The Cambridge Colors: Forest Green

By Les Hansen
Issue #367 - November 2003

Forest Green is the final color to be discussed of the six colors developed by Henry Hellmers during his employment by the Cambridge Glass Company from 1930 to 1932. The Colors in Cambridge Glass book indicates that Forest Green was introduced in September 1931. The Colors book further states, "It is a soft, cool, dark shade of transparent green that tends toward yellow." Furthermore, "Forest Green is often confused with the dark Emerald color [Late Dark Emerald] of the later period of production." Although both colors are deep transparent green, generally speaking, Forest Green leans somewhat more toward yellow than does Late Dark Emerald.

The primary source of information for this series of articles has been Henry T. Hellmers' Batch Book of Glass Formulae that was published in 2002 by J. W. Courter, Kevil, KY, who became a personal friend of Hellmers. Formulas in the Hellmers batch book for Forest Green and another transparent green, Light Emerald, are dated 9/1/1932. Earlier and later formulas exist for both of these transparent green colors in the Hellmers batch book, as well as in other batch books that have surfaced; however, the formulas differ only slightly from these 1932 formulas. Once again, this demonstrates that more than one formula was used through time for Cambridge colors. Availability and cost of materials, as well as new knowledge on materials, likely determined what ingredients were used at any point in time to produce a particular color of glass.

Light Emerald, of course, was introduced in the early 1920s, long before Henry Hellmers was employed by the Cambridge Glass Company, and it remained in production during Hellmers' tenure at Cambridge and well into the 1940s. The ingredients for the two formulas are (units are pounds except as noted):

  Forest Green Light Emerald
Sand 800 850
Soda 340 330
Feldspar 100 100
Lime 80 42
Nitrate 50 50
Lead 40 36
Borax 40 --
Arsenic 10 10
Bichromate 3 --
Iron Oxide 2 --
Copper Oxide 1 13 oz
Uranium -- 43 oz

Many of these ingredients have been reviewed in past articles. Sand is the base material for all glass, and soda and lime add fluidity to molten glass. Feldspar is a source of alumina, which improves the durability of glass. Nitrate (sodium nitrate) accelerates the melting of glass in a batch. Lead acts as a flux in glass melting and, of course, increases the density (weight) of glass. Arsenic is important to remove seeds (bubbles) in glass. These seven ingredients are common to both formulas in remarkably similar amounts. Bichromate (in the form of potassium dichromate), iron oxide, and copper oxide are the colorants in Forest Green. All three of these colorants are metallic oxides, and borax is a solvent for metallic oxides, which explains the 10 lbs of borax in the formula for Forest Green. The book, Modern Glass Practice by Scholes and Greene, indicates that bichromate is a source of chromium, which is a powerful colorant that imparts an intense green color to glass (there was only 3 lbs of bichromate in the formula). Iron oxide (only 2 lbs in the formula), when melted with arsenic, gives a yellow-green color to glass, and copper oxide (only 1 lb in the Forest Green formula) produces a greenish-blue color. The colorants in this formula for Forest Green totaled only 6 lbs in the 1466-pound batch of Forest Green, which is only 0.4% of the weight of the formula.

The colorants for the 1421.5-lb formula for Light Emerald were only 13 ounces of copper oxide (a greenish-blue colorant for both Forest Green and Light Emerald) and 43 ounces (2 lbs, 11 ounces) of uranium. Uranium produces a strong yellow in glass that counterbalanced the blue influence of copper oxide. The result was a light, sparkly green. Uranium is an interesting colorant for glass, and this ingredient will be the topic of a future article on colors of Cambridge glass (Primrose, Jade, Ivory, Topaz, Gold Krystol, and Pistachio all contain uranium).

Based on its availability to glass collectors today, Forest Green was a popular seller by the Cambridge Glass Company when the glass was manufactured. However, unless etched, Forest Green items usually don't command the prices today of the other four transparent colors (Carmen, Royal Blue, Amethyst, and Heatherbloom) developed by Henry Hellmers. Crown Tuscan, the opaque pink developed by Hellmers, was likely the top-selling colored glass produced by the Cambridge Glass Company. Crown Tuscan items with gold encrustations or enamel decorations do tremendous damage to the personal finances of serious glass collectors today. The popularity of all six of these colors demonstrates the impact that Henry Hellmers had on the success of the Cambridge Glass Company.