Put some cold water in your kitchen sink. Then place a few pennies on a pan and sit them in a hot oven for about ten minutes. Pull them out and drop them in the cold water and you'll see some nice blue toning. There are too many ways to create toning in coins. That's why I personally stay away from toned coins.
Virtually every cent that spends a few years in circulation is toned. It's unavoidable. Copper is highly reactive.