Patina

In everyday language, the word "patina" covers a wide range of phenomena, most often linked to changes in the surface of objects due to the action of time, the environment or surface treatments.

What they have in common semantically is the modification of the appearance of objects, in particular their color.

What they have in common scientifically is the existence, on the surface of the objects, of a layer or deposit of color (or texture) different from that of the "bare" material.

We speak of patinated stone, patinated plaster, even paint patinas and, of course, metal patinas. The origin of these surface modifications may be involuntary or voluntary. The former is generally due to the action of the environment over more or less long periods, while the latter belongs to the class of surface treatments.

For metal objects, the only ones considered here, patinas are defined as the formation on the surface of compounds resulting from chemical reactions between the material and the external environment, either as a consequence of the action of a natural environment, in which case they are referred to as corrosion patinas or natural patinas, or as having been intentionally applied during a surface treatment, in which case they are referred to as intentional patinas.

As far as natural patinas are concerned, a further distinction is sometimes made between so-called "noble" patinas, whose presence preserves the object's original form, and so-called "vile" patinas, which are in fact a more or less regular accumulation of corrosion products in the course of evolution. It would seem more accurate to speak of protective patinas and non-protective corrosion.

Prized by art lovers, patina - a term that only appeared in the 17th century and was not applied to metal until the following century - is also often regarded as a guarantee of authenticity. It's hardly surprising, then, that very early on, artificial patinas sought to imitate natural patinas.

The two effects can be combined, either voluntarily or involuntarily: some intentional patinas are obtained through the combined action of a chemical surface treatment and exposure to a natural environment; conversely, the existence of an intentional patina on certain archaeological objects, for example, can be masked by subsequent degradation.

In addition, some corrosion patinas are deliberately obtained by choosing a material which, through the action of the natural environment, leads to a desired change in appearance and color (see the example of patinable...