Dr. LaCourse Chromium Statement
Does trivalent chromium convert to hexavalent chromium in a glass furnace?
Many of the green glasses Bullseye makes include Chromium compounds that contain trivalent chromium (Cr+3), a naturally occurring element. Cr+3 can oxidize to dangerous hexavalent chromium (Cr+6) under certain conditions. However, if that happens the glass doesn't come out green, so our manufacturing process includes steps to keep the chromium in its safe trivalent state.
For more information on the subject, here is a post made by Dr. William LaCourse, a professor of glass science at Alfred State University in New York State, to a discussion on Eastside Portland Air Coalition Facebook Group on March 13, 2016.
My name is Bill LaCourse. I am Professor of Glass Science at the New York State College at Alfred University, Alfred NY. (About 50 miles west of Corning). We are the only University in the USA offering degrees in Glass Engineering and Science (BS, MS, and Ph.D). We also have the #2 ranked Master of Fine Arts Glass Sculpture degree program in the U.S.A. Over the past 45 years (yeah I'm ancient) I have taught several advanced courses in "glass" and this spring am teaching a course in Glass Manufacturing, a senior level engineering course. I am also writing a book, "Glass Science for Glass Artists" in which I will cover topics that are of interest here.
I joined this page about a week ago in order to follow the Uroboros/Bullseye story. I also follow the state web page that published the soil study. I have read that as well. I am not employed by either company (or the government), never have been and will not be in the future. I did contact Bullseye a couple weeks ago at the request of a glass artist who uses some of their glass products. Also, our college glass art program buys a lot of their glass. I have talked with 3 different people there and requested some information from them, which they sent. I used this story in my manufacturing course as a "textbook" example of issues glass engineers often face on the job. I have not yet talked with anyone from Uroboros, but will probably do so.
I want to add a little to the discussion of what happens when Chrome oxides are added to a glass batch and then heated to form the glass melt. I know that one post raised a concern/fear that the Cr in a glass batch would convert to Cr+6 when added to the glass batch and then heated. Further, a newspaper article suggested that the melting conditions used by some companies made it worse. I disagree with the analysis in that post and article as it relates to glass melting. The glass batch is much different from the conditions assumed by those articles. Generally, the Cr+3 does not oxidize to the +6 state unless the glass engineer wants it to. He/she has a lot control over what happens. If it did convert, the glass would not have the right color and would be useless.
The engineer controls what state is present through the batch composition and adjustment of the furnace burners. The batch typically contains sodium carbonate (soda), calcium carbonate (lime), and silica sand, along with some borax or other borate compound, and a little alumina and Mg carbonate. Green glass also contains iron oxide and chrome compounds. Often a little carbon containing material is added to prevent any Cr+6 formation. (Sugar, urea, graphite, etc -- the carbon "steals" oxygen from the Cr forcing it to reduce to Cr+3 and becomes CO2 gas. The "best" green colors result when the Cr is in the +3 state although some special ultraviolet absorbing compositions contain a substantial fraction of Cr+6 since it absorbs UV, but does not give a green color. I should add that some greens also contain increased Fe oxides, which also steals oxygen from the Cr and keeps the Cr as Cr+3.
Since the color depends on the amount of Cr+3 and +6, glass makers must control the batch AND the firing atmosphere. As noted, the glassmaker does this by raw material selection and using a reducing (low oxygen to fuel ratio). If the atmosphere above the melt is reducing, Cr+6 will reduce to Cr+3. Importantly, even if a company uses pure oxygen instead of air, ( many do because it is more efficient, and prevents formation of the pollutant NOx, which produces acid rain) the atmosphere can still be reducing, and generally must be, to produce the best green.
The net result is that in green glass most Cr is +3, and the best way to tell is by measuring the color.
I hope this is helpful. Glass color formation and melting are each complex processes and their interdependence makes Ly more so. That's why I love glass and have taught the subject for 40+ years. Glass Engineering is a great career with high paying jobs for those with a science bent. Sorry for the plug, but, it's the Prof. in me.... can't help it. We need more Glass Engineers to make sure that glass producers, users and the environment are well-served.
If anyone has question, I'd be happy to answer them, or I'll ask one of my many glass scientist friends to answer it, or tell you where to get the info. After about 40 years I still don't know all the answers, but I know a lot of nooks and crannies where they might be hiding.
William C. LaCourse, PhD
In response to a later post, Dr. LaCourse added this note:
Frank, I want to commend you on the way you approached and analyzed the issue of Cr. I wish we could have talked earlier. I have read the paper, and in my opinion the conditions of the experiments have little relevance to glass making. In order for the Cr to go from 3 to +6, Oxygen must be available. Carbonates do not provide that oxygen. If the glass maker wanted to create Cr+6 they would replace some of the carbonate with sodium sulfate. The SO3 gives off oxygen and goes to SO2. Sodium nitrate also yields a small amt of oxygen. If you would like, I will send you some notes which detail the batch reaction that occur as the temperature is increased, and a little bit on how one controls the conc. Cr+3/Cr+6 ratio. If you want to do it more privately we can go via messenger or email.