A crucible furnace preheating burner fed with propane consumes on the order of 7.96 kg/hr for a 25 kg charge raised across 920 K in 25 minutes, per the April 2026 Firgelli heat-input calculator (assumed 8.93% overall efficiency) [S2]. Propane's lower heating value of 46.4 MJ/kg is the same constant the calculator uses to back out kg/hr from a stated BTU/hr fuel input [S2].
Natural gas and propane are functionally interchangeable in a refractory-lined chamber, but the burner orifice, jet sizing, and feed pressure are not the same: hobby-scale propane foundry burners run at 10-15 PSI, while modern residential-style natural gas regulators deliver 2-3 PSI downstream of the meter, requiring a re-ported jet to keep stoichiometry valid [S5].
Fuel properties: heating value, density, and flame behaviour
Propane (C3H8) carries roughly 46.4 MJ/kg and about 86 MJ/m³ at 15 °C, 1 atm; pipeline natural gas (mostly CH4) carries about 50 MJ/kg and 35-38 MJ/m³ depending on composition [S2][S6]. On a per-kg basis propane is close, on a per-m³ basis propane is denser in energy, and that ratio is what drives the burner-orifice change-out rule when converting a forge from propane to natural gas [S5].
Propane vaporises at -42 °C and is stored as a liquid under its own vapor pressure (about 110 PSI at 20 °C, dropping with temperature), so the same bottle feeds a portable crucible furnace without pumps, regulators aside [S3][S5]. Natural gas arrives at the meter pre-pressurised in the 2-3 PSI range, and downstream regulators at the appliance step that down to roughly 0.7 PSI (around 19 inches water column) for the burner manifold [S5].
Burner hardware: orifice sizing, pressure, and preheat geometry
Switching fuels without changing the jet is the most common field failure: a propane orifice passes too much volume of natural gas at the same manifold pressure, drives the mixture lean, and the flame lifts off the port or fails to ignite reliably [S5]. The community rule of thumb is to upsize the natural gas jet roughly in proportion to the energy density ratio, then trim by flue-gas colour and melt rate [S5].
Tangential burner entries on a refractory-lined chamber direct hot gas around the outside of the crucible; the calculator in [S2] uses that geometry with an 8.93% assumed overall efficiency and reaches 60 kg/hr melt rate at 102.6 kW burner input for a 25 kg aluminium-style charge, which is the same order of magnitude hobby forges report for 5-10 kg melts [S2][S3]. For LPG burners in this geometry, regulator feed is held at 10-15 PSI before the air-mixer; natural gas burners in the same geometry run at 0.7 PSI at the manifold after the appliance regulator [S5].
Cost per BTU: where the fuel choice actually swings
Per-BTU cost is the only number most buyers compare. In a 2016 Australian data point, a 45 kg propane bottle cost $118 delivered, which sets the per-kg propane figure at roughly $2.62/kg (or about $0.057/MJ) and, using 46.4 MJ/kg, an effective $56/MWh thermal [S1]. Pipeline natural gas on a U.S. residential tariff in 2026 typically lands at $1.20-$1.80/therm (around $12-$18/MWh thermal), which is roughly 3-5× cheaper per delivered BTU where the main exists [S6][S8].
The crossover rule is: if a natural gas main is already within reach of the furnace pad, pipeline natural gas almost always wins on operating cost; if it isn't, the trenching and meter-set fee typically erases 3-7 years of fuel savings for a small foundry, and propane stays the right answer [S6][S8]. For job-shop shops running a single 25-100 kg gas crucible furnace one or two shifts, propane tank rental remains the lower-capex path [S4][S6].
Preheating the furnace: why fuel choice changes the cold-start sequence
A cold crucible furnace with a fresh refractory lining must be driven up through water-vapour release around 200-300 °C before any serious metal charge goes in, otherwise steam spalling cracks the lining. Propane burners at 10-15 PSI give a hotter, more concentrated flame that lifts chamber temperature faster; natural gas burners at 0.7 PSI manifold pressure give a softer flame and a longer preheat ramp, which can be safer for a new lining but cuts into shift productivity [S3][S5].
For oil-drip or residue-laden chambers, the 2025 hobby-thread guidance recommends forced-air burners plus a preheated furnace to vaporise oil films, which works on either fuel but is easier to stage on propane because the higher pressure gives finer air-fuel ratio control at low fire [S5].
Selection criteria: who propane is for, who natural gas is for
Propane is the right pick for: portable or relocatable foundries without a utility main, schools and small batch shops, low-throughput operations where capital cost matters more than per-BTU cost, and any installation running a tilting gas aluminum melting furnace on a remote pad [S4][S6]. Natural gas is the right pick for: permanent installations with a main at the gate, two-or-three-shift operations, and any site where local gas utility pricing under $1.50/therm is available [S6][S8].
Neither is right for a precious-metal refinery using oil or diesel: those fuels need atomising, not just pressure regulation, and the burner geometry diverges from a gas crucible furnace entirely [S7]. The Firgelli calculator's fixed-property assumptions (cp 0.38 kJ/kg-K, latent heat 200 kJ/kg) are illustrative, not certified, so for any production-cost sign-off a measured burn at the actual alloy is required [S2].
Decision matrix: propane vs natural gas on four criteria
On installed cost, propane wins for any new pad without a main: a 45 kg cylinder plus two-stage regulator is a few hundred dollars and a same-week install, while a natural gas tap, meter set, and trenching routinely runs into four figures before the first BTU is burned [S1][S6]. On per-BTU operating cost, natural gas wins by 3-5× wherever it is piped, against a per-kg propane figure that has tracked $1.50-$3.00/kg in hobby and small-shop data through 2025-2026 [S1][S6][S8].
On portability, propane wins outright: a forklift-moved tank with a regulator is the entire fuel train, and a gas crucible furnace can be relocated in an afternoon; natural gas ties the furnace to a buried service line [S4][S6]. On flame control during preheat, propane at 10-15 PSI gives finer low-fire modulation than natural gas at 0.7 PSI manifold, so propane is typically easier to drive gently through the 200-300 °C steam-out range where too-rapid ramp damages a new lining [S3][S5].
Limitations and failure modes in the field
Converting a propane forge to natural gas without re-jetting gives a lean mixture: visible symptoms are a flame that lifts off the port, lazy ignition at the burner, and aluminium or brass melts that stall 50-100 °C below target because the actual energy input is below the rated burner value [S5]. Converting the other way, natural gas to propane, risks a rich mixture and soot because the propane orifice is too small at 0.7 PSI, so the same advice applies in reverse: change the jet, not just the label [S5].
The 8.93% efficiency figure used in the April 2026 calculator is an average for a small gas-fired crucible furnace on aluminium, not a guarantee; real installations will run anywhere from 8% to 25% depending on lining age, charge mass, and whether the burner is also being used as a holding holding furnace [S2][S4]. A cross-check on the per-melt fuel number against an actual bottle or meter reading is the only way to validate the model for a given site [S2].