Where sun and wind are too weak for ponds, make salt by boiling seawater in wide, shallow pans over a wood fire, topping up as it boils away, then drying the crystals that form. The parent article counts 8-10 lbs of dry wood for each gallon boiled away, which makes fuel the main cost, and pre-concentrating the seawater in the wind before it reaches the pan can cut the water you must boil by about three quarters.
Part of Salt Production & Sourcing
Boiling is the fire-driven way to make salt from seawater, independent of weather but costly in wood.
The Method, As The Parent Gives It
- Collect seawater in barrels.
- Boil it in wide, shallow metal pans over a wood fire.
- As water evaporates, add more seawater to keep the pan full.
- When the brine is highly concentrated (thick, syrupy), reduce the heat.
- Crystals form as the remaining water evaporates.
- Scrape the salt from the pan, drain and dry.
The cost is the fuel. The parent’s rule is 8-10 lbs of dry wood per gallon boiled away. A gallon of seawater weighs about 8.55 lbs, so at the parent’s rule you burn roughly as much wood as the water you boil. That rule fits the physics at a poor-to-modest pan efficiency: a gallon of seawater needs about 10 MJ to heat from cool and boil away (worked out in Yield calculations), and 8-10 lbs of wood at about 15 MJ per kg of dry wood supplies 54-68 MJ, so the pan delivers about 15% to 18.5% of the fire’s heat to the water. The rest goes up the chimney or into the pan walls and the air.
Cutting The Fuel Bill
The parent’s tips:
- Pre-concentrate the brine by running seawater through salt-grass or straw bundles in the wind, so the sun and wind do part of the work before the fire. Starting at 2.74% sodium chloride and reaching 10%, the water to boil per pound of salt falls from (100 − 2.74) ÷ 2.74 = 35.5 to 9, a cut of 1 − 9 ÷ 35.5 = 75%.
- Use pans in series, with waste heat from one warming the next.
- Boil on windy days, which the parent says raises evaporation.
These and other ideas are in Fuel efficiency and large-batch processing.
What You Need
- Wide, shallow pans of a metal that tolerates hot brine (Evaporation pan design)
- A fire pit or firebox that holds the pans, with a chimney or draft
- Barrels to store seawater and a supply of dry wood (Seasoning & Drying)
- Skimmers, scrapers, baskets and a drying place
Method: A Boiling Day
Step 1: Fill the pan, light the fire and bring the brine to a steady boil.
Step 2: Top up with fresh seawater as it boils down, so the pan never dries.
Step 3: Skim scum as it forms.
Step 4: When the brine is thick, cut the heat and let the crystals form; scrape them out and drain.
Step 5: Keep the leftover liquid (bitterns) apart; dry and then purify the salt.
Common Mistakes
- Letting the pan boil dry. The pan can warp or crack, and the last bitter salts bake onto the salt.
- Boiling all the way to dryness. The bitterns end up in the salt.
- Using wet wood. It spends heat boiling its own water.
Safety
Scalding brine, steam and fire
Boiling brine spatters and causes severe scalds, and a pan fire sits under a large pan of hot liquid. Keep children out of the area, never move a full pan by hand, and ventilate the boiling space: a great deal of steam condenses in a closed building and corrodes everything. Use only pans of a metal that cannot leach poison into the brine.
What’s Next
- Learn next: Evaporation pan design — pan size, shape and materials
- Combine with: Solar evaporation ponds — the cheaper route when the climate allows