Choose a heavy pot with a tight-fitting lid, fill it at least two-thirds full, and put it in an outer container at least 15 cm larger than the pot in every direction. For a pot about 20 cm across and 17 cm tall, that means a box roughly 50 cm wide and 47 cm tall, near 117 litres, so the insulation is many times the volume of the food.
Part of Retained Heat Cooking
The pot sets how much heat is stored. The outer container sets how much insulation can surround it.
The Pot Is The Store
The overview’s pot requirements:
- A tight-fitting lid. Loose lids lose steam and heat.
- Heavy pots (cast iron, thick stainless steel) retain heat better.
- Fill at least two-thirds full. More thermal mass cools more slowly.
- Metal works better than ceramic, since metal transfers heat to the food faster in the initial boil.
The Box Is The Shell
Outer container options from the overview: wooden crate or box with lid; cardboard box lined with foil; plastic cooler or ice chest (excellent if salvageable); woven basket with lid; a hole dug in the ground (the original earth oven); a metal drum or bucket. It must be at least 15 cm larger than the pot in all directions.
Worked Example: How Big Is the Box?
Take a pot 20 cm across and 17 cm tall (assumed), which holds π × 10² × 17 = 5,340 cm³ ≈ 5.3 litres.
- Width: 20 + 15 + 15 = 50 cm.
- Height: 17 + 15 (below) + 15 (above) = 47 cm.
- Box volume: 50 × 50 × 47 = 117,500 cm³ ≈ 117 litres.
- Insulation volume: 117 − 5.3 ≈ 112 litres, about 21 times the pot’s volume.
So the box is far bigger than the pot. A smaller box means thinner insulation and a shorter hold.
Common Mistakes
- Using a thin pot. It has little mass to store heat.
- A box too tight. Less insulation, faster cooling.
- A box too large for the pot. A pot floating in a big cavity is cooled by air gaps.
Safety
Hot pot against the wrong box
A boiling pot touching a cardboard box, a thin plastic cooler or loose paper can scorch, melt or ignite it. Insulation must separate them. Lift full pots with both hands and mitts to avoid scalds.
What’s Next
- Learn next: Insulation Materials Ranked — filling the shell
- Combine with: Lid Sealing & Nest Construction — fitting the pot