A permanent rocket stove can be built entirely from cob, a mix of clay, sand and straw in the ratio 1 : 3 : 0.5, shaped around a cardboard or newspaper form with walls at least 8 cm thick. It must dry slowly over 3 to 5 days to avoid cracking, and its mass keeps radiating warmth after the fire dies. Light the first fires small, because wet cob can crack or spall.
Part of Rocket Stove Design
Cob, a mix of clay, sand and straw, makes a permanent stove whose thick walls store heat and keep radiating it after the fire. It must dry slowly before it is fired.
Mass Stores Heat; It Also Slows The Start
The overview’s build:
- Mix cob from clay, sand and straw 1 : 3 : 0.5. The overview does not say whether by volume or weight; measure each batch the same way and test a small block.
- Form the L-shaped combustion chamber around a cardboard or newspaper form.
- Build walls at least 8 cm thick.
- Let it dry slowly, 3 to 5 days, to prevent cracking.
- The mass continues radiating warmth after the fire dies.
- It can be integrated into an earthen bench (a rocket mass heater); see Rocket Mass Heater.
Dense walls soak up heat the burning gases need, so a cob chamber starts slower and burns less cleanly than a lightly insulated one. Line the chamber with a lighter insulator, or keep the cob outside an insulating lining, if clean burning matters more than mass; see Insulated Burn Chamber.
Method
Step 1: Prepare the form. Make the cardboard or newspaper L-shaped form to the burn tunnel’s size.
Step 2: Build up. Press cob around the form in layers, keeping every wall at least 8 cm thick.
Step 3: Dry. Leave it in shade out of rain and wind for 3 to 5 days. Fine surface cracks can be pressed closed; deep ones mean it dried too fast.
Step 4: First fires. Burn small, dry fires first; the form burns away. Wet cob heated hard can crack or throw off pieces.
Common Mistakes
- Drying in direct sun or wind. It cracks.
- Thin walls. Under 8 cm they crack and leak.
- Cooking before the stove is dry. Steam and cracks follow.
Safety
Cracks leak smoke
A cracked cob stove or bench leaks smoke and carbon monoxide, which has no smell and whose first signs (headache, drowsiness) feel like ordinary tiredness. Seal cracks with fresh cob after drying and fire it outdoors, or with the exhaust piped outside. See Carbon Monoxide Risks.
What’s Next
- Learn next: Rocket Mass Heater — using the mass for heat
- Combine with: Cob Building — the material in detail