for a realistic coal effect build up a mound of modeling gravel (i use games workshop, its great)
eg...glue, layer...wait to dry....glue,another layer...etc till you get a nice mound.
Wait for it all to dry and paint it with Tamiya X-18 (semi-gloss black)
it gives the carbony/semi-shiny effect of coal
...although the flash on my camera has brought up all the bits iv missed!
