An electrochemical process that thickens the oxide layer on aluminium, which can then be coloured with dyes or metal salts and sealed.
The aluminium part is the anode in an acid bath; the porous oxide that grows can absorb dyes or electrolytically deposited metals, or be coloured by interference, before sealing closes the pores. Alloy composition and recycled content affect how evenly it colours.
Anodised colour is part of the surface, not a paint coat, so it cannot be touched up and matching between batches depends on the metal as well as the process.
Bronze-anodised window frames from two production runs can differ visibly in shade.
It explains why aluminium colour matching is harder than paint matching, especially with recycled alloys.
An anodic coating is specified by its thickness, its sealing and its colour. Sulphuric-acid anodising gives a porous oxide from about 2 to 25 micrometres thick, with pores roughly 10 to 150 nanometres across that take up dye. Colour is measured with a spectrophotometer against agreed light and dark limit samples, because a single target is unrealistic: the result depends on the alloy, and more iron in the metal darkens the coating. Electrolytic colouring deposits metal in the pores and gives bronzes to black; interference colouring shifts through blue, green and yellow to red as the deposit thickens.
Anodised window frames, cladding panels and trims for one building are produced in separate batches or from different alloys.
The parts differ visibly in shade and no touch-up exists. Specifying one alloy and temper, ordering from one production lot and approving a range between limit samples are the standard precautions.
A brightly dyed red or blue anodised part is used outdoors.
Organic dyes in the pores vary widely in lightfastness, and reds and blues are among the first to fade. Blacks, inorganic golds and electrolytic bronzes last far better in sunlight.