Colors of Glass
Karen M. Vincent
Minnetrista Director of Collections
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The next time that you’re in the Center Building at Minnetrista, stop in the Heritage Collection Gallery to see glass, and lots of it. There is the sublime—the beautiful paperweight made by St. Clair Glass Company—to the supremely useful—an insulator made by Hemingray Glass Company. What do these two very different pieces of glass have in common? Sand! Yep, sand. Plus a few other ingredients, such as limestone, and soda ash. So what determines the color of glass? Again, sand.
The natural color of glass varies. When you mix and melt the components that make up glass, the resulting color will be some shade of aquamarine. The exact hue will depend upon trace impurities in the sand. The almost pure blue shade of aquamarine is commonly known as Ball blue. Ball Brothers Glass Manufacturing Company, along with other glass companies, used sand from the Hoosier Slide dune that was once located on the southern shore of Lake Michigan in Michigan City, Indiana from 1912 until the late 1930s. Many of the several billion jars made by Ball during this time were this trademark color.
Now, what about clear glass? How is that made? In the early days of glassmaking, manganese dioxide was added to the glass mixture as a de-colorizer. While adding manganese ensured clear glass, it has an interesting property. The glass is perfectly clear when first made, but exposure to sunlight will cause the manganese dioxide to turn a darker shade of amethyst. How deep the color is depends on the amount of de-colorizer is used. Currently, selenium is used to make glass clear. No more sun-colored amethyst.
Flaws in the glass mixture can provide beautiful colors. Swirls or streaks in glass are almost always caused by impurities in the glass. The impurities, usually caused by pieces of iron, like nails, are mixed into the raw ingredients and don’t usually dissolve before the glass is removed from the furnace. Coal has the same effect on glass, turning it a beautiful amber color.
Other colors of glass were created using different minerals. For example, a deep blue can result from the additional of cobalt oxide, while ruby red is created by adding gold chloride. Black glass can be created by adding a mixture of manganese, cobalt and iron. The list of mineral additions goes on and on. It’s a fascinating study of those useful and beautiful pieces of glass.