The Charcoal Budget of a Small Forge: Fuel per Hardening, the Temperature a Bed and Blast Can Hold, and the Combinations That Never Reach Hardening Heat

A small charcoal forge is usually described by anecdote: a bed of a certain size, a blower turned up until the colour looks right, and a bag of charcoal that lasts an uncertain number of blades. This paper replaces the anecdote with an energy balance. For a 0.25 kg steel blade that must reach 850 °C, the blade itself needs only 121.2 kJ, yet a working forge delivers that energy at an efficiency of 0.49 % on a cold start and 3.26 % per repeat blade in the reference case, because nearly all of the heat leaves with the flue gas and by radiation from the open bed. Across 72 combinations of bed diameter (0.20–0.45 m) and blast rate (2–30 L/s), 47 reach the 850 °C target, 6 reach only the 800 °C minimum, and 19 never reach hardening temperature at all, however long they burn. Whether a combination works is governed by one quantity: blast per unit bed area. Every failing combination sits at or below a superficial air velocity of 0.057 m/s, and every combination that reaches the target sits at or above 0.075 m/s, which gives a practical sizing rule of at least 85 litres of air per second per square metre of bed. In the reference case (0.30 m bed, 10 L/s) the bed holds 1111 °C, the workpiece 969 °C, heat-up from cold takes 9.8 minutes, and one hardening costs 0.855 kg of charcoal (1.03 US dollars in bulk, 2.57 dollars at retail bag prices), while each further blade from the hot bed costs 0.128 kg. The most expensive working combination, a 0.45 m bed at its minimum working blast of 12 L/s, costs 1.883 kg per first hardening. A large bed on a weak blast is the worst purchase of all: a 0.45 m bed at 8 L/s burns 3.56 kg of charcoal per hour at a heat release of 24.40 kW and still leaves the workpiece at 778 °C, below hardening temperature.