Heat a clean, dense, dry stone or brick in the cooking fire, and when you move the pot to the haybox, set the hot stone in the bottom of the nest beneath the pot; the overview says this extends the time above 60 degrees by 1 to 2 hours. The stone must sit on a non-combustible base, because a stone from a fire is far hotter than hay, paper or polystyrene can stand.
Part of Retained Heat Cooking
A hot stone is a second heat store beside the pot, adding to the food’s own reserve at a cost of care and a heavy, hot object.
How Much Heat Does A Stone Add?
Specific heat of brick and most dense rocks is about 0.84 kJ/kg·K (typical value, not from the overview). Take a 2 kg stone, heated to an assumed 200 °C, giving up heat as it cools to 75 °C:
- Stone: 2 kg × 0.84 kJ/kg·K × (200 − 75) K = 210 kJ.
- A 5 kg pot of water cooling from 100 to 75 °C: 5 × 4.186 × 25 = 523 kJ.
- So the stone adds 210 ÷ 523 ≈ 40 percent to the pot’s own reserve.
A stone from a fire may be much hotter than 200 degrees, and the figure depends on how well heat moves from stone to pot.
Cross-Check Against The Overview’s 1 To 2 Hours
Using the cooling model from the Time Limits page (room 20 °C, a box reaching 75 °C after 4 to 6 hours, time constants of 10.7 and 16.0 hours), the stone’s 210 kJ raises a 5 kg pot’s equivalent starting temperature by 210 ÷ (5 × 4.186) = 10 K, from 100 to about 110 °C. The cooling time to 60 °C becomes τ × ln(90 ÷ 40) = 0.811 τ = 8.7 or 13.0 hours, against 7.4 or 11.1 hours without it, a gain of 1.3 to 1.9 hours. That is in line with the overview’s 1 to 2 hours. It is a rough model with assumed inputs.
Method
Step 1: Choose the stone. Use dense, dry stone or fired brick. See Hot Stone Method. The overview warns that wet or porous river stones can explode when heated.
Step 2: Heat it in the fire while the food boils.
Step 3: Lift it with thick leather gloves or tongs onto a metal tray or a bed of dry sand or ash in the bottom of the nest.
Step 4: Seat the pot on it, close up and note the time.
Common Mistakes
- Setting a hot stone straight onto hay, paper or polystyrene. It scorches or melts them.
- Using a damp stone. It may burst.
- Counting on the stone to rescue a poor box.
Safety
Burns, bursting stones and fire in the box
A stone from a fire can cause severe burns and, if damp or porous, can burst and throw hot fragments. Hay, newspaper, leaves, sawdust and polystyrene burn or melt on contact with a hot stone. Use a non-combustible base and open the box slowly in the first hour.
What’s Next
- Learn next: Vacuum Insulation (Salvaged Thermos) — a different upgrade
- Combine with: Stone Boiling — using hot stones for cooking itself