I've never had the stones to put mine in the washing machine.
Some do it and swear by it. I'd use a tumbler (front load) and not an agitator, tho.
Nikwax Tech Wash for the soap.

I soak mine in warm water/mild detergent, hose off and drip dry.
Sometimes I just hose it down and drip dry.
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NO STEP ON SNEK