I am studying on carbon removal from the glass melts. In most of glasses, by more heating in the molten state, carbon will combusted. But I overheated a magnesium aluminosislicate glass and the carbon did not combusted and remained in the glass and the color was deep dark! Does anybody has a new idea for solving this problem?
What temperature are you using?
Carbon is quite easy to remove, but you need to verify if the carbon is not reaction with your crucible.... A slow heat rate could help you.
Where does your carbon come from? Do you add it purposely in your batch?
What atmosphere do you use during melting? In reducing atmosphere, the carbon will be removed slower. Maybe melting in oxidizing atmosphere could help removing it faster.
Carbon also influences the redox state of the glass, and if the glass also contains iron and sulfur (even in low concentrations), you may form amber (amber chromophore Fe3+-S2- formed in reduced glasses), leading to a glass with a dark coloration.
Thank you for your answers. We are studying the manufacturing of the glasses using induction melting. We used graphite crucible and suceptor for melting. We can increase the temperature up to 2200 C. But the glass is still black! We blow air into it and decrease the carbon content in the glass below 0.04%. Just we did not use oxygen. I know that in sodalime glasses carbon burns by increasing the temperature but in MAS glass does not burn. Do you have any experience of carbon removal from the melt?
I had once a SLS glass contaminated by carbon particles from a graphite crucible (glass melted in more conventional electric furnace however). The glass presented important dark volutes due to this contamination. I remelted the glass and bubbled the melt with oxygen (sending pure oxygen via a small tube plunging in the melt at high temperature) for some tens of minutes, and after cooling down the glass, all graphite contamination was gone. However, this procedure changes the redox state of the glass, so if this is of importance for you, you have to consider it. For example, for the glass I mention, the original (contaminated) glass was blue, as it was a reduced glass with chromium and iron, which were then in their reduced forms Cr2+ and Fe2+. After bubbling with oxygen, the glass became green, as the coloring ions Cr and Fe got oxidized into Cr3+ and Fe3+ during the bubbling. But the carbon contamination was gone...
Thank you Dr Hubert. I know that oxygen blowing is more effective than air blowing but do you think that we can remove all of carbon by oxygen blowing while we could not remove it by air blowing?
I don't know for sure but it would probably help increase the efficiency of carbon removal. At least it is worth the try. Or maybe, if oxygen bubbling is more complicated for you, increasing the time of bubbling with air might help. Have you try longer times with air?
There are a couple of papers by Shaw and Shelby on coloration in barium gallosilicate glasses for melts made in carbon crucibles. The papers are in Physics and Chemistry of Glasses. Since I'm retired, I don't have copies anymore, but I think they were in the 1980's. I think they might be relevant. These glasses are roughly analogous to your glasses.
Thank you professor Shelby. I will search more and surely will find the articles.
I can advise to cover the inner surface of your carbon/graphite crucibles with BN coatings, it is very easy to do by employing BN-bakelite compositions and simple brushes.
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