Model builders, creators of replica items and metal sculptors sometimes wish to make metal objects look aged. Ageing metal involves speeding the natural process of air and water interacting with and degrading the surface of the metal. To achieve an aged look for aluminium objects, the finish on the aluminium must be darkened. While it may be possible to age other metals with household chemicals, a prepared commercial chemical must be used for aluminium darkening.
Clean the aluminium with a solvent that leaves no residue. Examples include lacquer thinner, acetone and alcohol. Solvents such as kerosene or white spirit leave residue and should not be used.
Abrade the aluminium surface if discolouration from oxidation is present. Use an abrasive pad or other method.
Apply an even, thin coat of chemical aluminium darkener onto the surface of the aluminium according to the manufacturer's directions. Chemical treatments can be brushed on, applied with a swab or cloth. Larger surfaces can be sprayed using a spray bottle.
Observe the chemical reaction and reapply another coat of the chemical darkener over the first one when it appears that the chemical reaction has slowed. Continue this process until a uniform darkened finish has appeared on the aluminium.
Dip a natural sponge in warm water and squeeze out excess water. Sponge off the chemical darkener with the damp sponge. Frequently rinse the sponge in clean water.
Mix 2 tablespoons of baking soda in a gallon of warm water to rinse large or complex aluminium shapes. This will neutralise the chemical reaction.
Dry the aluminium with compressed air or a hair dryer set to warm.
Seal the aluminium with lacquer or wax to preserve the darkened finish. Larger hobby stores may carry chemical darkener. You can also find it online at such places as Jax Chemicals (jaxchemicals.com) and Birchwood Casey (birchwoodcasey.com).
Remove all jewellery before using metal darkener. Wear protective gloves and safety eyeglasses when using harsh chemicals.