Potassium and Sodium salts in optics have long story. When the Europe accepted the Christianity , they wanted to make churches and decorate with glass panes. Constantine was the first Roman Emperor who accepted the Christianity for Roman Empire and than he came to Istanbul and built East Roman Empire than the Byzantine Empire. At AD 573 , Byzantians built Hagia Sophia - the biggest building in the world for coming 1000 years - and decorated the walls with Gold leafed , Red , Green and Blue mosaics. The secret of Byzantine mosaics and their long life for the coming 1500 years was potassium salts were produced by Istanbul sea weed catched from Bosphorus. There is only one place you can find that weed is Venice another glass capital. Potassium salts lowers the melting point of glass batch and protects the glass from elements for a long time.
At that time Europe was using sodium salts to lower the melting temperature of glass batch and sodium harmed the glass and all european church glasses start to deteoriates.
Until mid ages , they could not solve the problem and source of potassium. Later they discovered if you burn some mid european alges and put the burned ashes to the glass , they are the potassium source.
Sourcing potassium was a battle for Europe for 1000 years until the mid ages.