What causes soot to build up in the wall furnace chimney that goes out through the attic.?

through the summer small bit,s of black soot came down through the furnace, and landed on the living room floor. we went through the process of tapping the pipe,s but only got black dust, no chunk,s but now after a week of furnace being lit , small chunks are coming out again. is this a danger of a fire starting?