Fire was used at least 1.5 million years ago in stone hearths. Wood was probably the first fuel, but humanity has exploited a much wider range of combustible materials.

Wood burns in two stages. The first stage, with visible flames, produces temperatures of 500-600°F. As the wood burns, it slowly decomposes into charcoal, which leads to the second stage. Charred wood burns without flames but at a far higher temperature: 1,000-1,100°F. Firewood is collected from fallen branches and dead trees where possible, or cut and left to season. It’s easier to gather dry branches from the ground than to cut down living trees. Fresh wood produces 25% less useful heat per pound than dry wood, so it’s burned only as a last resort. In terms of Making Things Burn (p. B433), starting a fire is easiest with dry wood, which is flammable; harder with seasoned wood, which is resistant; and most difficult with green wood, which is highly resistant.

Charcoal – used by TL1 if not earlier – is wood that has been heated in a low-oxygen atmosphere to remove water, tar, and other impurities, reducing it to near-pure carbon. It’s manufactured by stacking wood or putting it in a pit, lighting it, covering the fire, and letting the wood smolder for several days. The result is a lightweight substance (25-40% of original weight) that burns long and hot (charcoal provides nearly twice as much heat per pound as dry wood), and without impurities or flames. The low weight makes it easier to transport from the forests where it’s produced to the foundries and potteries where it’s used; the lack of impurities makes it superior for metallurgy. Charcoal’s main drawback is the quantity of wood needed to produce it – growing metallurgical industries mean widespread deforestation.

Wood and charcoal filled most fuel needs at TL0-4, but several other substances saw use under special circumstances.

Fats, Oils, and Waxes (TL0). Animal fats and vegetable oils have a higher energy density than wood, but don’t burn any hotter, can’t be converted into charcoal, and are much more expensive. They were used almost exclusively for lighting. Animal fat fueled Paleolithic lamps (TL0). Vegetable oils became available after the rise of agriculture (TL1). Vegetable oils cost $2 per pound. Low-quality animal oils and fats that burn with a strong smell or lots of smoke are $4 per pound. Clean-burning waxes are $8 per pound.

Agricultural Waste (TL1). Straw, dried grass, olive pressings, and other flammable debris can be used to build fires. Agricultural waste – which on average provides a bit less energy than wood, and is harder to collect – sees household rather than industrial use, but there’s some speculation that oil-rich olive pressings could be used effectively in firing pottery and processing metals. Waste is usually free; if it must be purchased, it costs $0.25 per pound.

Dung (TL1). The dung of large herbivores contains a high proportion of undigested vegetable matter, and is combustible when dried – although it doesn’t provide as much energy as wood. It burns slowly and evenly, letting cooks leave fires unattended for an hour or two while food cooks. While it’s often more valuable to farmers as fertilizer, it’s a convenient fuel for herdsmen and nomads. Dung is normally free wherever animals are herded; if it must be purchased, it costs $0.25 per pound.

Peat (TL1). Compressed, partially decomposed vegetation mined from wetlands, peat is essentially a precursor of coal. Dried peat blocks provide about as much energy as wood on a pound-for-pound basis, but because peat is harder to extract, it’s typically a second-choice fuel source. $0.60 per pound.

Coal (TL4). Before TL5, small amounts of coal were used where it was available on the surface, but coal-mining wasn’t widely practiced. Coal often contains foul-smelling impurities that make it undesirable, and mining frequently costs more than cutting trees. Some coal-rich regions, notably China, began to experiment more with coal at TL4, but widespread use didn’t come until the Industrial Revolution. If available, coal costs $0.60 per pound.