I would be apprehensive of using rubber mulch as an ingredient of something I would be working with regularly. There is some concern that rubber chips in soccer fields is a contributing factor for soccer goalies getting cancer. http://washingtonpost.com/blogs/early-lead/… Actually rubber is heavier than water as rubber tires sink in lakes and rivers. If you could get natural charcoal (no additives) used in barbecues, it would last a long time as well. All the best, Michael Homick Gardener's Pride Botanicals http://www.gpbotanicals.com/ Stevinson, CA