A salt evaporation pan is wide and shallow, with a large base over the fire and a large surface for steam to leave, made of a metal that hot brine does not dissolve into the salt, which rules out lead, zinc, galvanised steel and cadmium-plated metal. At a pan efficiency of 15%, one kilogram of dry wood boils off about one kilogram of water, and at 30% about two, so the pan must be sized from a measured test boil, not from a formula.
Part of Salt Production & Sourcing
An evaporation pan is the large, shallow vessel in which brine or seawater is boiled down to salt, sized and built from metal that survives hot brine without contaminating it.
Why Wide And Shallow
Boiling needs heat to get in through the base and steam to get out through the surface. A deep pot has little base for its volume and a small surface; a wide, shallow pan has the most of both. The parent specifies wide, shallow metal pans for exactly this reason. Shallow depth also lets the brine reach crystallising concentration quickly after the last top-up, and makes the crystals easy to rake.
A Rule Of Thumb From The Latent Heat
The useful heat to boil water away is the latent heat, 2.257 MJ/kg (recalled). Dry wood gives about 15 MJ/kg (recalled), so a kilogram of wood burned at pan efficiency η delivers 15 × η MJ:
- at 15%: 15 × 0.15 = 2.25 MJ, which is 2.25 ÷ 2.257 = about 1.0 kg of water boiled off
- at 30%: 15 × 0.30 = 4.5 MJ, about 2.0 kg
These ignore the heat to bring cold feed to the boil, which adds about 16% to the fuel for seawater (561 ÷ 3,605) (Yield calculations). The rule explains why the parent’s 8-10 lbs of wood per gallon (8.55 lbs of seawater) is consistent: about one pound of wood for each pound of water at the lower efficiency.
What a formula cannot give you is the pan area. How fast a given pan boils depends on the fire, the draft, the thickness and conductivity of the metal and the scale on its base. Measure it: boil a known quantity of water in the pan, weigh the wood and note the time, and scale from that.
Materials: What Not To Use
Hot brine is corrosive and dissolves some metals that you then eat.
- Never lead, which was used for salt pans historically and poisoned the product, and never solder or sheet that may contain lead.
- Never zinc or galvanised steel, whose coating dissolves in hot brine; galvanised metal also gives off harmful fumes when heated.
- Never cadmium-plated metal.
- Avoid aluminium, which hot chloride brine attacks.
- Copper and brass dissolve copper into the brine and make verdigris.
- Iron and carbon steel are usable and common, but rust, discolour the salt and wear away; expect to replace the base over time.
- Stainless steel resists better but can pit in hot chloride brine over time.
- Fired clay has been used historically, but cracks under direct heat.
If you do not know a metal, do not use it for pans.
Scale
Calcium sulfate separates as the brine concentrates and bakes onto the hot base as scale, insulating it and cutting heat transfer (Removing impurities and bitter minerals). Plan to scrape it off between batches, or to use an early settling or first-stage pan to take the scale, with the clean brine passing on.
What You Need
- A metal sheet or pan of a known, safe metal
- A firebox and chimney matched to the pan
- A scale for wood and a timer to measure efficiency
- Scrapers, rakes and baskets
Method: Size And Test A Pan
Step 1: Choose the metal. Reject anything you cannot identify.
Step 2: Build the pan wide and shallow and set it so the flame spreads under its whole base.
Step 3: Do a water test: boil a known quantity for a known time on a weighed amount of wood. Compute kg of water per kg of wood.
Step 4: Compare with the 1 to 2 kg of water per kg of wood from the rule above. A result far below means the pan is losing too much heat, or the fire is badly managed.
Step 5: Scale the pan count and the wood supply to the water that must be boiled.
Common Mistakes
- Using a salvaged pan of unknown metal.
- Making it deep. The surface is too small and the base too far from the top.
- Never descaling. The base overheats and warps.
- Letting it boil dry, which can crack or burn through the base.
Safety
A salt pan can poison the salt, and scald the operator
Lead, zinc, galvanised and cadmium-plated metals release poisons into hot brine and into the salt. Hot brine scalds; a pan at a rolling boil spatters well beyond its edge. Keep the fire box and chimney clear of overhanging roof timbers, and keep children out of the building.
What’s Next
- Learn next: Fuel efficiency and large-batch processing — squeezing more water out of each load of wood
- Combine with: Brine Boiling — the pan in use