The Cambridge Colors: Amethyst & Mulberry

By Les Hansen
September 2003 - Issue #365

Henry Hellmers developed the formulas for six colors of glass during his brief period of employment by the Cambridge Glass Company from 1930 to 1932. To this point, we have reviewed the formulas and related information for four of those colors: Crown Tuscan, Carmen, Royal Blue, and Heatherbloom. Amethyst is the topic of this article, and that will leave Forest Green as the topic for a future article.

The key source of information for all of these articles is Henry T. Hellmers' Batch Book of Glass Formulae that was published in 2002 by J. W. Courter, Kevil, KY. Dr. Courter donated two copies of the Hellmers batch book to NCC during the 2002 convention - one copy for the NCC museum and another copy to be used as a fundraiser. As mentioned in the August issue of the Crystal Ball, the "fundraiser" copy topped the silent auction that was held during the 2003 convention of NCC.

According to the book, Colors in Cambridge Glass, Amethyst was introduced in February 1931. The Colors book further states, "This very rich color, although quite deep in tone, is typical of the softness of appearance that exemplifies the darker colors of Cambridge."

Previously, during the 1920s, Cambridge produced another transparent amethyst-colored glass named Mulberry. The Colors book indicates that, "Mulberry, introduced in 1923, is a medium to deep shade of amethyst in a rather dull transparent color. It will not show the sparkling beauty found in the later issue of Amethyst".

A formula for Amethyst appears in the Hellmers batch book, and the formula is dated to 1932 and has a notation that it was used for both pressed and blown ware. Another batch book provides the formula for Mulberry from the 1920s, and the ingredients for the two formulas are (units are pounds except as noted):

  Amethyst Mulberry
Sand 400 400
Soda 175 170
Feldspar 50 --
Lime 30 50
Nitrate 30 --
Red Lead 20 --
Borax 20 --
Manganese 13.5 12
Salt -- 7
Arsenic 5 3
Tin Oxide -- 1.5
Antimony Oxide -- 6 oz
Powder Blue 1 oz 1 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. These three ingredients are common to both formulas.

The major colorants in both formulas are manganese and powder blue. In conjunction with soda, manganese produces a reddish-violet color. Powder blue is a dilute form (about 5%) of cobalt oxide and, of course, imparts the color blue. Only 1 ounce of powder blue was used in both formulas and this, once again, demonstrates the tremendous coloring power of cobalt oxide. A secondary coloring agent in both formulas is arsenic, because arsenic counteracts the green-coloring properties of iron, which often is an impurity in ingredients. Furthermore, arsenic assists in removing bubbles from glass. For Mulberry, antimony oxide plays the same duel roles that arsenic plays. I have found no explanation for the small amount of tin oxide as an ingredient in Mulberry.

The major differences in the formulas of Amethyst and Mulberry are the substantial amounts of feldspar, nitrate, red lead, and borax in Amethyst, and the lack of all four of these ingredients in Mulberry. Feldspar is a source of alumina, which improves the durability of glass. Nitrate accelerates the melting of a batch of glass, and red lead helps to secure an oxidizing condition in molten glass. Borax is a solvent for all of the metallic oxides that are ingredients.

None of these four ingredients are essential to make glass, and none are direct colorants. All four of these ingredients, however, improve the "quality" of glass, especially by improving the melting environment. This probably explains the difference in Amethyst and Mulberry regarding depth and warmth of color and sparkle.

On the other hand, salt was an ingredient in Mulberry. According to the book, Modern Glass Practice by Scholes and Greene, salt prevents scum from floating in molten glass. Without the "quality" ingredients of feldspar, nitrate, red lead, and borax, Mulberry probably required salt to prevent scum from forming in the batch.

Obviously, Amethyst was a much more expensive color of glass to produce than Mulberry. The demand among collectors today for Amethyst compared to Mulberry once again demonstrates the impact that Henry Hellmers' glass formulas had on the long-term success of the Cambridge Glass Company.