I'd soak them in a plastic or glass container with vinegar inside. Check them in 2 hours and rub them with baking soda to remove some of the crud and corrosion. Put them back in the bath if they need more and repeat the process until you can determine the year and value. If it's a common date and/or has little collector value, keep cleaning it slowly until it's the way you want it to look. If you want it a copper color, use an aluminum pie pan or the end of a soda can as your soaking tray. If you find it's rare/valuable, you may want to stop and get some advice as to whether it's better to leave it alone or continue cleaning.
Nonsense. He can't even determine the date, so, you have to do something to first find out if they're valuable or not. If you do this method slowly until the dates can be read, it's not going to negatively effect their value. After the date is determined, then is the time to decide on how much cleaning you want to do based on projected value. I haven't seen a crusty, unreadable Indian cent yet that was hurt by this method. Most of the time when they're in that condition, nothing you do is going to hurt or improve their "collectible" value. Not many coin collectors will buy pitted, and/or "as dug" coins anyway. This method doesn't apply to all coins, but, most copper it does.Please do this if you wish to ruin ANY resale value, collector's value, or attractiveness... Perhaps the worst advise I've seen in the Coin section, bar none.