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Aluminum has a high strength-to-weight ratio, and its patina is the same color as the metal. Therefore, many industries use aluminum, in place of steel or plastic, for strong, lightweight parts that resist corrosion. However, the aluminum surface does oxidize, and oxidation weakens the metal. Scratching and incising the softened, oxidized layer creates undulation in the patina, but the surface color will not vary, unless you rub pigment over the incisions. Aluminum oxidation hastens with an abundance of ions washed over the aluminum surface.
Mix equal parts salt and water to create a saline slurry. The water partially dissolves the salt, allowing the slurry to stick to the aluminum.
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Rub the slurry over the aluminum surface and allow it to set for one week.
Clean the aluminum surface with water and a wire brush. Remove all of the crusted salt.
Rub the slurry over the aluminum surface again. Do not disturb the surface for one additional week.
Rinse the aluminum surface with water to dissolve the remaining salt. Allow the aluminum to dry. A patina, from oxidation, will have formed on the aluminum surface, although there will not be a change in the color or finish of the surface.
This method of oxidizing aluminum is not the same as anodizing aluminum. Anodizing is a process of artificial oxidation using an acid and electricity to create a strong surface on the metal. Dye can be added to the acid to change the surface color of anodized aluminum. Anodizing is dangerous, because of the introduction of acid and electricity.