The copper we use to build our copper stills is 99.9% Pure Copper. 20 oz. versus 16 oz. seems to be the question everyone asks when deciding what thickness of moonshine still to run. 16 oz. is thick enough, but 20 oz. is better. They are both good, and going thicker doesn't necessarily mean that you will run a better still, or a better product. It simply means that your still is thicker and will take longer to heat up..... Personal preference trumps status quo, so you be the judge. Over the years, run different thicknesses and see which ones you like better. Many people prefer stainless over copper anyways. They mainly choose stainless because it is stronger and easier to clean. You can find 20 Oz. copper stills for sale on our website www.moonshinestills.com which we manufacture in Manchester, New Hampshire, USA.