Brief description

A device constructed to furnish a strong blast of air. It is the lungs of the forge, essential for reaching temperatures high enough to work or melt metal.

Use / Function

Its primary function is air supply.

• Combustion Boost: Delivers a concentrated stream of oxygen to the fuel (charcoal/coal), drastically increasing the burn rate and temperature.
• Ventilation: Can be used to clear smoke or fumes from mine shafts (historically).

Operating principle

The bellows relies on volume displacement and valves.

• Intake: When the handles are pulled apart, the internal volume increases, creating low pressure. Air rushes in through a one-way flap valve.
• Exhaust: When the handles are pushed together, the volume decreases, creating high pressure. The intake valve closes, and air is forced out through the nozzle (tuyere).

How to create it

Simple Bag Bellows (Primitive)

1. Bag: Use a whole animal skin (goat/sheep) or sew a leather bag.
2. Nozzle: Attach a pipe (wood/bamboo/clay) to one leg or a specific opening.
3. Operation: Lift the bag to fill with air (open top), then close and press down to expel air.

Double-Lung Bellows (Standard Blacksmith)

1. Boards: Cut two tear-drop shaped wooden boards.
2. Leather: Nail flexible leather around the edges to connect the boards, creating an accordion-like chamber.
3. Valve: Cut a hole in the bottom board and cover it with a leather flap on the inside. This allows air in but not out.
4. Nozzle: Attach a pipe to the narrow end.

Box Bellows (Piston)

1. Box: Build a rectangular wooden box.
2. Piston: Create a tight-fitting plunger wrapped in fur or feathers (to seal against the walls).
3. Valves: Use flap valves to direct airflow.
4. Efficiency: Often double-acting (pumps air on both push and pull strokes), providing a constant blast.

Materials needed

• Essential:
  • Chamber: Wood or Leather.
  • Nozzle: Metal pipe or fire-hardened clay to resist the heat near the fire.
  • Seal: Fat/Grease to keep leather flexible and airtight.

Variants and improvements

• Great Bellows: Massive, lever-operated bellows powered by a water wheel for blast furnaces.
• Rotary Blower: A mechanical fan (modern) that provides a steady stream of air with a hand crank.

Limits and risks

• Backfire: If the handle is released suddenly or the valve fails, hot gas/flame can be sucked back into the bellows, causing an explosion or fire.
• Maintenance: Leather must be kept oiled to prevent cracking.
• Fatigue: Operating bellows by hand for hours is exhausting work (often the job of an apprentice).

Related Knowledge

• Forging