Source: http://www.thedailybeast.com/articles/20...

Why stills made out of copper? The distinctive qualities of copper make it perfect for whisky stills—and thus for developing the full-bodied flavor of fine single malts. This is partly due to the metal’s malleable nature and ability to cope with wear—not to mention its exceptional capacity to conduct heat. But the copper performs another important function: working as a catalyst in the distillation process. In this way, it assists with the formation of “esters”: sweet, naturally forming chemical compounds which, when combined with water, react to create alcohol. What’s more, this process keeps impurities to a minimum, helping to give us the delectable aroma The Macallan is known for.