Making sea salt from saltwater is very easy, although quite time consuming. It's not necessarily cost effective, but it's a lot of fun and a rewarding learning experience.
How much salt are you going to get? I read on one website that the rule of thumb is 5 gallons of seawater to 4 cups of salt, which would mean a 5% salt solution. I did 4 gallon batches, and got between 2.5 and 4 cups per batch, which is a bit of a lower yield. However, my small batch of 2 quarts gave me 1 full cup of salt, which is much higher. So it all depends on the original saltiness of the seawater and the method used to evaporate it, and how careful you are not to lose any salt to the floor in the process (I speak from experience—I lost more than I'd like through careless pouring).
Overall, I processed 16.5 gallons of seawater, and I ended up with over 9 cups of salt. This is a total of about a 3.5% yield by volume of salt from saltwater, but your mileage may vary. I documented my own experience on my blog.
Let's get started!
Step 1: The Gather
The first step is to get yourself a lot of salty seawater. I used 5-gallon buckets with lids, but I've seen others use milk jugs (for smaller batches) and coolers (for larger batches).
Go on a road trip to the nearest saltwater source with your containers. You may want to do some research into the pollution along your coast so you don't end up with dirty water that won't give good salt (and may be very bad for you!). Generally, the farther from civilization, the better.
Gather as much saltwater as you like. The more you get, the longer the process will take, but the more salt you'll end up with.
Step 2: The Filter
Let your seawater sit for a while to allow any sediment and particles to settle.
Get your stockpot out and ready for the boil. Then siphon or scoop water from the top of the container, careful not to disturb the sediment at the bottom, through a filter into your stockpot.
I used a dishcloth in a sieve for my filter, which provided a good fine mesh to get out any sand or other particles.
I scooped out water with a 1-quart measuring cup so I could easily keep track of how large my batches were. I did 4 gallons per batch.
Step 3: The Boil: Method 1
The quickest way to turn your saltwater into salt is by boiling it. But be careful, you don't want to scorch the salt! Be prepared to spend all day in the kitchen with the stove on.
There seem to be several schools of thought on how to evaporate your seawater, so I will go over each method briefly. I will leave it up to you to experiment and develop your own technique.
The first method, which I found described most frequently in my research, uses a constant boil. The danger with this method is how easy it is to scorch your salt. The advantage, though, is that it is the quickest method.
Bring your seawater to a boil. Once about half of the water is evaporated, or when you start seeing the salt form in the water, you'll need to start stirring—infrequently at first, but more and more frequently as more water evaporates until you are continuously stirring. The goal here is to get your salt to the consistency of wet sand.
Note: there can still be water in the pot when the sand reaches the right consistency. This part confused me when I was doing my research, but once I actually did it, it made more sense. Try scooping out the salt with your stirring spoon to check the consistency. When it's like wet sand, thick but still pourable, then you're done.
Remember: Don't scorch the salt!
Step 4: The Boil: Method 2
The second method that I read about involved a much longer process that actually does not involve boiling, but rather simmering. The idea with this method is to evaporate the salt without risk of scorching by using low heat. However, depending on the amount of salt water you have, this could take multiple days with the stove on, and thus will cost more in energy.
With your pot of seawater on the stove, turn on the burner to the lowest heat setting, and let it evaporate slowly. As with the first method, you'll probably need to stir your water once it gets low, although it won't be as necessary as if you were boiling it. And again, you'll want to get your salt to the consistency of wet sand.
Remember: Don't scorch the salt!
Step 5: The Boil: Method 3
After some experimentation, and trial and error, I developed a technique that seems to be a good balance of amount of time spent and avoiding scorching the salt.
I started off with a full boil at high heat, until about half of the water had evaporated. Then I turned the burner down to the medium-high heat. As the water evaporated, I turned the heat lower and lower until I was down to medium-low. This gradual decrease in heat helps prevent scorching without requiring you to stir constantly. It will take longer than boiling the whole time, but it does take less interaction on your part.
And as with the other methods, you want to get your salt to a consistency of wet sand. Don't worry if there is still water in the pot; the only thing you're concerned about is the consistency of the salt itself.
Remember: Don't scorch the salt!
Step 6: The Dry
This is the longest step of the whole process. Once you've boiled down your seawater and your salt is the consistency of wet sand, you'll want to scoop it out into a pan to dry. The larger the pan, the thinner you can spread it, and the sooner it will dry.
If you still have water in the pot (which you probably do), I would suggest trying to pour the water into a separate dish, so the majority of your salt is not sitting in water and can thus dry faster.
You can place your pan(s) by a window so that the sun can speed up the drying process. Depending on how much salt you have, and how wet the salt is, this can take anywhere from a few days to a few weeks. Some of my batches came out too wet (I didn't let them boil down enough) and they're still in the process of drying after two weeks. But a smaller batch that I did was not as wet, and it dried much quicker.
Alternatively, you can place the pan in the oven on the lowest heat (the lowest on my oven is 170 ºF) to speed up evaporation. Some people seem to think that the oven technique changes the makeup of the salt; it seems to me to just affect the way the crystals form. Either way, it's worth experimenting with. Just be careful not to burn the salt.
Step 7: The Final Product
You're done! Now that your salt is dry, you can pour it into spice jars, mason jars, or other containers to store and use. If your salt crystals came out larger, you can put them in a grinder for freshly ground salt when you need it, or grind it all ahead of time before storing it.
I've read that sea salt is better suited for flavoring food after cooking, rather than for the actual cooking, but you can experiment and see what works for you. After all, it's just salt! You can also mix your salt with herbs or spices to make seasoning salts.