< BASIC PATINATION TECHNIQUE
BROWN (PENNY BRONZE)
This cold application patina is the classic foundry finish – it results in a fine patina in it’s own right, and also makes an excellent base or GROUND for an overlaid patina (especially hot torch cupric and ferric patinas).
1. Dissolve about 1/2 level tea spoon finely crushed lump of POTASSIUM SULPHIDE [POLY] (sometimes called LIVER OF SULPHUR) into about 1/2 pint of warm water (or other quantity to suit).
2. Brush or spray (plant water diffusers are suitable), the dissolved chemical over a shot-blasted or lightly buffed cast surface. Allow the applied chemical to settle and partially dry. It is normal for the patina to be somewhat patchy at this stage, this variation can be evened out later by wire brushing.
3. Using a soft engineer’s brass brush (similar to a brass seude brush but larger), vigorously work the film of deposited liver into the surface of the cast, taking particular care to even out any uneveness.
4. Once the wire brushed deposit is evenly distributed and has completely dried off (note: total dryness is essential), re-dip the brush into the liver solution, flicking off any excess moisture. Using the semi-wet brush, stipple the surface of the cast to produce an second, even patina deposit. Recharge the brush as it dries out and continue until the sculpture’s surface has been completely re-coated. Do not overwork the stipple deposit and do not wire brush the cast’s surface after this application.
5. Alow to dry off. If the penny bronze colour is intended as a finish patina, apply a protective wax coating to a gently heated cast.
BLACK
There are many ways of obtaining a black patina on copper alloys. One simple technique is to mix up a strong solution of POTASSIUM SULPHIDE (see above), and apply it to a gently heated cast. This produces a matt black/brown on a shotblasted surface, though too strong a solution may produce a ‘lead’ like finish. Strong potassium sulphide deposits can also be prone to flaking away from the sculpture’s surface to reveal the underlying cast metal. The resulting patina should be waxed soon afterwards, to prevent unslightly patches developing.
Alternatively, a VAT solution of about one tablespoon of potassium sulphide dissolved per gallon cold water, will develop a black patina on many copper alloys within 60-120 seconds. Again, this method can result in a lead like appearance and the solution will loose it’s potency quickly if you treat several sculptures in sucession.
Another method is to apply a strong CUPRIC NITRATE solution to the cast (see GREEN below), then scorch the patina by overheating with a torch. This can produce a pleasing finish on brass and bronze, selected rubbing of the surface before waxing can encourage a deep brown variation in the patina if desired. To inpart a black oxide coating on copper that mimics the natural weathering process, dissolve 30g (1oz) SODIUM CHLORITE (not sodium chloride!), and 60g (2oz) SODIUM HYDROXIDE in 500ml (0.9pt) parts distilled water. Heat to 158°F (70°C) and apply to a keyed and degreased copper surface using a naylon (Scotchbrite [TM] type) scouring pad. Allow to dry, preferably overnight until a black deposit develops. Repeat if necessary. Note, this is an extremely CAUSTIC SOLUTION which should be handled with great care, wearing full protective equipment.
The application of a cupric nitrate deposit is also the first stage of the classic RENASISSANCE BLACK fume/burial method. This technique requires a hot or cold applied cupric coating to cool (or dry), on the cast’s surface. The cast then is exposed to heavy smoke, usually evolved from a fire fuelled by damp straw or similar. The cast is evenly rotated in the smoke’s precipitate until the required effect is achieved all over. Once cooled, the cast is treated with a weak ammonium chloride solution, then buried in soft clean sand for about fourteen days. Every 48 hours or so the sand should be saturated with fresh milk, this helps fix the soot deposit. After fourteen days the cast is removed and dusted with chalk or fine talc, before repeating the burial/milk saturation process for a further 20+ days. The cast can then be removed and cleared of loose material. Cold waxing is reccommended [ref].
AMMONIUM SULPHIDE is a useful chemical for achieving a black patina deposit. Bear in mind this chemical is an especially volatile and noxious substance, quite capable of inducing a fume patina when left exposed in proximity to copper alloys. This chemical should be used both sparingly, and with caution because any evolved hydrogen sulphide gas will tend to scavenge available oxygen. In addition to hot torch and fume techniques, this chemical can also be used as a hot water immersion bath by adding small quantities of SODIUM CHLORIDE into the solution.
1. Prepare the cast by shot blasting, though this procedure is not essential, provided the metallic surface is free of grease.
2. Mix a strong potassium sulphide solution (as described for a penny bronze), but not too strong as heavy deposits are prone to flaking off the cast later, especially when waxed and polished. Apply the solution to a gently heated metal surface via brush or spray. The preparation should take easily to a warmed cast on contact (without sizzling).
3. Once the potassium sulphide application has been evenly deposited, mix a weak solution of AMMONIUM SULPHIDE (liquid form). About 1-2 tea spoons per 1/2 pint of water should suffice. Apply this solution to the gently heated surface of the cast.
4. The cast will turn a dark, matt blue-black. The patina deposit can be finished with a wax protection when dry. Some founders use a black pigment impregnated wax polish to impart a deep and even gloss sheen.
5. Rather than apply the POTASSIUM SULPHIDE and AMMONIUM SULPHIDE solutions separately, a tea spoonful of ammonium sulphide can be added direct to the prepared polysulphide solution, then applied to the warmed cast in a single application.
PATINATION (GREEN) >
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