Good afternoon Dr. Greg Merkel,

I am wondering if you would be kind enough to address the possibility of the same color having a slightly different shade or tone due to the reagent qualities of the batch chemicals used as colorants. I have always assumed that this could have some effect on the finalized pot metal.

Kind regards,
David P. Donaldson, MFA
Orlando, FL

Answer from Greg

In answer to Mr. Donaldson’s question:

There are certainly a great many sources of color variability, depending on the specific glass in question. Although variable amounts of impurities in the commercial “reagent” batch ingredients is one factor, it may be small when compared to other causes, which include the following: (1) weighment error, due either to the very small quantities of the colorant or to moisture absorbed by the raw material (so that part of the weight is water rather than the dry inorganic compound); (2) purity of the cullet, which can bring in iron (as metal or oxide) from the blowpipe or pontil rod; (3) contaminants from the melting pot, which depend on the purity of the clays used to make the pot, the extent to which the pot was cleaned of the previous glass that was melted in it (often of a different composition), and the composition of the glass that was previously melted in that pot; (4) the oxidation state of the glass, which depends on the raw materials that were used and the atmosphere of the furnace; (5) the annealing treatment; and (6) intentional changes in the amounts of colorants from one batch to the next, and even changes in the elements that were used to achieve the color (as in the examples of the various recipes for Antique Green in Issue #1938.

Sometimes it’s amazing that glassmakers got the same color twice.

Best regards,
Greg Merkel,
Corning