It was something I was taught in school that complete and efficient combustion of a carbon-based fuel will produce a blue flame, and incomplete combustion will produce a bright yellow flame. You can easily see this as an acetylene flame cuttter is not yellow when cutting due to the oxygen supplied to it, but an acetylene lamp is very bright as it only has the oxygen in the air to react with. Although the soot will only mark walls with streaky marks if the flame is played directly on the wall, the soot is still being generated when the lamp is simply being used away from any surface. The microscopic particles will eventually settle onto upward facing surfaces. This takes many many years to accumulate. It can be seen in long-established show caves where naked flame lamps were once used, and in many old mines where candles or flame lamps were used by the miners. The amount of soot generated and settled out onto calcite etc may not be that noticeable in many cases, but it will be there to some degree. How important is this as a conservation matter, given that much greater damage has been inflicted over the years by careless trampling and muddy hands?
