Muffle kilns were the most common kiln. Certain kilns were renowned over others and prices depended on the reputation of the kiln. By going beyond kiln drying wood, heat treatment may make timber more durable. Wood is air-dried or dried in a purpose built oven (kiln). Fired bricks are burned in a kiln which makes them durable. Bricks were made in an onsite kiln. Cobalt(II) oxide has for centuries used as a coloring agent on kiln fired pottery. The first ever cement kiln is still in Northfleet today in a cardboard factory. An updraft kiln has shelves that trap heat. Historically, pearl ash was created by baking potash in a kiln to remove impurities. Other sources of pollution are exhaust gases from old vehicles, stone crushers and brick kiln. Worthy of note is the last remaining conical kiln of the Fulham Pottery. The barley is dried in a closed kiln fired by natural gas (formerly anthracite coal). The regulations on the potters working in the imperial kiln were severe. Shards recovered from archaeological Eastern Han kiln sites estimated firing temperature ranged from .