It's a bit tedious but you don't really need to trade so hard to get silver coins. From the moment you invented pirate's coin, do money laundry by buying stuffs with pirate coins, followed by selling them, repeatedly. Once you decide to stop and let the day slip, the exchange rate between silver coin and pirate's coin will slump, so that you can no longer do it with a satisfying efficiency. You'll need a while to let it recover then the next laundry.