Another traditional means of stump reduction was to set it ablaze, usually using petroleum-based accelerants or charcoal briquets. Stumps must be dried before they will sustain ignition. Other woody parts of the tree which are already dry can be used to burn for heating the stump, driving off the water, and igniting the stump. Stumps are in contact with moist soil, have limited surface area open to the air, and are covered with bark, all of which slows drying. Petroleum products, resin soaked wood, torches, charcoal briquets, or other external heat sources can be used to ignite and sustain stump burning. Partial excavation or extraction can aid in drying the stump and allow enough oxygen to reach applied fuels to burn the stump. Extensive large-bore drilling into stumps is cited as essential by many stump removal products. Burning rapidly breaks apart wood bonds, releasing energy. The parts of the stump which do not burn will be covered with a charred layer, or be partially burned to charcoal, which slows the decay of these residual parts. Burning can be assisted by products which either kill and dry the stump faster (salts), or make the wood more flammable (diffusible chemical fire accelerant). There have been a number of stump removal products marketed that are a burning pre-treatment. Note in many parts of the nation burning stumps, smoke generation, and accelerant products are not allowed for stump removal.