"I've looked at the archives and it seemed to me that the prevailing
wisdom was to put the stones in the dishwasher (when SWMBO is not home
preferably) and give them a good bath."

I would never be game for this, and others report problems cleaning the
dishwasher afterwards. With very dirty old stones what I know works (have
used) is:

(1) boil the stones in an old saucepan (bit of towel on bottom) over a
camping stove outside with dishwashing liquid + dishwasher powder (whatever
concentration you feel is ok). Change the liquid a few times for really
dirty stones. You know you have finished when boiling in plain water
generates little junk

(2) the same as above but using an old urn that was thrown out, but still
worked

Cheers
Peter Evans
Sydney, Australia
Skype peterrevans

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