Cold metal casting is a technique that uses real metal powder encapsulated in a resin. These Metals can be Aluminium, Brass, Copper and Bronze metal powders. For the best effect it is better to oxidise the Bronze or copper powder before using them to cast. By oxidising the powder it will darken quite considerably giving great low lights and when burnished with steel wool will high light the metal to a nice shine but leaving the darker low lights resulting in a beautiful contrast of varying shades. Oxidising the metal powders can be done by using a metal paint pot, a metal spoon and a gas flame.
a) Pour the metal powder into the metal paint pot.
b) Place metal paint pot onto gas flame on full heat
c) Stir powder in paint pot as it heats in a constant slow stirring action.
d) Metal tray
e) A metal sieve
f) Heat resistant gloves
g) Dust mask
The metal powder must reach about 300 degrees C, so a high gas flame is essential. (A sturdy camping stove would do). As the powder heats up to start oxidisation, keep stirring as this will reduce the powder sticking together. The powder as it heats up will go through a number of colour changes, starting with oranges to browns until it reaches a chocolate brown. It is possible to continuing the heating process until a blue/black is achieved. Once the desired colour is achieved pour metal powder from the metal paint pot through the metal sieve and onto the metal tray to cool. Once the metal powder has completely cooled it is ready to use.
NOTE: Please remember that extreme heat is used, so ensure that you take the greatest care. Ensure that you operate in the right environment that is safe to use naked flame, use protective clothing such as heat resistant gloves, safety glasses etc. Be aware that transferring hot metal powder from Pot, through sieve and onto tray, ensure the tray is on a surface, that will not be affected by the extreme heat transference. Operate in a well ventilated room. Wear a good quality dust mask when using powders. Bronze powder comes in two common mesh sizes 200 mesh & 300 mesh. Choose a metal sieve size mesh that will allow bronze to pass through.
Bronze powder comes in spherical or irregular. I prefer to use a 50/50 blend of the two as it gives a better final finish. There are two techniques used when producing cold metal casting
1) Adding metal powders to the resin as a concentrated filler system
2) Dusting mould with metal powders.
I will concentrate on the Dusting technique as the filled system is self explanatory. Dusting the mould is more cost effective and the finish is as good if not better. As it is pure bronze on the surface of the casting it is easier to burnish with steel wool and add a finishes. Use a condensation cure silicone (TIN CURE SILICONE) as this will allow the bronze powder to adhere to the silicone surface more readily than Addition cure silicones (PLATINUM CURE SILICONES), Take the condensation cure silicone mould and fill the cavity of the mould fully. Then pour the bronze powder out of the mould, ensuring that all loose powder is out of the mould by shaking and lightly tapping the silicone mould. The bronze powder will give you about a 30 micron coating on the surface of the mould.
If the silicone does not give you an even coating of bronze, which usually happens toward the end of the silicone moulds life, a light spray of a mould release would help adhesion of the bronze powder to the surface. Secure the mould for casting then cast resin in a slow continuous pour until full. The resin used whether it is Polyurethane or polyester, to get the best effect colour the resin as close to the colour of the bronze by using black and brown pigments. Using pigmented resins will enhance the tonal value. If you use filled polyester resin or Polyurethane leave the casting to cure out of the mould for 3 days to harden, so that the bronze powder has set into the resin to give you the best shine to the burnish. Use a medium grade steel wool to burnish the cold cast bronze casting in a single stroke motion. Be gentle with sharp details on a casting as it is easier to cut through the bronze coating when burnishing that flat areas, to obtain the burnish sheen, remember that this is a bronzed coating of 30 microns thick.