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There is a chart somewhere that shows different methods of applying smoke that work best for rings.. tells about hot burning wood..pellets..chips..charcoal and lump coals.. low and slow wood and charcoal.. what it all does to give better or worse results... pretty interesting stuff.
There are 339 pages of this subject here to sort through.. it's kinda hard to see what title is from anyone other than just a user talking about smoke rings.