That Antique Purple Glass Might Not Have Started Out Purple

88

A purple vase in the Corning Museum of Glass has an unusual distinction. The museum identifies it as authentic Lalique, but its purple color is a later alteration. The Pierrefonds design dates to 1926, and this particular example was probably colorless before someone exposed it to gamma rays after it left the factory.

Asking whether a piece is real and, in fact, old is useful, but it may leave something important unanswered: Is this the color its maker intended? Here’s how purple glass can acquire its color and what to check before treating an unusual shade as a special or valuable find.

Three Ways Glass Can Become Purple

It Was Made That Color

Purple is a legitimate glass color, not automatic evidence of alteration. Corning lists manganese among the ingredients that can produce amethyst glass. But how that ingredient behaves depends on the glassmaking recipe and conditions, so finding manganese does not automatically tell you why an object is purple today.

One authentic purple piece is this New England vase from 1763–74 at the Metropolitan Museum of Art. It’s blown pattern-molded amethyst glass, but it doesn’t authenticate another vase simply because the two look alike.

Sunlight Changed It

Some glassmakers used manganese to counter an unwanted tint in glass. Subsequent sunlight exposure can turn susceptible glass purple, a process called solarization. Collectors also use the terms sun-purpled glass and sun-colored amethyst, often shortened to SCA.¹

The important distinction is that sunlight changed an existing object. A lavender tint does not necessarily indicate an original factory color, even if the piece is old.

Someone Altered It Later

The Lalique example mentioned in the intro shows why “authentic” and “unaltered” are separate questions. Its later purple color does not turn the vase into a newly manufactured reproduction. It does change how the object should be described.

The National Insulator Association’s experimental program found that deliberate irradiation can turn manganese-bearing glass a dense, dark purple, though radiation and heat can also produce subtler color shifts. Even two seemingly identical insulators may react differently because their glass batches contain different additives and impurities.²

Does a Purple Tint Make Glass More Valuable?

Purple glass does not by itself make glass more expensive, though some dealers might try to trick people into thinking so. In a 1974 article later reprinted in Heisey News, J. D. Lokay argued that sun-purpling should reduce the value of glass originally intended to be clear. That is a collector’s assessment, not a formula for calculating today’s prices.

For a purchase, I would separate two decisions: what the object is worth to you as a display piece and what evidence supports the seller’s claim of collectible value. You can enjoy an altered piece while asking for an accurate description. An appealing color alone is not a good reason to pay an original-color premium.

How to Investigate a Suspicious Purple Color

Identify Other Characteristics

Start with the object rather than the intensity of its color. Identify the maker, pattern, approximate production date, and documented factory colors. If museum records or original catalogs show a particular form only in clear glass, a deep-purple example deserves more scrutiny. That evidence is much more useful than deciding the shade simply “looks natural.”

Consider the Color Distribution

Next, examine how the color is distributed. Prolonged sunlight can leave glass unevenly colored when one area received more exposure than another. However, Dwayne Anthony explains in the National Insulator Association’s research on altered glass that deliberate treatments can also create two-tone effects. Glass exposed to high heat may have slumped areas, a flattened base, or rack marks, though these features can also appear on unaltered glass and should be treated only as supporting clues.

Use a Black Light

A long-wave black light may provide another clue when manganese is present. In the association’s experiments, many irradiated purple insulators produced a weaker yellow-green glow than authentic purple examples. Anthony describes the testing as ongoing, and its findings apply specifically to insulators, so fluorescence cannot provide a universal verdict for bottles, vases, or table glass. The strongest conclusion comes from combining physical clues with documented colors and a traceable history.

Sources

1. Lockhart, Bill. “The Color Purple: Dating Solarized Amethyst Container Glass.” Historical Archaeology, vol. 40, no. 2, 2006, pp. 45–56. Historic Glass Bottle Identification & Information Website. ↩︎
2. Anthony, Dwayne. “Artificially Induced Colors.” National Insulator Association, revised Jan. 2006. ↩︎