Double-chamber bellows
Continuous forced air — the component that decides whether you get metal or roasted rock.
Home: Metalworking, Level 2 · Forge, level 1 of 2 in its track. Serves 3 steps across 2 subjects.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
A single-chamber bellows delivers air in gasps: blast, pause, blast. That is fine for a cooking fire and useless for smelting, because the hot zone in a furnace has almost no thermal inertia and cools during every pause.
The fix is old and elegant. Two chambers stacked. The operator pumps the lower one, which does not feed the furnace at all — it feeds the upper one through a one-way valve. The upper chamber is a reservoir with a weighted top board, and it feeds the nozzle, squeezed steadily by the weight regardless of what the operator is doing. Air goes in intermittently and comes out continuously.
Everything here is ordinary carpentry and leatherwork except the valves, which are three pieces of leather and the whole reason the machine works. Build this before the furnace: it is the component most likely to be inadequate, and testing it alone is easy while diagnosing it during a failing smelt is not.
Keyed parts
| 1 | Weighted top board | The weight is the pressure regulator. More weight, more blast pressure, harder pumping. |
| 2 | Upper chamber — the reservoir | Stores a pumped stroke and releases it steadily. This is what makes the blast continuous. |
| 3 | Fixed middle board | Rigidly mounted to the frame. Both chambers work against it. |
| 4 | Transfer valve — opens upward | Lets air from the lower chamber into the reservoir and refuses to let it back. |
| 5 | Lower chamber — the pump | Never connected to the nozzle. Its only job is to refill the reservoir. |
| 6 | Intake valve — opens inward | Draws outside air on the down stroke; seals on the up stroke. |
| 7 | Moving bottom board and handle | Hinged at the nose so the stroke is an arc, not a slide. |
| 8 | Nozzle, taking off the reservoir | Sized to the tuyère. The nozzle must be fed by the reservoir, never by the pump. |
| 9 | Leather sides, pleated and battened | The flexible walls. Tacked between batten strips so no single tack tears out. |
Specifications
| Board size | 1.20 × 0.55 m | Tapered in plan from the hinge to the wide end |
| Stroke | 0.25 m at the handle | Arc, not slide — hinged at the nose |
| Swept volume | ≈ 80 L per stroke | Effective area × stroke, allowing for the taper |
| Delivery | 600–800 L/min | At 8–10 strokes per minute. Enough for a 280 mm bloomery |
| Top board weight | 15–25 kg | Sets the blast pressure. Add stones until the blast is right, then fix them |
| Nozzle bore | 25–30 mm | Match the tuyère. A mismatch wastes most of the pressure |
| Intake valve area | ≥ 2 × nozzle area | Undersized intake makes the pump stroke heavy and slow |
| Transfer valve area | ≥ 2 × nozzle area | Same reasoning; this is the commonest sizing mistake |
| Leather | Supple, 2–3 mm | Chrome-hard or thick sole leather will crack at the pleats |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Boards, 30–40 mm | 3 × 1.2 × 0.55 m | Flat, seasoned, knot-free. Warped boards will not seal against the leather. |
| Leather | ≈ 3 m² | Supple, oiled. Substitute: heavy oiled canvas doubled, or treated hide — both wear faster but work. |
| Battens | ≈ 8 m | Thin strips that clamp the leather to the boards, so load is spread across many tacks. |
| Tacks or small nails | ≈ 250 | Close-spaced, 25–30 mm. Substitute: hardwood pegs through pre-drilled holes. |
| Valve leather | 3 pieces ≈ 150 × 100 mm | Softer than the sides. These are consumables — make spares now. |
| Nozzle | 1 | Fired clay or sheet metal, tapering to bore. |
| Hinge pin and straps | 1 set | Metal if available; a greased hardwood pin in a hardwood block works. |
| Weights | 15–25 kg | Stones, sandbags or a timber block. Must be secured so they cannot slide off. |
| Frame timber | ≈ 6 m | The middle board must be held absolutely rigid — the frame does that. |
| Tallow or oil | small | Dressing the leather. Re-dress every few months or it dries, cracks and leaks. |
Tools: Saw and plane · Auger · Knife and leather punch · Hammer · Straightedge
Construction
Make the boards flat
Cut the ports and fit the valves
Mount the middle board rigidly
Fit the hinge at the nose
Fit the leather sides
Fit the nozzle to the upper chamber
Load the weights and test on a fire
Find the leaks
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Board flatness | no light under a straightedge | A hollow board leaks along the whole tack line and cannot be sealed afterwards. |
| Valve overlap | ≥ 20 mm all round | Leather curls as it dries; a marginal flap starts leaking within weeks. |
| Tack spacing | 25–30 mm | Wider spacing lets the leather lift between tacks under pressure. |
| Leather slack at full stroke | always slack | Taut leather tears at the fold, and it always tears at the worst moment. |
| Nozzle to tuyère bore | matched | A step change in bore throws away pressure right at the point of use. |
How this fails
How you know it worked
Scaling and variants
Smaller: a pair of bag bellows — two leather bags worked alternately by one person, one in each hand — needs no boards and no frame and gives a near-continuous blast. It is the right choice for a forge and for a first small smelt, and it can be made in a day.
Larger: beyond about a 400 mm furnace bore, human pumping cannot keep up regardless of design, and the answer is a water wheel driving the bellows through a cam and a lever. That is the historical step that made large furnaces possible, and it is the reason ironworks were sited on streams rather than on ore.
Two operators: the simplest capacity increase is a second bellows onto the same tuyère through a Y-piece, worked out of phase. Cheaper and far more robust than one large machine, and if one fails mid-smelt you still have air.