This article takes a fascinating look at the history of furnaces and the raw materials required to achieve ever-increasing efficiency gains. For millennia, glass melting furnaces were wood fired. Plant and wood ash was also the primary alkali source int he Middle Ages. Glassmakers needed 10/t wood per ton of glass - some estimates are even 10 times higher. In spite of attempts at sustainability such as coppicing, forestry destruction in the UK int he 16th century was so great that the government had to introduce restrictions. Timber for an efficient navy was far more important. The fuel heated the glass batch indirectly by radiation and convection from hot combustion gases. The batch was first fritted at 900 deg C then melted. Wood burning furnaces could at best achieve 1350 deg C. Batch rich in flux (ash) needed lower temperatures but reduced product quality. Potash rich ashes needed higher temperatures than soda ash based materials. Technological developments in the UK meant coal (plus some shale oil) became the next energy vector and this led to the banning of wood-firing in 1615.
