< COLD PATINATION
HOT PATINATION
Hot patination requires either the cast and/or the chemical solution to be heated during the application process. Any cast intended for a hot patina is first carefully inspected. If the cast’s internal volume has been sealed (ie. by welding a plate over it’s base), the interior should first be vented by drilling a small diameter relief hole through a discreet part of the cast. This vent will enable contained air to equalise as it’s volume expands upon heating. Failure to vent a sealed cast can potentially lead to a structural failure or in extreme cases, an explosion.
Most hot patina applications involve pre-heating the cast with a BLOW-TORCH. The torch should be fuelled by a clean burning LPG or town gas (this avoids any discolouration of the patina from the carbon deposits sometimes evolved by acetylene fuels). Alternatively, an oven or kiln can be used to heat up the sculpture.
The founder must draw upon their experience to judge the optimum application temperature – too hot or too cool a metal surface can adversely effect the quality of a patina deposit. Once a sufficient temperature has been reached, the prepared chemical solution is applied to the cast’s surface. In most cases the chemical solution is applied by brushing on with a STIPPLING action, though a diffused spraying of the chemical through an ATOMISER can produce equally pleasing effects. Many of the traditional red and green patina variations (usually obtained from applying ferric and cupric solutions respectively), are successfully applied to casts by a HOT TORCH technique.
Less common than the torch technique, is the heated VAT IMMERSION method. Vat immersion is usually limited to use on small scale casts, this is mainly because of the storage and disposal problems associated with holding very large volumes of chemicals in solution. The contained patina solution is heated up to a pre-determined temperature in a copper or plastic vat. The temperature of the solution is normally maintained whilst the suspended sculpture is fully immersed in the vat over a period of time. On fully developing the patina, the cast is removed from the vat, then washed and dried, before having a wax or other protective coating applied. Some chemical immersion patinas can be deposited using cold solutions however, neither the hot nor cold vat process is widely used in today’s art foundry environment.
It is also possible to impart a colour to a heated copper alloy without a direct application of chemical compounds. Heating an alloy to an elevated temperature will often be sufficient to cause a colour variation to develop – the resulting patina is relative to the specific alloy and the temperature attained in the cast. Residual colours from the heating of cast alloys can often be observed in new castings following a ‘knock out’ of the refractory mould. Unfortunately, the surface colours produced during casting can be patchy, localised and usually disappear over a period of time as the metal surface naturally oxidises. Colouring a copper alloy by heat alone, or with an additional application of oils (jade oil for instance), can produce very successful results, especially on polished material. PHOSPHOR BRONZE for example, can take on a deep rich pink/red colour through intense heating (with a blow torch), followed by a rapid quenching in boiling water (see RED PATINA). It is worth bearing in mind when considering the use of this type of patination process, that some HARDENING or ANNEALING of the alloy can occur, this kind of treatment will potentially have a metallurgical or structural effect on most cast alloys.
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