Help! Tumbled coins turned black/dull

[AC1955]

Been using the rock tumbler/aquarium gravel/dish soap method of cleaning coins. I separate out the clad and the copper and clean them separately from one another. My question is this, the first batch of clad I cleaned came out silver colored and shiny. Subsequent batches are coming out dull gray/black. Pennies are coming out black (separate from clad). I use plain Dawn dish washing liquid (no bleach and not anti-bacterial), natural rock gravel and warm water. Dirt and crud coming off, but coins look awful. Could I be overloading tumbler? Any suggestions on how to avoid this in the future? Will those coins clean up better if I run them again? Thanks for your help.

 

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[Frankn]

Try laundry detergent. It is made to cut dirt" water base" , Dish washing soap is made to cut grease. Also use fresh gravel each time.

 

[AC1955]

Thanks. Will try this on the next batch.