Twin Bellows Blast

Two hinged-board bellows rocked by one beam feed a charcoal shaft furnace: the stroke rate sets how much air goes in, and working two bags in turn keeps it steady.

Tell-tale flap: swings with the blast in the ductLeather bags: one fills while the other blowsShaded band on the flap's scale: the target air bandTwo hinged-board bellows, boards about 60 × 40 cm with a 30 cm stroke, each moving roughly 30–40 L a stroke; the guide gives about 400 L/min for one bellows at 12 strokes a minute, against roughly 300–500 L/min for a small bloomery shaft furnace (25–30 cm bore) and perhaps 100–200 L/min for a small crucible furnace. The stroke rate here is the lever's total, so one bag or two give the same mean air; what changes is the gap. With one bag the blast stops while it refills and the bed's glow sags between strokes; worked alternately, the bags hand the blast to each other and it stays steady. The tuyère enters about 20 cm above the furnace base, angled down, so the air passes through the charcoal, not over it. The guide calls these airflow figures order-of-magnitude estimates. The tell-tale flap and its scale are a bench-model instrument; the flap's swing law, how fast the bed follows the air and the steady/pulsing test are illustrative, and the glow shows air reaching the bed, not a temperature. Charcoal makes carbon monoxide: never burn it inside a sleeping space or any enclosed structure, and a slit plank is not eye protection.

What you're looking at

This is a hand-worked blowing rig of the kind used to make a charcoal fire hot enough to smelt iron or melt metal. On the left, two leather bags sit between hinged wooden boards on a trestle, and a see-saw beam on a post above them lifts one top board while it presses the other down; you can grab the grip at either end of the beam. Each bag blows through a pipe into a clay Y-shaped junction in the middle, where a small tell-tale flap behind a window swings with the air, and from there a clay nozzle (the tuyère) runs down into the furnace on the right, drawn cut in half so you can see the charcoal glowing at the nozzle's mouth. The round knob on the base in front of the bellows sets the stroke rate, and a lynch pin on the rod of the nearer bag lets you unhook that bag.

What it means

The guide's boards are about 60 × 40 cm with a 30 cm stroke, which moves roughly 30–40 litres of air a stroke, so about 12 strokes a minute gives around 400 L/min. A small bloomery shaft furnace with a 25–30 cm bore needs roughly 300–500 L/min, and a small crucible furnace perhaps 100–200 L/min; the guide calls these order-of-magnitude estimates. The figure keeps the mean air the same for one bag or two at the same stroke rate, and shows what changes: one bag stops blowing while it refills, so the fire cools between strokes, while two bags worked alternately hand the blast to each other. That is why the guide says to use two bellows, and why a furnace whose temperature swings wildly points to a single pulsing bellows.

Try this

  1. Turn the knob by dragging sideways across it, or click the figure and press the arrow keys (Shift moves in steps of 4, Home jumps to 4, End jumps to 20). The beam rocks faster or slower, the flap swings further or less, the glow at the tuyère brightens or dims, and the readout's strokes per minute and L/min change. Air delivered is simply litres per stroke times strokes per minute, so the rate is how you match the blast to the furnace.
  2. Click the ring of the lynch pin on the nearer bag's rod, or press Space or Enter. The pin pulls out, that bag's rod folds up and the bag rests shut; the flap now flicks and drops back each stroke, the glow sags between strokes, and the readout says 1 bellows and pulsing. With one bag the blast stops every time it refills, even though the average air is unchanged.
  3. Press B. The shaded band on the flap's scale moves, and the readout switches between the bloomery band and the crucible band, saying whether the air is in, below, above or at the edge of it. A crucible furnace needs far less air than a bloomery, so the same rate can be too little for one and too much for the other.
  4. Drag either grip at the end of the beam up and down to rock it yourself. The bags open and close under your hand, the flap and glow follow your strokes, and after about two seconds the readout shows your estimated rate, such as ≈12 strokes/min by hand; hold the beam still and it reads no blast. Press Escape to hand the beam back. It shows how much the blast depends on keeping an even, unbroken rhythm.

Reading the readout

The first line gives the mode (1 bellows, or 2 bellows alternating) and the stroke rate in strokes per minute; total means both bags' strokes counted together, and by hand it shows an estimate marked ≈. The second line multiplies the guide's 30–40 litres a stroke by that rate to give a range in litres per minute, then says whether that range is in, below, above or at the edge of the chosen bloomery or crucible band. The last word, steady or pulsing, says whether the air reaching the fire holds up between strokes or drops away; that test is an illustrative rule of the drawing, not a furnace measurement.

What the model uses

  • A small shaft furnace of 25–30 cm internal bore needs roughly 300–500 litres of air per minute to run a bloomery smelt; a small crucible furnace needs perhaps 100–200 L/min (guide text, uncited; the guide calls its airflow figures legacy order-of-magnitude planning estimates).
  • A pair of boards about 60 × 40 cm hinged at one end, with a stroke of 30 cm, moves roughly 30–40 litres per stroke; one bellows at 12 strokes a minute gives around 400 L/min (guide text, uncited). The drawn wedge sweeps 36 L.
  • Use two bellows worked alternately: a pulsing single bellows lets the fire cool between strokes (guide text, uncited).
  • If a furnace won't reach temperature, insufficient air is the most likely cause, followed by heat loss through uninsulated walls; wood ash between two clay shells is a free insulator (guide text, uncited).
  • Updraft shaft furnace: 0.6–1.5 m tall, internal bore 25–30 cm, walls 10–15 cm thick, tuyère about 20 cm above the base and angled slightly down, so the blast passes through burning charcoal rather than over it (guide text, uncited). The drawn shaft is 72 cm tall with a 28 cm bore and 14 cm walls.
  • Charcoal produces lethal carbon monoxide; odor, flame behavior and apparently adequate ventilation are not exposure tests. (OSHA carbon-monoxide guidance, cited by the guide)
  • Charcoal burning produces carbon monoxide, which is odorless and kills: never burn charcoal, or cool a charcoal pit, inside a sleeping space or any enclosed structure (guide text, Safety section, uncited).
  • A slit plank or mesh is not eye protection for hot work. (OSHA eye and face protection, cited by the guide)

The stroke is idealised as a wedge between flat boards and the lever as rocking at a steady speed with rounded reversals. The bed's 1.5 s lag, the flap's swing law and the steady test (the air reaching the bed never falling below 60% of its mean) are illustrative, so "steady" and "pulsing" compare two ways of working rather than measure a furnace. This is a code-drawn explanatory model for orientation, not an equipment drawing or a safety procedure; follow the guide's warnings.