Before commencing any patina application, a suitable work area should be prepared. This space should be dry, but well lit - ideally with strong natural light and well ventilated. If using a torch technique, ensure the entire work zone is clear of flammable substances. This can include removing materials such as wire wool (a material sometimes used for rubbing down applications of ferric compounds - it easily ignites in contact with a naked flame).
Any unstable sculpture should be securely mounted or supported. Temporary mounting for patination is normally made possible by using the fixing points already located on the cast (for securing the sculpture to a plinth etc). Successfully colouring a work with it’s base or ‘footprint’ resting directly on the floor or work bench is extremely difficult – having the cast in direct contact with floor surfaces often leads to the development of ‘tide marks’ and other unsightly disfigurements around the touching portions of the sculpture. Larger sculptures should be raised off floor level, with touching areas supported on fire bricks, inverted stainless steel angles or similar spacers. Avoid the use of mild steel for temporary fixings and supports, this material will almost certainly contaminate any applied patina (FERRIC solutions excepted).
The cast sculpture should itself be free from grease or debris, and exhibit a uniformly even surface finish (for instance shot/bead blasted, wire brushed, semi-polished [satin] and so forth). Particular care should be taken not to allow a newly cleaned cast to pick up sebaceous oils from the skin during handling, or to otherwise become ‘spotted’ by a contaminant – any of these could act as a RESIST to water soluble chemical preparations.
Prepare sufficient quantities of the chemical solution for the job, but take care not to mix too large an amount. Few casts are likely to need more than a pint or so of solution, most need much less. Wearing the appropriate personal safety equipment, mix the patina chemical in an ‘inert’ container, ie one made of plastic or glass, not metal. Available in a number of convenient sizes, a decorator’s plastic paint kettle can be ideal for chemicals mixing, these containers are also available with a sealable lid suitable for safe short term storage.
When preparing patination chemicals, be careful to avoid cross contamination, especially if the container has been previously used for storing other solutions. Ideally, purified water should be used for diluting the chemical base, though most of the preparations described later are tolerant of tap water. ‘Hand warm’ water is usually better than cold water for dispersing most crystallised and ‘lump’ forms of chemical; though be especially careful of using boiling water, this can cause the release of excessive or hazardous fume. Unless specifically directed otherwise, always add a chemical compound into the [water] solution, never pour water over quantities of concentrated chemical – especially ammonia and other volatile compounds. Before using the patina solution, ensure the added in chemical has been fully dissolved and is evenly dispersed. When using unfamiliar chemicals for the first time, mix a weak solution then test for results on a piece of scrap metal. Gradually add in more of the compound, until a satisfactory result is obtained.
Many of the standard foundry patinations (including most of those given later), are applied by a HOT TORCH TECHNIQUE. The following comments are made with this method of application in mind.
Contrary to the commonly held view of patination as a ‘black art’ of chemicals mixing, much of the skill in producing a satisfactory finish lies in the finer points of preparing metal surfaces and the [chemical] application technique deployed – though this is not to suggest that a sound knowledge of chemicals' preparation is not important.
When working a hot torch over a cast copper alloy, the patineur often has relatively few visual clues by which they can gauge the temperature of the heated metal. Some patina solutions are more temperature tolerant than others, though most will work best when applied to a surface which has been heated to within an optimum temperature range. The ideal surface temperature range for a hot torch patination is variable according to the specific chemical solution used by the founder.
Chemical solutions applied to too hot a workpiece will either boil off on contact and leave little or no deposit, or else discolour the metal. Applied too cold and again the patina will either discolour the metal, or the solution will run off leaving no significant deposit. Attaining and maintaining a correct and even temperature in the workpiece is additionally complicated by two further variables. The first is rather obvious, as the patina application progresses, so the heat input into the workpiece increases. This requires the patineur to adjust the play of the flame over the cast’s surfaces, carefully controlling metal temperature and the uptake of the patina.
The second variable is the sculpture’s wall thickness. This factor is most applicable to cast artworks, but may also affect patina applications to fabrications that are constructed from a variety of cross sectional thicknesses. Most hollow casts exhibit some variation in their wall thickness as a matter of course. If however, the wax worker or sand moulder has been less than careful in their work, the wall variations in a cast may be both large and unpredictable. In extreme cases, the patineur will start to get the measure of the job, only to come across with too thin or too thick an area on the cast. This can cause a localised hot spot (thin sections) or cold spot (thick sections), either of which can cause a discolouration of applied patina or the underlying metalwork.
In the event of the patina discolouring, it may be possible to retrieve the situation by quickly applying cold, clean water to the affected area with a fresh clean brush. The applied water can often dampen down any unwanted variation in tone and allow the patination solution to be successfully re-applied once the cast has cooled off slightly. The affected area can then be carefully blended back into the existing patina without the ‘repair’ becoming too obvious. This recovery technique can work especially well with CUPRIC (green) compounds, which can easily discolour if overheated. Uneven applications of FERRIC (red/brown) compounds, can be allowed to cool off first, then worked back to a more even finish by rubbing down with fine wire wool. If needs, be a further application of ferric solution can be added to the worked surface.
Avoid resorting to coloured waxes and pigments to make good a poor patina. Waxes have their place in the patineur’s repertoire, but will rarely recover an initially poor chemical application, coloured wax is in no way suitable as a long term solution. Unless first completely removed, an applied wax finish will also make any other method of effective repair to the patina near impossible to carry out.
There is no meaningful way to convey the judgements made to gauge the correct temperature of a cast, the subtleties involved in preparing and applying compounds and the ‘seat of the pants’ responses required of the patineur to obtain good results under sometimes difficult and demanding circumstances. Ultimately, as far as sucessful patination is concerned, patience, perseverance and lengthy experience counts for everything.
TIP: When using a hot torch, take care to continually 'play' the flame over the sculpture's surface. Prolonged heating in a concentrated area of the cast can lead to over heating and the development of 'hot spots' that may lead to discolouration when chemical is applied.