While some purple sea glass started out as intentionally purple pieces, many pieces of purple or amethyst sea glass were originally clear glass. They changed color over time in a process called solarization, due to the chemical composition of the glass.
All glass is made of silica, but sometimes there are impurities in the sand, such as iron, that can cause a light green discoloration. To offset the discoloration, glassmakers add different ingredients to the molten glass. One of these decolorizers is manganese dioxide, sometimes called “glassmakers’ soap.” Manganese dioxide has been used for thousands of years for this purpose, but new compounds became more popular in the early 20th century. When manganese is exposed to sunlight or UV light, it gradually takes on a light pink or lavender color.
Sea glass isn’t the only glass you can find that has been solarized. Anywhere glass treated with manganese dioxide sits in the sun for enough time, you can find it. Old door knobs, glass left out in the desert sun, and even the windows on early-American houses on Beacon Hill in Boston. In fact, having these violet windows shows that your home has its original windows and is quite prestigious…so much so that some people even include fake violet windows in their homes.
This lamp, shown with some clear and some purple sea glass, was passed down through my family and has definitely spent time in sunny windows since its time as an oil lamp, and now as an electric lamp sitting on the desk in the sunny window of my studio.
So, when you find purple sea glass, you can be confident that it’s probably pretty old. In addition to being smoothed by the waves, it also spent some time sunning on the beach!
You can read more about solarized glass on the Corning Museum of Glass website ›