The most common glass in use today, including float (window) and most art (Bullseye, Spectrum, etc.) is soda-lime glass. The basic recipe for soda-lime glass is silica, sodium oxide (soda ash or sodium oxide), and lime.
Potassium-lime (also called potash-lime) glass replaces the sodium-oxide with potash. Potash is typically potassium carbonate derived from wood ashes. Compared with soda-lime glass, potassium-lime glass is harder and more brilliant and has a shorter working range (the temperatures between which the glass changes from solid to molten).