The patina can be thermal or chemical. The chemical would generally be an acid with specific salts and sometimes heat too, for a specific material. Differant materials = differant processes ! The most beautyful would be damask but this is a forging folding welding process. The simplest would be snake oiling or heating to a dull glow of iron or steel then plunging into sulphurous oil, which makes for a very anti rust treatment. I still have some tools treated this way that my grandfather did in the early 1900's that look very fresh and new ! but it isn't really a patina surface. you will need to pick the results wanted for a particular base metal then ask for a process to meet your needs.