In your case, the charcoal may not need to smolder for 12 hours. It may only need to smolder for long enough to get the wood decking to smolder and propagate to some other material/boundary condition that will allow a smoldering to flaming transition to occur. Similar to a cigarette and upholstered furniture. This transition can occur as a result of increased air flow velocity across the smoldering surface. This increase in velocity is possible when the smoldering propagation transitions from a horizontal surface to a vertical surface (e.g. decking to siding of house). Thus, the "ruling out" of the hypothesis of smoldering charcoal based on smoldering times, you may produce a scientifically unreliable determination.

Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com