In January, 1980, the History Museum at the Museums at Stony Brook was the scene of a puff-back, an emission of uncombusted fuel oil and soot, caused by a break-down of the heat exchanger of one of its two oil-fired hot air furnaces. All surfaces in the gallery, storage and work areas, as well as approximately 15,660 collection items, were covered with a thin layer of contaminating particulate matter. The phenomenon of a puff-back is explained as well as its causes and effects. Precautions that can be taken to prevent this problem are stated; the various conservation techniques necessary to repair or minimize the hazard are reviewed. Disasters involving soot are both possible and probable, but there has been little research thus far on this problem and few individuals and centers are equipped to deal with it.