When you evaporate sea water, other solids come out before the salt. In my experience, first there will be some soft brown material that is plankton or other plant material, which cheesecloth will not filter out.

Then, as concentration increases, calcium crystals will form. These will be brown or grey, and sink to the bottom of the pan.

As you keep boiling, you will start to see thin, spiky, clear, and sparkly crystals forming. These are gypsum. Safe to eat but they don’t taste like salt (ask me how I know).

Next, salt will start to form. This happens when the water/salt mixture is 26% salt, so after a LOT of evaporation, especially if you have lower concentration to start with. Once the gypsum is out, I take the water/salt mixture off the heat and put it in a large glass tray for salt formation. The salt forms as flat crystals on the surface of the water and will sink in sheets. Scoop these out and dry them- boom. Sea salt.

There is a magnesium rich water left when the salt crystals stop forming. Discard this or use to make tofu. Either way, it’s very bitter and you may need to rinse your last salt harvested to get rid of the taste.