Tiedoux+ Coal or clinkers (ashes) will Defiantly cause detectors to be unstable. Detectors without ground balance are usually useless, somewhat depending on the amount of coal or ashes and clinkers present. Clinkers and ashes from burning Anthracite coal (hard coal), will frequently defeat a detector with ground balance. I have a hundred and fifty year old church, I have permission to hunt in Missouri (where I live). The church had been heated with a coal burning stove, The ashes had been spread all the way around the mud (dirt) church yard, to provide more stable parking for buggies and cars 100 to 150 ft.out from the church, for 75 to 100 years. I have been in there with Minelabs, Garretts, whites, and fishers, nothing worked. the ashes were 1 to 2 inches under the top soil, going down to three ft. in depth. Have had the same experience in some of the Pennsylvania coal fields, and other old churches and schools in a number of other states and other buildings in Missouri. If the coal and ashes are not too concerated a ground ballance may help, you will still have chatter, but may be able to hunt, your depth will be reduced. I have not tried to hunt this old church in the last 10 to 12 years, if I can get current permission I may try again, to see if a more recent detector would work. This has been my experience with coal and ashes, hope I have answered your question Steve.