The Rockwell Museum’s Reifschlager Gallery Pre-2000

Back To Mirror Black

Photos, courtesy of The Rockwell Museum, Corning, NY

Prior to Thanksgiving, the Gazette published a photo of a selection of Mirror Black glass from the Reifschlager Gallery at the Rockwell Museum. This photo was accompanied by an excerpt from Eric Ericson’s two volume set A Guide to Colored Steuben Glass 1903-1933. In describing the transmission of light through Mirror Black, Ericson contends “light will appear pinpointed as a potassium permanganate…” In a subsequent Gazette, Rande Bly asserts that “Steuben black is potassium permanganate in color.” Unfortunately, both Ericson and Bly leave the misleading impression that potassium permanganate has something to do with the color of Mirror Black. It does not.

I asked Greg Merkel to explain the chemical basis for the color Mirror Black. Greg has been a frequent speaker at the Club’s Symposiums because of his research of Carder chemical formulations and his work analyzing Carder glass using Xray fluorescence. Here is that explanation:

Mirror Black and Manganese-Containing Glasses

In view of recent comments made on Steuben’s black glassware, and on manganese-containing glasses more broadly, I would like to introduce the following information:

Steuben produced a number of different black glasses at different times and for various purposes, depending upon whether the black was intended to be the body of the object or, alternatively, joined with (or cased upon) Crystal, Alabaster, or Ivory. Recipes in Carder’s notebooks from the 1920s, coupled with XRF analyses of numerous objects, confirm that the color that has become known as Mirror Black was colored by additions of manganese, cobalt, chromium, and nickel, not by manganese alone. The recipe for this black was eventually given the code “F.G. 10-46.”

I would also like to add that the purple to violet color of manganese-containing glasses in general (appearing “black” at sufficiently high concentrations of manganese) is not due to “permanganate,” which is a combination of manganese and oxygen in which the manganese has a charge of 7+. Manganese cannot exist in such a highly-charged state in silicate glasses. Indeed, even though manganese was typically added to the batch as the mineral pyrolusite (MnO2), in which the manganese has a charge of 4+, it actually becomes further “reduced” in the molten glass, existing as a mixture of Mn2+ and Mn3+. It is the triply-charged manganese ion, Mn3+, which produces the purple to violet color in such glasses. The similarity of the colors produced by Mn3+ and Mn7+ is simply a coincidence of nature.

I’m compelled to reiterate my position that Ericson’s books contain so much misinformation on Carder’s glasses that they are best left on the shelf.

Greg Merkel, 11/21/2017

I share Greg’s opinion of the Ericson texts. The more than 50 year old texts have been superseded by subsequent research, analysis and scholarship.

Scott Hansen