< PATINATION (BROWN & BLACK)
GREEN (LIGHT)
A multitude of greens can be obtained on cast COPPER ALLOYS. This colouring is particularly sympathetic to copper based alloys (green eventually develops as a natural product of COPPER OXIDE). The following describes a fairly standard hot torch technique.
1. Prepare the cast by lightly SHOT BLASTING (or rubbing over with fine abrasive cloth) to KEY the metal’s surface.
2. For best results apply a GROUND PATINA, first follow either steps 1-3 (PENNY BRONZE patina) or 1-2 (hot torch POTASSIUM SULPHIDE), according to preference. Doing this provides a good base for building up a finish deposit, though a hot torch copper nitrate solution can be directly applied to a clean metallic surface.
3. Dissolve about 1 table spoon CUPRIC (copper) NITRATE crystals in 1/2 pt warm water, or use other quantities to suit (note: some cupric preparations are available as pre-diluted proprietory solutions).
4. Pre-heat the cast, working the torch flame from the base of the sculpture, up towards the top. Do not keep the torch in one area as this will encourage HOT SPOTS which may discolour the patina. Bear in mind it can be especially difficult to judge the correct temperature of the underlying metal cast, especially when working over a dark ground patina.
5. Apply the dissolved cupric solution by brush or spray diffuser. The solution should just ‘sizzle’ on the cast’s surface if the application temperature is correct, leaving a light green deposit. If the cast is too cold when the chemical is applied, a black smear may appear, too hot and the preparation will instantly boil off and nearby areas may discolour to yellow (as noted in the introduction, this situation can sometimes be retrieved by allowing the cast to cool slightly, then brush over affected areas with clean water).
6. Once completely cooled (this is essential), the cast can be carefully given a cold wax protection. The wax is lightly stippled on with a soft brush. Waxing temporarily darkens down the patina, which should regain it’s original, lighter tone on drying. The wax application may need repeating a number of times if the patina deposit is heavy (cupric deposits can be very absorbent). The cast can then be dusted with plain talc and later polished very gently with a soft cloth. Before polishing, the sculpture should first be set aside to stand, allowing the wax deposit to settle and harden, otherwise the wax and patina may lift off when rubbed.
GREEN (DARK)
1. Follow steps 1-5 as for a light green above.
2. Prepare a weak solution of POTASSIUM SULPHIDE (see PENNY BRONZE), or a very weak AMMONIUM SULPHIDE solution. (note: if ammonium sulphide is used the patina may continue to darken some considerable time after finishing, allow for this when arresting the patina, then thoroughly wash down the finished cast with clean cold water before waxing).
3. Follow step 4 (penny bronze), for the application of a stippled brown effect. Applying the polysulphide solution on top of the cupric deposit darkens the light green patina, producing a mottled green-brown to green-black variation, according to the strength of the solution. Start with a very weak solution, adding to it’s strength as needed.
4. Applying a wax protection whilst the cast is still warm, will further darken the patina.
APPLE GREEN
1. Follow steps1-4 for a light green, omitting step 2 – the deposit of a dark PENNY BRONZE ground.
2. Prepare a weak solution of FERRIC NITRATE; about 1 level tea spoon dissolved in 1/2 pt water, or in increasing strength to suit.
3. Gently warm the cast and apply the solution over the deposited green patina. Take care to avoid overheating the cast, this can lead to concentrated blotches of red patina.
4. Allow the cast to cool off and then wax.
BLUE/PALE BLUE-GREEN
This is a cold application patina, very easy to mix and apply. Mix equal measures of a 15% AMMONIA solution and strong CUPRIC NITRATE solution to make up about 1/2 pint. Add in about 1 generous tea spoon of AMMONIUM CHLORIDE (powder), then top up with vinegar (ACETIC ACID) to make 3/4 pint. Stir gently, but thoroughly.
Stipple the mixed solution either directly onto a clean cast surface, or else over a PENNY BRONZE ground. The applied solution dries to a mottled mid-sky blue. Once thoroughly dried off, further treatments can be applied as needed. The patina covers well and will even bite on polished material – but less so on BRASS. The resulting deposit tends to be exceptionally stable for a cold applied patina, though may fade more towards a light green over time. Allow to settle for a period of time before applying a cold wax (allow to dry and dust lightly with chalk powder before polishing with a fine soft brush).
PATINATION (REDS) >
|
