Bellows Building

Part of Metalworking

Constructing bellows for forge air supply — the lungs of the forge that make ironworking possible.

Why This Matters

A forge without forced air is just a campfire. Iron melts at 1,538°C, and even forging requires sustained temperatures above 900°C. An open charcoal fire, fed only by natural draft, rarely exceeds 700°C. The difference between a useful forge and an expensive campfire is the bellows — a device that forces air into the fire at controlled rates, raising temperature by 300–500°C and concentrating heat exactly where it is needed.

Without bellows (or a mechanical blower), your community cannot smelt iron from ore, forge-weld two pieces together, or heat metal to the bright yellow necessary for efficient shaping. Every metalworking operation described in the other articles in this series depends on having a reliable, sustainable air supply.

Bellows are also one of the first “machines” a community builds — they convert human effort into controlled pneumatic output, a principle that extends to organ pipes, blast furnaces, and eventually pneumatic tools. Building your first pair of bellows teaches woodworking, leatherwork, and valve design in a single project.

Types of Bellows

Several designs have been used historically. Choose based on available materials and intended use.

Type	Output	Effort	Materials	Best For
Single-chamber	Pulsing	Moderate	Wood, leather	Light forging, brazing
Double-chamber	Near-continuous	Moderate	Wood, leather	General blacksmithing
Great bellows	High volume, continuous	High (two-person)	Heavy timber, leather, counterweights	Smelting, heavy forging
Box bellows (Chinese)	Continuous	Moderate	Wood only	When leather is unavailable
Bag bellows (African)	Pulsing	High	Goat skin, clay nozzle	Minimal materials needed

For a general-purpose blacksmith shop, the double-chamber bellows is the standard choice. It delivers near-continuous airflow with manageable effort by one person.

Building a Double-Chamber Bellows

A double-chamber bellows has an upper and lower chamber, each with a one-way intake valve. When you pump the top board down, the lower chamber compresses and forces air out through the nozzle. Simultaneously, the upper chamber expands and draws in fresh air. On the upstroke, the upper chamber compresses while the lower refills. The result is airflow on both strokes.

Materials

• Boards: 3 boards of hardwood (oak, maple, or similar), each approximately 60 × 30 cm, 2 cm thick. These form the top, middle, and bottom of the bellows.
• Leather: approximately 1.5 m² of thick, flexible leather (cow hide, 2–3 mm thick). Chrome-tanned leather resists heat better, but vegetable-tanned works.
• Nails or tacks: about 80–100 small tacks or nails for securing leather.
• Nozzle: a tapered wooden or clay tube, about 3 cm diameter at the bellows end, tapering to 1.5 cm at the forge end, 15–20 cm long.
• Hinge material: leather strips or iron strap hinges.
• Valve leather: two pieces of thin, supple leather for flap valves, about 10 × 8 cm each.

Board Preparation

1. Shape the boards. Traditional bellows boards are teardrop-shaped — wider at the back (about 30 cm), tapering to about 8 cm at the front where the nozzle attaches. You can use rectangular boards, but the teardrop shape is more efficient and easier to seal with leather.

2. Cut the nozzle hole. In the front (narrow end) of the middle board, bore a hole matching your nozzle’s large end diameter (~3 cm). The nozzle will insert here and deliver air from both chambers.

3. Cut intake holes. In the bottom board and the middle board, cut rectangular openings approximately 8 × 5 cm each. These will be covered by the flap valves. Position them about 10 cm from the back (wide end) of the bellows.

4. Make the flap valves. Cut thin leather pieces about 10 × 8 cm. Nail each piece over its intake hole on the inside of the chamber, attached at the rear edge only. The flap must be free to lift (allowing air in) and fall flat (sealing the hole when air is compressed out). The valve on the bottom board opens upward into the lower chamber. The valve on the middle board opens upward into the upper chamber.

Valve Quality

The valves are the most critical component. They must seal completely when closed and open freely when suction is applied. Use the softest, most supple leather available. Test each valve by blowing and sucking through the hole — it should pass air freely in one direction and block completely in the other.

Assembly

1. Attach leather sides. Cut a long strip of leather wide enough to span the gap between boards when fully opened (typically 15–20 cm wide). Tack this leather around the perimeter of the bottom and middle boards, creating an airtight bag between them — this is the lower chamber. Repeat for the middle and top boards — this is the upper chamber.

2. Fold the leather. The leather sides must accordion-fold to allow the chambers to expand and compress. Create 2–3 pleats in the leather, evenly spaced, so it collapses neatly when boards are pressed together and expands smoothly when pulled apart.

3. Install the nozzle. Insert the tapered nozzle tube through the hole in the middle board. Seal around it with leather wrapping and glue (hide glue works). Both chambers must exhaust through this single nozzle.

4. Add hinges. The three boards hinge together at the back (wide end). Use leather strap hinges — strips of heavy leather nailed across the joint. The top board pivots up and down relative to the middle board. The middle board can be fixed to a frame, or the bottom board can be fixed and the middle board floats.

