In a weird sequence of amnesiac time-travel events, you are transported to 16th century Germany where the only practical fuel (or energy source) is firewood. Suppose you are given a piece of Copper which contains silver in small quantities. As luck would have it, you have a good quantity of Lead at your disposal. In this scenario, which one of the following metallurgical methods would you choose to refine the impure copper?
Liquation
Liquation is a method for separating metals from an ore or alloy. The material is placed in a sloping hearth and heated. Eventually, a temperature is reached until one of the metals starts to melt and drain away from the other and can be collected.
In this case, we should melt the silver-rich copper with 2.5 – 3 times its weight in Lead. This is carried out in a sloping hearth and the temperature is gradually raised by burning more firewood (in a non-oxidizing environment). Eventually the temperature is high enough to melt the lead which also removes the traces of Silver from the copper. This is because Silver has a much greater affinity for lead than Copper.
Clearly options a b and c are beyond your means given the constraints.