'The Valkyrie', Hans Makart, 1877

The alchemy symbol for potassium typically features a rectangle or open box ("goalpost" shape). Potassium is not found as a free element, so alchemists used it in the form of potash, which is potassium carbonate.

There were several different symbols for the metal magnesium. The element itself is not found in pure or native form; rather, the alchemists used it in the form of "magnesia alba," which was magnesium carbonate (MgCO3).

"Philosopher's wool" was zinc oxide, sometimes called nix alba (white snow). There were different alchemy symbols for the metal zinc; some of them resembled the letter "Z."

These are the Egyptian alchemical symbols for the metals. From Lepsius, Metals in Egyptian Inscriptions, 1860.

Although alchemists in different parts of the world worked with many of the same elements, they didn't all use the same symbols. For example, the Egyptian symbols are hieroglyphs.

These are some of the alchemical symbols used by Carl Wilhelm Scheele, a German-Swedish chemist who discovered several elements and other chemical substances. H.T. Scheffer, Chemiske forelasningar, Upsalla, 1775.
One alchemist, Carl Wilhelm Scheele, used his own code. Here's Scheele's "key" for the meanings of the symbols used in his work.