The ageing process of copper sheet is the most dangerous and unpredictable job. I developed this procedure during my work experience and I like keep it simple and relay on the power of nature to get every time different wonderful colours.
First of all, the sheet has to be perfectly clean. For this reason I usually brash it with a special abrasive sponge.
Then the copper sheet has to be exposed to chemicals in close and safe container. This step takes almost half a day. Eventually, using safety tools and clothes, I take off the copper sheet and expose it to open air (but not directly to rain, sun or wind). The grade of ageing and the final colour nuance depends on several variables: tickness of copper sheet, position during the exposure at the chemicals, the amount of humidity, and so on.
This means that each piece of copper will have a peculiar ageing process and, in the end, a different kind of pattern, that is, a different colour nuance.