Base Metal Clays, like bronze, copper, iron, brass etc, when fired exposed to air create oxidation. Oxidized parts cannot sinter correctly and they are not solid when coming out of the kiln. Gold and silver are no base metal clays.
To avoid oxidation, the parts must be buried into activated carbon which reduces the amount of oxygen in the firing pot. It is very important to use the correct carbon.
Most base metal clays must be fired in two steps:
Step 1: binder burn off. The piece must be exposed to air for this step because most organic binders need oxygen for burn off.
Step 2: sintering. Fill a steel container with activated carbon (2-3 cm), place your piece(s) onto the bed of carbon and fill the container completely with carbon. Close the container with the lid.
Content: 450g of activated carbon coconut based.
Activated carbon is an organic product which can be used many times. Use a sieve to sort out the dust from time to time. Fill it up with new carbon if necessary. Activated carbon can be recycled.