Coatings, media and adhesives are found as normal constituents of most furniture items.
Consolidants when present have usually been added by a restorer or conservator. A wide range of materials has been used to fulfil these functions, with synthetic materials having been added more recently to the range of naturally occurring materials used historically. Both groups are chemically complex, mostly organic, polymers but natural materials tend to be much more variable in composition and properties than their synthetic counterparts. These substances possess qualities of cohe-siveness and adhesiveness and share a range of other properties, discussed below. In practice, the same substance was often used for different functions. An adhesive substance found to be useful in joining wood is also likely to adhere when applied in a thin layer as a coating, to bind pigment particles together or incorporate dyes and stick them to a surface as a binding or paint medium. Therefore, while it is convenient to discuss these categories separately, it is important to realize that there is considerable overlap between them.
To be useful, materials used as coatings, binding media, adhesives and consolidants need to be liquid at some point in their application and then to solidify. There are four basic mechanisms by which this occurs. Setting can occur through change in temperature alone (e.g. waxes), or through change of temperature and loss of solvent together (e.g. animal glues). It can also occur through loss of solvent or liquid phase without change in temperature (e.g. modern synthetic thermoplastics). Lastly, setting can occur by chemical reaction (e.g. thermosetting polymers, urushi, linseed oil). Chemical reaction may involve loss of a small molecule, such as water (e.g. urea formaldehyde) or may proceed without loss of volatile matter (epoxies, polyesters). Sometimes more than one setting mechanism may exist for a given chemical type. The way in which a material sets plays a large part in the way it can be used. Elimination products cause contraction. Epoxies, because they do not produce elimination products do not contract appreciably on setting.
A basic (ideal) requirement for all coatings, media, adhesives and consolidants is that they should be stable, fully compatible with the object material with which they are in contact (that is there should be no adverse chemical or physical interaction) and that they should remain so. They should be durable under their intended service conditions and should not discolour, degrade, or crosslink. For conservation materials added to objects these are important considerations that govern our ability to undo treatments and to retreat at a later date, for example in the case of applying a coating to a paint layer. Materials must be safe to use and have the desired working properties to facilitate handling and application. Sensitivity of these materials to solvents remains important after their application.
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