Been doing a bit of testing and best bang for buck and minimal effort has been Hydrochloric acid so far. I tried some silver/gold/copper coins, took about 30 seconds in solution to clean all the black/dirt away.
I'd probably dilute it for a large lot and let it sit for a couple of minutes with a bit of agitation, then rinse with water.
Note it will slightly pit some coins. This is for clean and spend only, make sure the 20 isn't a florin! I guess composition changed some years because some of the same denomination coins are bright and some are dull!
I know this is a ancient thread but I read somewhere all Pre-Decimal coins are 50% silver and a fair bit of nickel, where as current silver coins are 75% copper and a bit of nickel and a few other small amounts different mixtures on top
Apologies if it's been covered before.