It's a sworn duty that I leave little or no trace behind me. Lighting wood fires outdoors is a great passion for me, and my usual procedure is to use a folding shovel to dig a 1 metre squared hole, about 8 inches deep, and line it with some logs. I've tried this method on a grassy hillside, and managed to leave no trace a fire was ever there. The embers should be put into another smaller hole dug within the fireplace, so they are at least 12-16 inches below ground level. Before putting the original soil and sods back the scorched earth should be ploughed up a little bit to break the crust. This way the grass will show little sign of withering in 2 weeks time.