Wood can deteriorate from long term exposure to heat and that can cause it to ignite at lower temperature. I don't recall the time or temperature required but it was covered in a fire science class.
If you decide to go with wood, only ever use it in a location where it will not set your house, garage, car etc on fire should it light up. For that matter, I always keep burning stuff away from the house and garage and cars. I don't want to be one mistake away from burning my house down.
citation: from http://www.inspectionnews.net/home_i...ace-flues.html
"Wood exposed to heat, such as wood framing too close to a metal chimney in a building, is chemically transformed in an important way: its ignition point or combustion point is lowered
- the wood can actually catch fire at a lower temperature. This means that by pyrolosis, wood and some other combustibles found in buildings are chemically changed by exposure to even relatively low but warm temperatures over time." (emphasis mine)