The Cambridge Colors: Carmen

By Les Hansen
March 2003 - Issue #359

Two colors made by Cambridge were extremely difficult to make because they required reheating of the glass after it was pressed or blown to create the desired color. Those two colors are Rubina and Carmen. The topic of this article is Carmen, a color that Henry Hellmers was very proud to have developed for the Cambridge Glass Company. Carmen was the first ruby glass commercially sold by the company and, according to the Colors in Cambridge book, it was introduced in February 1931.

The glass formula for Carmen is rather simple compared to the formula for many colors, and the formula differed only slightly for pressed and blown ware. Pressed glass items are typically much thicker than blown glass items.

The formulas for all Cambridge colors changed somewhat over time; however, the basic formulas for Carmen (dated 9/1/32) from Henry T. Hellmer's Batch Book of Glass Formulae were:

  Pressed Ware Blown Ware
Sand 400 400
Soda 170 190
Potash 20 40
Lime 3 3
Borax 8 8
Zinc Oxide 60 60
Cadmium Sulfide 11 10
Selenium 6 6.25

That's right - there is no gold in Carmen. I have heard the rumor that Carmen glass contains gold, and this rumor was probably started because gold indeed can be used as a red colorant for glass. However, as stated in the book, Modern Glass Practice, "Needless to say, the cost of gold ruby excludes it from all products except luxury glassware." Cambridge glass was high-quality glassware, but not luxury glassware, at the time it was produced (although a Carmen goblet gold encrusted Rosepoint would certainly be considered luxury glassware today).

The Hellmers batch book has 51 formulas for gold ruby, but most of them date to the late 1800s and only one of them has a notation indicating it was a Cambridge formula. That single formula is dated 12/29/1932, nearly two years after Carmen was introduced, and has the name "Cherry Red" attached to it. Why this formula was developed and for what purpose will likely remain a mystery. There are 159 formulas for "selenium" ruby glass (which Carmen is) in the Hellmers batch book. Twenty-six of them have a notation indicating they were Cambridge formulas.

Only eight raw materials were needed to make Carmen and, of course, sand is the base material for all glass. Soda, potash, and lime are standard materials for glass and add flux (fluidity) to the melted glass. Borax is a solvent for metallic oxides (the zinc oxide). Zinc oxide was needed to keep the selenium in the molten glass, because selenium becomes very volatile when heated.

The coloring agents in Carmen were cadmium sulfide and selenium. Cadmium sulfide will produce glass with a yellow color, and selenium gives glass a pink color (it was the coloring agent in Crown Tuscan). However, when the two materials are combined in the same formula, a red glass might result. I say a red glass "might" result because, according to the book, Modern Glass Practice, cadmium sulfide will only develop color when reheated. The creation of a color by reheating is called "striking" or "warming in" the color.

Carmen probably was colorless or pink when it was cooled from the molten state after the routine processes of pressing or blowing. The glass items developed their red color only when they were subsequently reheated, piece by piece, in the "glory hole". However, if reheating is continued too long or done at too high a temperature, selenium ruby (Carmen) glass becomes cloudy or opaque. Notes in the Hellmers batch book and in another batch book from a Cambridge worker suggest that Carmen was a difficult color to make because of its sensitivity to the length of time and the temperature for both the original heating as well as the subsequent reheating.

Some Cambridge promotional materials provided to retailers list the wholesale prices of the various colors of the 1930s. All transparent colors were priced the same, except Carmen which was priced 50% to 100% higher than the other colors. This higher wholesale cost has led to conjecture that there was more breakage of Carmen glass at the time of cooling than of glass of the other colors. Perhaps selenium ruby (Carmen) glass had more breakage than glass of other colors; however, there is little doubt that the color of Carmen was difficult to achieve consistently from batch to batch and from piece to piece because of the essential reheating process.

Therefore, one could expect a higher rejection rate of Carmen items than other colors. Many of the numerous formulas for Carmen have a tremendous amount (more than 50% of the formula) of cullett, which is reused glass; therefore, many Carmen items must have been rejected upon inspection and, therefore, were recycled as cullett. The need to reheat the pressed and blown items for just the right length of time and at just the right temperature to yield the Carmen color probably resulted in greater labor costs, too, which also could have contributed to higher wholesale prices for this color. Despite the greater wholesale cost compared to the other transparent glass colors produced by the Cambridge Glass Company, Carmen must have been a popular seller with the public based on the availability of items of this color for today's collectors.

One of the easiest colors of glass to manufacture is cobalt. Next, we will discuss the production of Royal Blue, which was another color developed by Henry Hellmers for the Cambridge Glass Company.