafer seals, making them attractive to silverfish. [1]

Appearance like puff pastry

This is a specific problem occurring on wax seals. The seal appears brittle and split into individual layers. Different crystalline areas have formed within the wax, leading to a decomposition of seal in the worst case. In Germany it is compared to puff pastry due its appearance. During her Master's thesis, Dümmler has developed a method for consolidating these seals. [7]

References

[1] Landesarchivdirektion Baden-Württemberg, Institut für Erhaltung von Archiv- und Bibliotheksgut: 4. Restauratorenfortbildung, Siegelrestaurierung, vom 10. bis 12.04.2002.
[2] Seal (German):
https://de.wikipedia.org/wiki/Siegel
[3] Signet (German):
https://de.wikipedia.org/wiki/Petschaft
[4] Bulla (German):
https://de.wikipedia.org/wiki/Bulle_(Siegel)
[5] Conditions for mold formation (German):
http://www.bauphysik-zimmer.de/leistungen/ursachenermittlung_von_schimme...
[6] Restoration sponsorships (German):
http://historischesarchivkoeln.de/de/patenschaften/id/68?pic=3
[7] Dümmler, Maren: „Blätterteigsiegel“ – Schadensphänomen und Restaurierung, Köln, 2013.
http://www.hornemann-institut.de/german/epubl_ha_ausgabe.php?haid=2009&l...