Are chrome-containing glasses prone to devit?

[Josh Bernbaum]

Sorry I haven't added anything to this part of the forum in a while, but I'm really glad we now have this section specifically on color chemistry etc. So, I'm just curious if my new hunch might be accurate that chrome-containing glasses might be prone to devit? Here's why I'm asking: I've recently been doing some pieces where the color in the murrini cross-sections I'm using is uncased and 'exposed'. I've done several pieces using the same methods and the first and only-to-that-point devit I'd seen on the surface was in murrini made with one of Gaffer's transparent greens (Lime green they call it). I'm first rolling up a sheet of the murrini slices and making a 'cup' out of it to stuff with more clear glass later in making the final piece. I first noticed the devit on the surface of the cup, just slightly. After reheating and stuffing and making the piece, the devit was slightly more noticeable. So a while later I tried a similar color green made by Reichenbach, R-24. I was a bit surprised to see the same kind of devit. I don't want to case this murrine layer, that might be a solution here but not the kind of look and feel I'm after in these pieces. This is all kind of subtle, but it's there and it sort of makes that section of the piece look etched. I don't totally hate the look but it's not what I intended and I'm mostly just curious what's going on with these transparent greens. I'm guessing both are done on a lead base. One other thing to add which might be contrary to my hunch about the chrome is that I've done one or two murrine pieces made with one of my homemade chrome melts in the past and didn't notice devit. But my chrome content was only about 1-2% if that's a factor at all, and this melt would have been dichromate added to SP batch as the base.

[Pete VanderLaan]

I do not know. It's a good question for John. I can say that I've certainly seen lots of iridescence on green bottle glasses that have solarized and I think them to be chrome based. I do know that the EU has become concerned with Chrome leaching from glass in landfills.

I've always thought of lead glasses as allowing what I would call "packing " of metallic oxides into glasses. When making the black, extra lead has to go into the goop to get the metals into solution. My black has a significant addition of chrome. I grind it finely with lithium and sodium before it goes in the mixer. I don't think of 2% as being a small addition at all. Chrome seems capable of causing significant expansion shift as well.

[Josh Bernbaum]

I'll definitely try to pick John's brain about this at the class this April. One other interesting tidbit that I forgot to mention in the previous post was that I had a friend (Jerry Kung, a glass blower/tool maker from Oakland who some of you may know) visiting this past year, we used this same green murrine on a piece we made together. He is always thinking outside the box, and looking for different ways of doing things. So as an alternative to the 'cup-stuffing' I've been doing, he wanted to try doing a Swedish overlay with the murrine rollup. I remember seeing this devit on the surface after the rollup, but after we smeared the Swedish overlay onto another clear bubble and then made the piece, no devit detected. That formerly inside surface of the rollup, which was now on the outside, still had the exposed green on the surface but no devit. That seemed odd to me, not sure what got rid of or inhibited the devit by doing this.

[Eben Horton]

I’m gonna go out on a limb and suggest that your chrome is coming out of solution and is not devitrifying. As Pete suggests with using lead, the lead will give something for the chrome to hang onto.

[Pete VanderLaan]

That's not quite what I suggested. I do know that chrome will create crystals as in "Aventurine" in the right concentrations at the right temperatures but it's not a glass I have much experience with. I do think anytime you have a color hanging around 16-1700F, it is in the happy range for crystals forming. Color rods certainly have to go through that range. I'd still ask John.

[Jordan Kube]

Chrome does weird stuff in boro. Crystal growth, etc.