The Cambridge Colors: Heatherbloom

By Les Hansen
July 2003 - Issue #363

A short-lived color of glass produced by Cambridge in the first half of the 1930s was Heatherbloom. Like its "sister" colors of Alexandrite by Heisey and Wisteria by Fostoria, Heatherbloom is a pale lavender, which changes color based on the thickness of the glass and the source of lighting. Heatherbloom is yet another color developed by Henry Hellmers for the Cambridge Glass Company in the early 1930s. Furthermore, Hellmers consulted with many glass manufacturers during the 1920s and 1930s, and he claimed to have developed the formula for the Alexandrite color made by Heisey.

The color of Heatherbloom hinges on an ingredient called neodymium (pronounced nee-eh-DIM-i-em), which was first used as a coloring agent for glassware in 1925. Neodymium is a silvery-white element, which is costly to separate from its base materials and is categorized as a scarce "rare earth" element. Therefore, neodymium was (in the 1930s), and remains today, a very expensive ingredient for making glass. A web search to prepare this article provided a current market value of $1 per gram for neodymium, which is the equivalent of approximately $450 per pound! Granted, this current estimate of cost would be for a small amount of neodymium, rather than for the bulk amounts (with likely lower cost per unit) required as an ingredient for glass.

The Colors in Cambridge book indicates, "Heatherbloom was introduced in November 1931, and the last advertising reference to the color was found in 1935. This color can be deceiving. When viewed in natural light (daylight or incandescent) it is a very delicate pale orchid or lavender. When viewed under fluorescent light, it generally takes on a light blue or gray appearance." The Colors book continues, "Caution! This color could be confused with Crystal which has changed to a lavender shade by prolonged exposure to sunlight."

Personally, I travel frequently to Phoenix, AZ, and the antique malls in that city have many booths with ONLY sun-colored (lavender) crystal glass. Apparently, residents in that part of the country simply place crystal glass on shelving on the rooftops of homes to acquire a uniform lavender color - after all, the sun shines almost every day of the year in that part of the country! So, buyers should beware when pur-chasing Cambridge glass that has a light lavender color. Heatherbloom items with a crystal stem or crystal foot can provide some level of comfort regarding color. I have over-heard many debates among NCC members regarding whether a particular piece of glass is true Heatherbloom or is sun-colored crystal.

Modern Glass Practice, by Scholes and Greene, states, "The violet-pink shade produced by a few parts of neodymium oxide in glass is so attractive that it is used in expensive ornamental ware." The textbook continues, "The hue of glass colored with neodymium varies with the thickness or with concentration of the colorant from a light pink in thin sections to a beautiful blue-violet in thicker pieces. This change of hue with thickness is known as dichromism. Lastly, the textbook points out, "The color of neodymium would be useful for masking the yellow-green of iron [as a contaminant in raw materials] if economic factors did not interfere."

The formulas for Heatherbloom (for both pressed and blown ware) of Cambridge and Alexandrite of Heisey according to Henry T. Hellmers' Batch Book of Glass Formulae follow. Units are pounds, except for the powder blue. The formulas in the Hellmers batch book tend to be provided with either 450 base pounds (small pot) or 1000 base pounds (large pot) of sand. The formulas for Heatherbloom and Alexandrite are for the larger volumes.

  Heatherbloom Alexandrite
Sand 1000 1000
Soda 250 360
Potash 140 --
Lime 110 70
Sodium Nitrate 140 35
Feldspar 125 --
Borax 40 75
Arsenic 7.5 2.5
Neodymium 20 90
Selenium 1.25 --
Powder Blue 2.5 ounces --

Sand is the base material for glass, and soda, potash, and lime are standard ingredients to add fluidity to melted glass. Sodium nitrate accelerates the melting of glass, feldspar improves the durability of glass, and borax is a solvent for metallic oxides (all of the colorants). Arsenic eliminates seeds (bubbles) in glass and counteracts the yellow-green coloring properties of iron, which might be an impurity in the sand and the feldspar.

Besides the neodymium (the sole coloring agent in Alexandrite), the formula for Heatherbloom contained trace amounts of selenium and powder blue as colorants. Selenium imparts a pink color to glass, and powder blue is a dilute form (about 5%) of cobalt oxide. The 2.5 ounces of powder blue in the formula would contain only about .125 ounce of cobalt oxide for the 1830-pound pot of glass, which once again demonstrates the power of cobalt oxide to impart blue color to glass.

The comparatively large amount of neodymium in the formula for Alexandrite by Heisey (four and a half times as much as the amount in Heatherbloom) is remarkable, especially when cost is considered. More than likely, Hellmers chose to supplement the coloring properties of neodymium with selenium and powder blue to bring down the production cost of Heatherbloom compared to Alexandrite.

Next, we will discuss another formula for a colored glass developed by Hellmers for Cambridge - Amethyst - and compare its formula with the formula of an earlier purple glass produced by Cambridge called Mulberry.