Used in stills to aid in producing a smooth, clean end product. Fermfast Copper Mesh 1lb Roll. Copper has always been preferred to to any other metal due to its ability to bind with the sulfur compounds from the yeast, making hydrogen-sulfide which in turn, forms copper sulfate. (The same way you can remove some hydrogen-sulfide by running wine through copper.) The copper sulfate sticks to the inside of your still and gets washed down the drain instead of going into your whiskey and leaving a bad taste in your mouth.