Last Updated: Aug 08, 2018 Views: 20904
The purple hues are likely due to solarization of glass. A great resource is the article "Solarized Glass" in All About Glass on The 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 contain manganese 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 the manganese 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.
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.
- It's Manganese Dioxide
Thanks for catching the typo - I'll correct it!
- I found this article because I dug up a water/ milk glass from an old army base and it has a yellow tint to it that I cant get rid of -makes it look kinda dirty. But, my question is this -ive dug up a lot of old glass and quite often, I find a fair amount of purple hued glass -I always thought it was just a popular glass color at one point (looks pretty), but might I assume that it wasnt always purple glass, but in fact clear at one time?
- Bobby Glass, it might be and it may have been glass that was purple originally. One possible way to tell would take time and I don't know how much time. We do restoration work on churches and often run into glass that has had parts of pieces covered by lead or moldings and the glass is still perfectly clear where the lead or moldings around the leader panel kept that section from UV exposure. So you could take some and tape off a bit with some light safe tape ( probably electrical, duck, etc. and leave it where it would get maximum sun exposure and then check it after???
Although, I also don't know if the glass reaches a certain stage of purple and then never gets any darker? I'm thinking that may be the case as they are always pretty close to that same shade.
- Any idea why, when heated in a torch, the purple disappears and the glass turns clear again? Same happened with red glass (though I'm certain the red started out that color, and wasn't discoloration due to UV).
- Thank you all for your comments! If you have follow-up questions that haven't been answered by library staff or other commenters or through your own research, please feel free to contact us via Ask a Glass Question (http://libanswers.cmog.org/) or by email (firstname.lastname@example.org) or phone (607-438-5300).
- I have a pair of glass candlestick holders that display a purple tone to them. Does this mean that they would have been made in late 19th to early 20th century?