Glass Coloring May Seem MagicalBy the late Jan Rutland who served as Executive Director of the National Bottle Museum until her passing in October 2010
"Sun Amethyst" or "Sun Colored"
Most collectors know that the natural color of old glass is aqua - either blue-green or green-blue. This is caused by impurities in the sand used in the batch. Iron impurities produced the colors blue and green.
By mixing decolorizing agents which produce the opposite colors of yellow, red and purple, a neutral color was achieved which has the visual effect of no color at all. The popularity of clear, or "clarified" glass jars and bottles was brought about by a demand for clear food containers in the 18180's. The most popular decolorizing agent was Manganese, until 1915. Manganese -- in small proportions --produced just enough amethyst coloring to neutralize the aqua color.
Glass which was neutralized by the addition of Manganese is admired and collected as "Sun Amethyst" or "Sun Colored" glass. When exposed to the ultra violet rays of the sun, The Manganese in the glass oxidizes, or combines with oxygen, and turns to rust. The longer it is exposed to ultra violet rays from the sun, or from an ultra violet lamp, the deeper the color. If the glass is reheated, it will return to a clear or clarified state. World War I cut off the main source of Manganese, which was Germany. By 1916, the glassmaking industry was using Selenium as a clarifying agent. Selenium is more expensive to use, but a more stable agent. This too, however, will change color with prolonged exposure to ultra violet rays. Glass clarified with Selenium turns pale amber in time. The use of Selenium was replaced by the use of arsenic around 1930.
Glass which has been colored by adding Uranium Oxide or Uranium Dioxide to the batch can be canary yellow in color. It is called "Canary Glass" in this country. It can also be a striking yellow-green in color, and this is known as "Vaseline Glass" because of its supposed resemblance to petroleum jelly. Depression era Uranium Glass is typically green. The foolproof test for Uranium content is exposure to ultra violet or "black light." All Uranium Glass fluoresces under ultra violet light. Also, a Geiger Counter will register the emission of small amounts of gamma rays. All other radioactivity is locked into the cooled glass. Only gamma rays escape the cooled glass, in safe and relatively small amounts. Pity the poor glassblowers of the 18th, 19th and early 20th centuries however, who were exposed to radioactive gasses in lethal amounts, just four feet from their mouths.
Glassblowers of the past who blew molten Uranium Glass were exposed to radon gas, as well as alpha, beta and gamma radiation in lethal amounts.
Generally speaking, the use of Uranium compounds as a coloring agent was most popular between 1850 - 1950. The "Heyday" for Vaseline Glass was the Victorian Era. Under the soft light produced by kerosene lamps or gas lights, Baseline Glass appeared to glow. When electric lighting became commonplace in the home, the glass lost a great deal of its charm. Gaslight and kerosene lamps have varying light frequencies, creating the glowing appearance, but incandescent and fluorescent lights have a more steady frequency, eliminating some of the characteristic "magic" once attributed to Vaseline.
Vaseline Glass was used in the production of tableware, whimsies and apothecary, perfume and cologne bottles. Its special qualities made it ideal for various containers and contraptions associated with the "quack-Cure industry." Strange machines and contraptions were used to defraud the public, including devices which ran electrical charges through gases in colored tubes. About all they did was make noises and create impressive light shows, all the more impressive if the tubes were made of Vaseline Glass. These devices are known today as "Whiz-Bangs." When Radium was first discovered, it was thought to have curative powers. It was marked to and ingested by the unwary public as an "elixir" or "vitalizer", and often dispensed in Vaseline Glass containers.
Today, regardless of age, be it original or reproduction, all Vaseline Glass is collectable. The many regulations governing creation of the "batch" and the conditions under which it may be worked, make it impractical for mass production. It is the one glass that can be proven authentic, and verified beyond a shadow of a doubt. In my personal collection, I have broadened my sphere of interest to include all Uranium Glass. I see no reason, for instance, to turn my nose up at some of the green colored Depression era glass which may have especially pleasing qualities in shape and form. The fluorescence is still so eye-catching, and nothing is more exciting than wandering through a flea market and spotting a piece 100 yards away. I swear, it sings to me!