Firing through the temperature range

Table of contents:

There are different requirements, and so different firing styles, as one goes from a cold kiln to top temperature.

Phase A · Cold start → 500 °C

Fuel strategy: Small, split wood - maximum surface area. Moisture evaporation dominates early, so dry wood is very important. Avoid large logs.
There is no real draw to the kiln in this temperature range, so in many kiln designs you start the fire right at the entrance to the firebox by the air holes, and the amount of available air limits how fast you can fire. A smaller, hotter, more intense fire is better than a larger, smouldering one.
Combustion quality: Worst phase. Cold, damp kiln walls, low draft, high CO and VOC risk regardless of wood type.
Softwood advantage: Ignites faster, achieves higher flame temp earlier due to resin content. Better for this phase.
Emissions: CO highest here. PM and PAH elevated. Ignition batch can be 4× higher PAH than steady state.
Fire vs. kiln ΔT: Run fire as hot as possible. Low kiln T means poor radiation return; you need convection. Hotter fire → faster climb, cleaner combustion.
Efficiency: Necessarily poor - kiln mass absorbing heat, no insulation benefit yet. This phase uses more fuel per °C gained than any other.

Phase B · 500 - 800°C (climbing)

Fuel strategy: Moderate log size. Still benefit from split wood. Stoking rhythm matters — avoid temperature dips.
Combustion quality: Improving. At 600 °C kiln/flue, VOCs begin to auto-combust in the kiln atmosphere. CO/CO₂ ratio drops.
Softwood vs. hardwood: Softwood still climbs faster; hardwood gives more steady heat. Switch toward hardwood if holding at 700–800°C for any time.
Emissions: CO falling; PM still elevated if fire is unsteady; NOx starting to rise (higher T, more ember phase).
Fire vs. kiln ΔT: A larger differential (200–300°C) still accelerates climb efficiently. Radiation transfer beginning to matter more.
Efficiency: Improving. Kiln mass now thermally saturated to 500°C, heat going into temperature rise of ware and atmosphere.

Phase C · 800 - 1000 °C (mid-high fire)

Fuel strategy: Dense hardwood logs increasingly useful for sustained heat. Temperature maintenance without huge stoking bursts.
Combustion quality: Good. Hot kiln acts as secondary combustion chamber - VOCs and CO largely oxidised before leaving. Near-complete combustion achievable.
Softwood vs. hardwood: Hardwood's advantage becomes clearer - slower, longer-burning logs maintain temperature between stokes. Softwood needs more frequent loading.
Emissions: CO low. PM lower. NOx peaks in ember phase. PAH effectively destroyed by hot kiln atmosphere. Flue gas dominated by CO₂, H₂O, N₂.
Fire vs. kiln ΔT: Reduce ΔT. Kiln radiation is now intense; a 100°C differential transfers as much heat as a 300 °C differential did at 500 °C. Excess fire = excess flue loss.
Efficiency: Best phase. Radiant heat transfer dominates; well-insulated kiln retains heat well. Optimal wood consumption per °C.

Phase D · 1000 - 1300°C (peak / hold)

Fuel strategy: Dense hardwood. Modest, controlled stokes. Damper adjustment critical — too much draft = flue loss; too little = CO spike and temperature stall.
Combustion quality: Excellent in principle. Kiln temperature alone oxidises most pollutants. Risk is over-stoking causing oxygen starvation (reduction atmosphere).
Softwood vs. hardwood: Hardwood strongly preferred for holds. Softwood's fast burn makes precise temperature control difficult; overshoots more likely.
Emissions: CO can spike if over-stoking creates reduction. Otherwise lowest CO and PM of any phase. NOx remains elevated from ember combustion.
Fire vs. kiln ΔT: Keep small - 50 - 100 °C sufficient to maintain temperature and compensate for heat loss. T⁴ radiation means the kiln is now a powerful heat source in its own right.
Efficiency: High but highly sensitive to damper control. Main losses are flue gas (if over-fired) or incomplete combustion (if under-aired). Correct O₂ in flue gas (10–13%) is the target.

Overall

To keep emissions down, the key factor is keeping the fire hot enough to burn off all the volatiles, which is a challenge below about 700 °C. Above that, the firebox and ware chamber are generally hot enough for that to happen, and the kiln will burn cleanly so long as it is kept in oxidation.
Wood type is a secondary factor. Softwood helps by giving off a bit more heat and burning faster at low temperatures, whereas at higher temperatures hardwood gives less frequent stoking and so a steadier set of burning conditions.