The Cambridge Colors: Royal Blue

By Les Hansen
June 2003 - Issue #362

Royal Blue is one of the most popular transparent colors made by the Cambridge Glass Company. Likely, most collectors would agree that the colors Royal Blue and Carmen probably command the highest average prices today. That would be particularly true for collectors of nude stems, flying lady bowls, and silver overlay items. In the last article in this series, it was pointed out that at the time of production, Carmen items were priced 50% to 100% higher than items in the other Cambridge colors produced during the 1930s. However, Royal Blue items were priced the same as items produced in the other transparent colors and crystal.

Numerous sources indicate that blue is the easiest color of glass to produce reliably. The textbook, Modern Glass Practice, by Scholes and Greene states that cobalt oxide "has an amazingly high coloring power", and "as little as one part in 5,000 produces a blue sufficiently intense for most ware." The color of Royal Blue is often referred to as "cobalt", even among Cambridge glass collectors, as is most transparent glass that is deep blue.

The Colors in Cambridge book describes Royal Blue in this way: "It is a deep color of transparent blue with a pleasant softness that will show highlights trending toward the reds." Furthermore, "The striking beauty of this rich color creates a desirability level among collectors that favorably affects prices." Royal Blue was one of the colors developed by Henry Hellmers for the Cambridge Glass Company, and it was introduced in mid-1931. The color was discontinued sometime in the first half of the 1940s, so it was produced for roughly a 12-year period of time.

Hellmers developed four deep transparent colors for Cambridge: Royal Blue, Carmen, Amethyst, and Forest Green; all were introduced in 1931. Carmen was discontinued for a period of time, but was reintroduced in 1950, and nude stems in Carmen were produced once again during the 1950s. Amethyst remained in production until the final years of the company, and a color similar to Forest Green - Late Dark Emerald - was introduced in 1949. However, Cambridge did not produce deep blue glass after Royal Blue was discontinued. Although an easy color to produce, Royal Blue apparently was not in demand by the consuming public for stemware, for dinnerware, or for display pieces in the late 1940s and into the 1950s.

Two formulas for Royal Blue are in Henry T. Hellmer's Batch Book of Glass Formulae. One formula is dated 1931, with the notation that it is for heat-resistant dinnerware and for both blown and pressed ware. Another formula with identical ingredients and just slightly different amounts of the ingredients is dated 1932. Those ingredients (pounds, except as noted):

Sand 450
Soda 180
Potash 14
Lime 46
Sodium Nitrate 34
Borax 20
Feldspar 50
Lead 20
Arsenic 5
Manganese 12 ounces
Cobalt Oxide 21 ounces

Feldspar is a source of aluminum oxide, which improves the durability of glass. However, feldspar must have been an optional ingredient in Royal Blue, because later formulas (1940 to 1942) for Royal Blue from another Cambridge batch book have the same ingredients with very similar volumes, except the feldspar was removed.

Sand is the base material for all glass. Soda, potash, and lime are standard materials for glass and add fluidity to melted glass. Sodium nitrate accelerates the melting of a batch of glass. Borax is a solvent for metallic oxides (feldspar, lead, arsenic manganese, and cobalt oxide are all metallic oxides). Lead increases the density of glass. Arsenic actually has a dual role as an ingredient - one role is to eliminate bubbles in glass and the other role is to counteract the green-coloring properties of iron, which often is an impurity in sand and feldspar. Royal Blue leans toward red rather than green, and arsenic would help insure that was the case.

Manganese and cobalt oxide were the coloring agents in Royal Blue. Manganese is often used as a decolorizer in crystal glass, because it imparts a violet tint to glass. Notice the incredibly small amounts of manganese and cobalt oxide required to obtain the color Royal Blue. Of the 820 pounds of ingredients in the formula, less than one pound (12 ounces) was manganese and somewhat more than one pound (21 ounces) was cobalt oxide.

The next article in this series will discuss another color developed by Henry Hellmers for Cambridge - Heatherbloom - which contained an extremely expensive coloring agent.