Fake Silvering?

What would be the climates+conditions that would best preserve an ancient coin?

Right--arid desert burials, away from exposed air are the absolute best. The most famous example may be Tutankhamun's burial, where bronze, wood, and even iron survived largely unscathed for 3300+ years! Under those conditions, a silver or perhaps even bronze coin might still show mint surfaces, even after 1000s of years.

Must be cleaned! Even most 'uncleaned' ancients have been cleaned. Lumps that have an inch of dirt used to have two inches of dirt. Some people are very skilled at looking, feeling and deciding which lumps contain salvageable coins and which are not worth the effort. They make a few mistakes and a few good coins slip through but coins that can be sorted, have been cleaned enough to be diagnosed. With experience, they can tell which encrustations will leave a rough surface when removed and which will look good.

IMHO the sad part is many new collectors fail to realize that there is a huge difference in cash value between rough and corroded coins and ones with pleasant surfaces. It is fine to love low grade coins but we should not pay close to high grade prices. I agree with jw that it is bad to deceive the buyer but I add one more level of improper deception. That is telling people that a coin has to look that way because it is old when the real reason is that we botched its conservation. Harsh cleaning methods may leave coins that can be identified but not coins that are attractive. The big skill in cleaning is knowing when to stop and sell the coin to someone else who will either love it as it is or reduce its value with poor cleaning.