hi alan
Do not use the foil method if it is plated, if they are silver then it should be ok.
If they are valuable then do a test first by putting in the tin foil, hot water, crystals then choose some silver, either jewelery of 1 piece of cutlery and dip part of it in and hold for 1 min, if ok and no reaction, hold for 2 mins etc and see if you get a result. As with anything valuable always proceed with caution.
As a side note, when they brought up the wreck of the Senhora de la Atocha off of key West (Florida) they used a similar method to clean 400 years of dirt off the jewels that were found (total value 100 million dollars)
The reaction between the crystals and the tin foil is a weak electrical current and it is this that loosens the dirt.
Please keep me posted.
Regards
Martin