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Old 04-12-2019, 12:46 PM
Pete VanderLaan's Avatar
Pete VanderLaan Pete VanderLaan is offline
The Old Gaffer
 
Join Date: Apr 2002
Location: Chocorua New Hampshire
Posts: 21,860
Pete VanderLaan is on a distinguished road
The questions surrounding what makes a good casting glass as opposed to a good blowing glass sound simple, but they're not.

Shawn is certainly on the money when he says SP87 is a lousy casting glass. It's quite true and making billets of it and then melting a second time is tweaking the nose of Buddha.

Modern Glass practices by Sam Scholes lays out the parameters for what different materials do to a glass and they are worth noting. In my color classes I try to stress that you can't make a good colored glass until you can make a good clear. I also think it's covered to a certain extent in Glassnotes.

If I wanted to make a casting glass that worked in the Anne Robinson process with the flower pots, I'd be looking at having materials that would lower the viscosity a good deal. There are the following to consider: Boron coming from borax, Fluorine from Fluorspar ( calcium fluoride) Lithium carb, and barium if you aren't doing lead.

It will need alumina in a ratio taking the total alkaline fluxes and dividing by eight, so, a 16% flux glass will need 2% alumina. You will need more than 16% though. Boron lowers viscosity but it also eats at your furnace. It is not a favored nation material in my toolbox. Fluorspar is powerful stuff and also attacks refractories as well as all electrical parts. Barium can be substituted in a glass for calcium or lead. It brightens the glass notably and helps reduce viscosity. At over 3% it will start to give you trouble. It really makes blue colors jump. I really like potassium in my glasses but they increase viscosity while increasing luster. It also really enriches color. I view Soda Ash and calcium as sort of trash materials. I use them but I don't lean on them beyond the basics. . Keep your silica content at 70% or so. Mine has a good deal of potassium in a nitrate form which helps with the melt. You need Antimony as well, not much but don't start without it.
My formula, available from Spruce Pine won't devitrify, has high luster but is a bit more viscous than SP87. It's the potassium that does that, so, it might take an increase in temps to flow. Beyond that, Gaffer makes great casting glasses as does Bullseye. Both are pricey. The colors from Gaffer are infinitely better.

That's a real nutshell. The subject takes much more time if you are picking at it. There's many things I've left out. I think a thorough search of the archives may surprise you. There's a lot in there from the last 18 years.
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