I tend to think they put some laquer on the pipe at production to keep
it shiny. I do outdoor sculpture with copper and first use fine steel
wool to get down to bare metal, chemically clean with good old malt
vinegar (how people use that stuff as food flavouring !) and then with
a bit of practice apply a cupric nitrate solution. Hey presto 3 years
worth of patination in an hour. If you want to keep it a plain old
brown just apply some wax to keep the acid rain away.