< PATINATION
APPLIED PATINATION PROCESSES
Naturally weathered patinas, the PATINA NOBILE, are capable of developing into some of the finest colourings possible on cast metal sculptures. In particular, a patina nobile avoids any of the contrived mannerisms and affectations which can occasionally afflict manually applied patinas. Unfortunately, the overwhelming disadvantage of the patina nobile is the extended period of time usually taken for a full development to occur. Furthermore, there is often is no reliable way of predicting the ultimate development or tonal values of a patina nobile. For these reasons it is usually impractical to install a cast artwork with the intention of having it develop a rounded and satisfactory patina, without first treating the cast surface to some form of accelertated chemical colouration under controlled conditions.
Of all the metals capable of taking patination, copper and it's alloys (brasses, bronzes and gunmetals), comprise the most important group. Exceptions in the copper alloy family include aluminium and nickel bronzes (see METALS), both of these are formulated to offer high resistance to corrosive conditions and are considered unsuitable for patination.
A very wide range of chemical compounds are capable of a producing a colour finish on most other copper alloys. In practice though, founders tend to use just a select few chemicals for the majority of metal colouring tasks. These standard patination preparations, when correctly applied to sculpture cast in an suitable alloy, produce predictable, stable and long lasting surface colourings. The basic chemical compound groups that form the backbone of most art foundry patinas are as follows:
• BLACKS ‑Potassium, sulphur, and ammonia compounds
• BROWNS ‑Potassium, sulphur and ferric compounds.
• REDS ‑Iron/ferric compounds
• BLUES & GREENS ‑Copper and ammonia compounds
By selecting individual compounds from the appropriate groups or mixing two or more together, a skilled patineur can in combination with application techniques, produce a multitude of colour finishes on a cast copper alloy surface.
Green variations for example, can be obtained in a broad chromatic range, from light blue-green through to deep black-green through the use of ammonia, cupric nitrate and potassium sulphide; applied either individually, mixed together, or by sequential layering over earlier treatments. Variations in effect are achieved by applying these chemicals in different orders and solution strengths, also by employing a range of application techniques. It is also possible to use quite different chemical compounds and techniques to produce a markedly similar visual result; for instance a brown colouring can be produced either by applying a cold potassium sulphide solution, or by burning on a hot cupric nitrate solution (see BLACK & BROWN PAINTA) .
It is not only the chemical’s composition and application method which influences the development of a patina. A large number of other factors, including the sculpture’s alloy composition and surface condition, as well as temperature, all play an important role in determining the precise colouring effect generated on the cast.
SURFACE CONDITION >
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