I have some experience with ancient copper and bronze coins. Copper sulfate 
seems to be a problem with some. You may be able to clean the coin with a bicarb. paste, 
but it will not remove all traces of copper sulfate from all the details of the coin and 
it may impair the surface.  I do not have a book on cleaning ancient coins, but most of 
what I said about circulated coins in "The Dirty Little Secrets About 
"Cleaned" Coins" probably holds true for ancients coins. That may be a 
good subject to address in a future release of this book. Caution: Washing any coin with 
distilled water and a mild liquid soap solution to remove dirt or using an ultra-sonic 
cleaner probably will not harm any coin. Do not use anything that will remove that 
natural patina that the coin has acquired over the years, such as dipping in acid or any 
commercial coin cleaner. Thanks for the question,

Why do some dealers buy foreign coins by the pound?

• The more establish countries like Great Britain, France, Russia and others have been minting coins for hundreds of years longer than the US, so their coins have to be much older and rarer to be worth much. Most of the coins people have in a box somewhere are too new to be valuable.
• Many countries come and gone on the world scene in the last few hundred years, so their coins are very new.
• No market! Very few collectors make it hard for a dealer to market them.
• Most of the pocket change coins that people bring back from over seas are made from non-precious metals: Copper, brass, bronze, aluminum, pewter, or steel.
• Large denomination coins that may be made out of silver or gold are easily spotted and remove before weighing. Smaller coins are hard to sort out.
• Mixed batches of coin are labor intensive. When a dealer is charging #25 to $35 an hour to appraise coins, it does not take much time, for the labor to out price the coins in value.
• The complexity involved. Just think of all the different countries and languages that these coins represent.
• The books that are used to price these coins are the size of a major cities phone book and are very expensive.  These books are only updated yearly and the prices are just a guess. Most dealers can't afford to buy a book every year.  There is just not a large enough market to justify the expense.
• Even if you find one coin in a pound of coins, which may have value as a collectable coin it may be on display for months or years before some one buys it, if at all. It is a gamble that any of the coins will be valuable. Coin dealers are in business to make money, so they can't afford to invest in coins that won't sell.
• Because of the labor involved the dealer may just put the coins in a box and let his customers sort through them for a minimal price per coin.