Before 1800 a great deal of coal was left in places as extraction was still primitive. As a result, in the deep Tyneside pits (300 to 1,000 ft deep) only about 40 percent of the coal could be extracted. The use of wooden pit props to support the roof was an innovation first introduced about 1800. The critical factor was circulation of air and control of dangerous explosive gases. At first fires were burned at the bottom of the "upcast" shaft to create air currents and circulate air, but replaced by fans driven by steam engines. Protection for miners came with the invention of the Davy lamp and Geordie lamp, where any firedamp (or methane) burnt harmlessly within the lamp. It was achieved by preventing the combustion spreading from the light chamber to the outside air with either metal gauze or fine tubes, but the illumination from such lamps was very poor. Great efforts were made to develop better safe lamps, such as the Mueseler produced in the Belgian pits near Liège.