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What are bitterns after salt has crystallized from brine?

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Cluster of translucent, white salt crystals with sharp cubic shapes viewed under a microscope at high magnification against a dark background. — salt Salt crystal magnified.

Bitterns are the highly concentrated, bitter-tasting liquid solutions that remain after sodium chloride (table salt) has been removed from brines or seawater through evaporation and crystallization.12

This final solution typically reaches a specific gravity of 1.25–1.26. It contains a concentrated mixture of various chemicals originally present in the source water, including:2

Magnesium compounds, such as magnesium chloride and magnesium sulfate (Epsom salts)12
Calcium chlorides and sulfates1
Bromides and iodides1

Because of this composition, bitterns serve as a valuable commercial source for the production of potash, bromine, and various magnesium-based chemicals.12