Caustic soda turns aluminium black and then dissolves the metal, in fact I think any soda does, I stand to be corrected on that.
I believe sugar soap has soda in it to Eamonn.
I wonder why he was dipping? To colour, to dull a shine or maybe it was a home made silver dip which will shine ally goodly.
Interested to see the result of your ads notes, no rush mate but please do keep us informed init?
Have you found em yet?