Left alone, brass, bronze and copper colour themselves over years. A patina finish does the same in an afternoon, by hand, and stops at the moment the colour is right.
How it is made
The metal is cleaned until it is completely bare. Then the colour is brought up with heat and with solutions that react with the copper in the alloy, applied by brush, sponge or spray, a little at a time. Browns and near-blacks come quickly; greens and blues take patience. The craftsman watches the colour, not a clock.
Why no two are alike
The result depends on the alloy, the temperature, the weather on the day and the hand that applies it. That is the point of it: a patina has depth and movement that a printed or painted finish does not. It is also why a patina is always agreed on a sample made from the same metal, and why panels that sit side by side are finished together, laid out in order, so the colour flows across the joints.
Keeping it
Once the colour is right it is sealed, with lacquer to hold it close to how it is, or with wax to let it keep deepening slowly. Sealed patina is cleaned with a soft cloth and nothing stronger; polish removes it. Outdoors, it keeps changing whatever is done, which is often the reason it was chosen.
