Heat: the fire and the bar in its colour heatsParts already in placeA side-blast charcoal hearth 1.1 × 0.95 m, 0.75 m high, blown by a great double bellows 1.6 m long. Pulling the rocker down lifts the bottom board and drives air through a flap valve into the upper chamber; its weighted top board presses the air out of the nozzle while the bottom board falls and refills, so the blast stays nearly steady. Pure iron forge-welds at nearly white heat, about 1,400–1,500 °C; charcoal under a forced blast reaches about 1,400 °C in a furnace. The model is a simple one, stated here: the fire runs from about 800 °C with the bellows still to 1,500 °C at a steady blast, and the bar follows it exponentially, with a time constant of 20 s in the fire, 100 s on the anvil and 6 s in the tub; from cold that is about 54 s to welding heat in a full fire. The heat runs 10 times fast on the page; the lever is drawn at an easy working pace. Glow starts near 525 °C and the colour names are the usual eye estimates. This is a guide to how the parts go together, not engineering drawings. Hot iron and scale burn at a touch and look no different from cold iron below about 500 °C: pick nothing up bare-handed. Wear eye protection against sparks and flying scale. Burning charcoal gives off carbon monoxide, which you cannot smell: forge only under a working hood and flue or in the open air, never in a closed room.