What causes the varied purple hues in old glass bottles? (2024)

Streetlight globes that have solarized to a purple color. Note that the screw threads have not turned purple because they were shielded from sunlight by the metal fixture.

The purple hues are likely dueto solarization of glass. A great resource is the article"Solarized Glass" inAll About Glass onThe Corning Museum of Glass website. Below is some additional information about the process, compiled by a colleague:

Solarization of Glass

Many glassmakers through the centuries have attempted to produce clear, colorless glass. Impurities, especially iron oxide, in the batch ingredients that were melted to make the glass often resulted in glass that was greenish instead of the desired "water clear."

An interesting characteristic of colorless glasses which containmanganese dioxide as a decolorizer is their tendency to turn different shades of purple when exposed to the rays of the sun or to other ultra-violet sources. It is a photochemical phenomenon that is not yet perfectly understood. It is generally accepted that the ultra-violet light initiates an electron exchange between the manganese and iron ions. This changes the manganese compound into a form that causes the glass to turn purple.

It was in the mid 19th century that manganese dioxide, popularly called "glassmaker's soap," began to be used by American glass manufacturers as a decolorizer. By including a small amount of this ingredient in the melt, they could produce glass that appeared virtually colorless. An 1899 publication by Benjamin Biser remarked,

The especial use of manganese in glass is to mask or neutralize the greenish color imparted to the glass by the protoxide of iron. Manganese imparts to glass a pink or red tint, which being complementary to green, neutralizes the color and permits the glass to transmit white light. Pellat refuted this theory, and claimed that the green tint of iron was not neutralized by the pink of manganese, and thus subduing it; but by the iron taking another charge of oxygen from the manganese and becoming per-oxide of iron, and producing a reddish yellow tint, while the protoxide produces a green tint.

Glass scientists today generally agree with Apsley Pellat, explaining that an ion exchange between the iron and themanganese molecules changes the observed color of the glass.

This process is sometimes reversible by gently heating the glass to about 200°C.

In the early 20th century, changes in manufacturing processes, as well as more pure batch materials, dictated different ways to decolorize glass, and the use of manganese oxide for this purpose dwindled.

Additional Resources

• Biser, Benjamin F. Elements of Glass and Glass Making. Pittsburgh: Glass and Pottery Publishing Co., 1899. pp. 43-44.
• Vogel, W. Chemistry of Glass. Columbus, OH: The American Ceramic Society, Inc., 1985. P. 282.
• Weyl, Woldemar A. Coloured Glasses. Sheffield, England: Society of Glass Technology, 1951, 1976. "The Solarisation of Glasses," pp. 4997-514.
• Zimmerman, Mary J. Sun Colored Glass. [Canyon, TX]: the Author, 1964.

More extensive lists of resources are available upon request. Please let us know if we can be of further assistance via Ask a Glass Question (https://libanswers.cmog.org/), email (rakow@cmog.org), phone (607-438-5300), or text (607-821-4029).

Please don't hesitate to contact us in the future with your glass-related questions!

Links & Files

• "Solarized Glass," All About Glass, The Corning Museum of Glass (Dec. 8, 2011)
• I have heard a rumor that some clear glasses produced in the last 100 years have turned a slight shade of purple. Is there any truth to this?
• Does colored glass fade?
• How much will a stained glass lampshade solarize?

Add a public comment to this entry. To ask a new question, submit form at libanswers.cmog.org.