The Chemicals Behind the Colors of Antique Glass

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    Glass Ingredients and Clear Glass

    Close-up of candle holders
    Carol Haynes / Getty Images

    Glass, whether pressed or mouth-blown, was made with three primary ingredients: sand, soda (also referred to as potash), and lime. Metallic oxides, such as lead, are added to remove impurities yielding clear glass. Manganese was also used for clarifying glass through 1917. When this variety of clear glass sits in the sun for a long period, such as an insulator on a telegraph pole, it will turn various shades of purple.

    When a clear glass is referred to as “lead crystal,” this indicates that lead oxide was added to the glass during manufacture to improve the quality. In the early 1800s, glass had flint added to it to improve clarity and durability. The term "flint glass" continues to be used to describe an antique glass of good quality made through the early 1900s, even though lead replaced flint as a clarifying additive in the mid-1800s.

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    Blue Glass

    Cobalt blue flint glass
    Morphy Auctions

    Colored glass is made similarly to uncolored glass. Blue antique glass was given its color by adding cobalt oxide to a molten glass mixture of sand, potash, and lime. This additive accounts for the deep blue glass being referred to as cobalt blue. Other shades of blue were achieved by lowering the amount of cobalt oxide added to the glass mixture.

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  • 03 of 06

    Amethyst Glass

    Amethyst glass apothecary jars
    Morphy Auctions

    Amethyst, or purple, glass was given its color by adding manganese oxide to a molten glass mixture of sand, potash, and lime.

    Amethyst glass should not be confused with clear antique glass that has changed color due to sun exposure. This type of clear glass was made with manganese as a clarifying agent, and the sun will turn it various shades of purple if enough time passes. When compared to the rich plum color of amethyst glass, it looks more violet in nature and is lighter in color.

    It's also wise to note that "sun purple" glass has been artificially produced by some unscrupulous sellers who expose antique clear glass pieces to ultraviolet light in order to change the color in an effort to make it more desirable.

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    Green Glass

    Olive green onion-type demijohn bottle
    Morphy Auctions

    The green antique glass was given its color by adding iron oxide to a molten glass mixture of sand, potash, and lime. Varying shades of green ranging from light to dark with an olive hue were achieved by lowering the amount of iron oxide added to the glass mixture.

    This type of glass should not be confused with green Depression glass containing uranium oxide, or vaseline glass, which is more yellow in color and will also fluoresce under ultraviolet light due to uranium content.

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    Amber Glass

    Two brown glass milk bottles
    Morphy Auctions

    Amber and brown antique glass were given their color by adding iron/sulfur oxide to a molten glass mixture of sand, potash, and lime.

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    Red Glass

    Ruby red and cranberry glass examples
    Morphy Auctions

    The red antique glass was given its color by adding gold oxide to a molten glass mixture of sand, potash, and lime. More in the mixed gold would result in deep red like the Mary Gregory style mug shown here. Other shades of red and pink, like that of the cranberry glass shaker depicted here, were achieved by lowering the amount of gold oxide added to the glass mixture.

    The other item shown here (at right) is clear frosted glass with painted red accents, also known as "flash" coloring in the antique business, rather than ruby glass.