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Shawn Watt
10-14-2017, 07:22 AM
We melt fenton red cullet on occasion. It is a cad/sel red. It was a casting glass for them. When I melt it I have always had to squeeze it and leave it around 2040 F or else we end up with lots of large bubbles( reboil is what I've heard this is called.) I would like to hold it at a higher temp than that. As well as I would like to know whats going on with the bubbles and where are they coming from. It doesn't seem like they will just rise out and go away. Thanks

Pete VanderLaan
10-14-2017, 08:18 AM
Sulfide Gas, and everyone who melts this stuff has the same issues. Get it too cold and it will switch valence, make tiny bubbles and change color.
Kenny always called them "Smokers".

Shawn Watt
10-14-2017, 12:47 PM
Is there higher temperature range that it would change valence and reabsorb the sulfide gas? Im at 2040 now and it just a bit slow

Pete VanderLaan
10-14-2017, 01:00 PM
No. The trouble with cad sel reds is the sulfur. Cadmium is colorless in glass without the sulfur. Remember it's a cadmium-sulfo selenium glass. It has to have all three and one is a pure sociopath. There's more of it than you need in Cadmium sulfide and it really is better made with some of the cad sulfide and some cad oxide to get the cadmium level right but it usually isn't done that way. As Chuck Savoie used to say, "sulfur is not your friend in a red glass.". Taking the glass down in temperature does help but obviously makes it more viscous. Go too far and the valence switches and it turns brown. It's not a cooperative glass and even worse if you are having to use someone else's cullet. I picked bubbles from mine in large pieces.

Shawn Watt
10-15-2017, 01:19 PM
Thanks for the info Pete

Jordan Kube
10-19-2017, 12:59 PM
Some of the boro guys are using encapsulated cadmium and getting great results. No off gassing, nice colors, and added to cullet.

Pete VanderLaan
10-19-2017, 03:15 PM
seems to me that encapsulated is a fancy term for such things as Cadmium selenate which is a compound. We did use it back when Chuck and I were doing the color rods but the difference wasn't really appreciable. The goal needs to be getting the sulfur content down to where it should be. Cad sulfide just has way more than you need. I would agree that the boro guys are getting good results . I suspect it's since the boro is so much more viscous than a zinc potash glass would be at a fixed temp. I think the other aspect of using the compounds is there is less volatilization. I haven't seen anyone really prove that out.

Jordan Kube
10-19-2017, 04:02 PM
No, it is zirconium silica encapsulation. Cool stuff.

Overview:
https://www.highwaterclays.com/index.cfm/category/296/encapsulated-stains.cfm

Fun stuff: click download to see full text
https://www.researchgate.net/profile/Kun_Li11/publication/287432522_Research_and_Development_of_Cadmium_Sulp hoselenide_Red_Pigment/links/567bb1c208ae051f9ade69e6/Research-and-Development-of-Cadmium-Sulphoselenide-Red-Pigment.pdf

My friend remelted some cadmium yellow from Northstar and experienced the smoking and off gassing one might expect. He mixed his own using the encapsulated cadmium and didn't get any off gassing. It was a really nice yellow too. He added some holmium and got a little bit of a red shift in the UV range. I'm not sure which manufacturer he is using.

It would be interesting to investigate in soft glass.

Pete VanderLaan
10-19-2017, 05:35 PM
When I've looked at the recent stuff, it's pretty closed tech. I've seen zinc, sodium and barium based stuff but not zircon at this point. Get after it.