5. Install the handle. Attach a wooden handle (a stick or shaped grip) to the center of the top board. This is what you pump. In operation, you push down (compressing the upper chamber, expanding the lower) and pull up (compressing the lower, expanding the upper).

Mounting

Mount the bellows on a sturdy frame or table at a comfortable working height, with the nozzle pointing horizontally into the forge through the tuyere (the opening in the forge wall or floor). The nozzle tip should sit at or slightly below the level of the firepot’s air inlet.

A common arrangement fixes the bottom board to a stand, allows the middle board to float on the leather, and attaches the pump handle to the top board. Some designs fix the middle board and allow both top and bottom to move — either works.

The Box Bellows Alternative

If leather is unavailable or in short supply, the Chinese box bellows uses no leather at all. It is a wooden box with a sliding piston inside — essentially a large syringe.

Construction

1. Build a rectangular wooden box, approximately 60 × 20 × 20 cm, open at both ends.
2. Make a piston board that fits snugly inside the box. Seal the edges with cloth, raw wool, or feathers wrapped around the piston edge (the “piston ring”).
3. Cut air inlet holes in both end walls of the box, each covered by a hinged wooden flap valve.
4. Cut an air outlet hole in one side wall (near the center), connected to a tube leading to the forge.

When you push the piston forward, it compresses air in front and expels it through the outlet while drawing fresh air in behind through the rear valve. When you pull back, the reverse occurs. Like the double bellows, it delivers air on both strokes.

Sealing the Piston

Wrap the piston edge with several layers of raw wool or cotton cloth, then pack grease (tallow or lard) around it. The piston should slide smoothly but with noticeable resistance. If air hisses past the piston, add more packing. Replace the packing every few weeks of regular use.

Connecting to the Forge

The Tuyere

The tuyere is the pipe or channel that delivers air from the bellows into the forge fire. It is the critical interface between air supply and fire.

• Material: clay pipe, iron pipe, or copper tube. Clay works for lower-temperature forging but may crack under sustained heat. Iron is best for permanent installations.
• Diameter: 2.5–4 cm internal diameter. Smaller gives higher velocity (hotter, more focused fire), larger gives higher volume (bigger fire, more metal heated at once).
• Position: the tuyere tip should enter the firepot at or slightly below the center of the charcoal mass. Too high and the air blows over the fire. Too low and it gets blocked by clinker and ash.
• Angle: slightly downward (5–10°) so slag and scale do not flow back into the tuyere.

Airflow Control

You need a way to regulate airflow. Options include:

1. Gate valve: a sliding plate in the air path between bellows and tuyere. Slide it partly closed to restrict flow.
2. Bleed valve: a hole in the air pipe between bellows and forge, covered by a sliding plate. Open it to vent excess air and reduce flow to the fire.
3. Pump speed: simply pump slower for less air. This is the simplest control but gives less precision.

For most forging work, pump speed regulation is sufficient. For precise heat control (tempering, brazing, forge welding), a gate valve gives finer control.

Maintenance and Longevity

Bellows are consumable equipment — the leather wears, the valves stiffen, and the wood eventually chars near the nozzle. Proper maintenance extends service life dramatically.

• Leather care: oil the leather sides and valve flaps with neat’s-foot oil or tallow every few weeks. Dry leather cracks and leaks air.
• Valve replacement: when valves stop sealing properly (you can hear air escaping backward), replace the leather flaps immediately. Leaky valves reduce output dramatically.
• Heat protection: keep the nozzle end of the bellows at least 30 cm from the forge opening. A heat shield (a flat stone or metal plate between the bellows and forge) prevents radiant heat from drying and cracking the leather.
• Spark protection: sparks and scale will fly back through the tuyere. A slight downward angle on the tuyere helps, and a spark screen (fine iron mesh) over the bellows nozzle prevents embers from reaching the leather.
• Storage: when not in use, leave the bellows slightly open (do not compress flat). This prevents the leather from sticking together and the folds from taking a permanent set.

Fire Risk

Bellows filled with air and situated near an open forge are a fire hazard. Never leave bellows unattended while the forge is hot. A single ember blown back through the tuyere can ignite the leather. Keep a bucket of water within arm’s reach at all times.

Performance Expectations

A well-built double bellows, pumped by one person at a comfortable rate (~30 strokes per minute), should deliver enough air to:

• Maintain a forge fire at 1000–1100°C continuously
• Heat a 3 cm diameter bar to forging temperature in 2–3 minutes
• Support forge welding temperatures (1200°C+) with vigorous pumping
• Run a small bloomery furnace for ore smelting (with sustained effort)

If your fire is sluggish, check for air leaks first (hold a candle near all seams and joints), then check valve function, then check tuyere position. Nine times out of ten, poor forge performance traces back to the bellows rather than the fuel or fire design.