Conditions for Conservation of Sealed Documents

Alterations and Conservation of Wax Seals

Detail of wax seal showing mould on the surface.
Wax seal showing saponification.

Generally the climatic conditions for the conservation of graphic documents equally apply to wax seals and bulls as long as the absence of atmospheric pollution is secured.

It is important that the conditions of conservation remain stable, in particular for the hygrometry, as the materials are extremely sensible to changes in humidity. A level of humidity that is too high may cause severe alterations such as mould, corrosion or hydrolysis. Considering that too low temperatures raise the environmental humidity, called in conservation relative humidity; however, a too warm place could provoke a desiccation of the parchment while the textiles are threatened by deformation, breakage, and disruption.

With wax seals, the loss of fatty acids and non-permanent alcohols causes saponification, so the material becomes both opaque and friable. Such organic materials are vulnerable against attacks of insects and microorganisms, which often become apparent through holes or mould.

The major principle of conservation is to handle the objects with greatest care. Most damages in collections are of mechanic character: cracks, missing pieces or pollution. Such damage is caused by poorly executed handling and inadequate organisation. It is recommended to arrange the objects individually, well-supported and free from dust. The fragile objects must not be touched or stored on top of each other.

Alterations and Conservation of Metals

Corrosion of lead on the surface of a papal bull.
Corrosion of iron in a metal seal container.

Although alterations in physical or chemical categories are rather rare, one can observe corrosions of the metals, discolorations or desiccation of organic materials. Metals, both that of bronze matrices and lead bulls, are extremely sensitive against humidity and volatile pollutants. They have to be stored in an absolutely dry environment. No traces of acid gas or sulphur (not even wool, carpet, etc.) should be found together with the object. Moreover, pieces of wood containing acid or glue (principally oak, plywood, chipboard), but also boxes and papers containing acid have to be removed.

Forms of active corrosion are easily identifiable. An altered metal presents with indentations, deformation, changes of colour and the loss of material in the form of matte powder. For those objects made of copper alloy, such as the bronze matrices (this is an alloy of copper and tin) one can observe green surfaces which derive from the acetates of copper. They remain from the corrosion of copper that is generally named ‘green rust’. On leaden objects the corrosion appears white and this specific carbonate of lead is commonly called ‘white lead’. Finally metals containing iron show grey or red areas known as rust. Metallic sigillographic objects should only be handled when wearing gloves, preferentially of nitrile rubber, because humidity and perspiration is enough to cause active corrosion (gloves made of latex are unsuitable as they contain powder that affects certain metals). In order to react quickly to degradation, it is important to check the condition of collections regularly.