I'm in trouble with bubbles. My batch contains Sb2O3 and SnO2 and I am mixing my batch with magnetic stirrer in ethanol. Also I am drying my batch before first melting in order to avoid humidity. I am crushing my cullets in an agate mortar after first melting and they become really fine powder. I am not able to stir my melt because the absence of platinum or quartz blades. Melt generally waits in the furnace at 1500 °C for 3 hours.
It has been a while since I have worked with glass, but if I remember correctly, antimony pentoxide can serve as a fining agent. Upon heating it will reduce to antimony troxide -- which should work with your composition -- and produce oxygen that will serve to grow the existing bubbles, which should then leave the melt via higher buoyancy. (If you batch nitrates with Sb2O3, then you should be getting Sb2O5 part way through heating the components.) Depending on the composition, you can also use NaCl or a sulfate as fining agents.
When fining, you will want to keep the melt at a high temperature (higher than what you use for casting the glass), so that solubility of gasses is low and the viscosity is low enough for the bubbles to escape. After this, you will need to decrease the temperature to re-dissolve the remaining gas bubbles. (For example, after fining for 3 hours at 1500 oC, bring it down to 1400-1450 oC for 15-30 minutes before casting).
Since you are not stirring, you homogenization will come from the bubble evolution during fining, and from diffusion. In that sense, time is your friend, so if you don't have anything in your batch that can react with Pt, feel free to let it sit at the fining temperature for longer.
Alternatively, you can agitate the batch by bubbling a gas through it via a Pt tube.
Answering that question without knowing the exact composition of your melt is not possible. You can use fining agents, as described above, raw materials which release gas, thus stirring the melt (carbonates, for example), bubbling with gas (helium is best), adding water to the batch as a hydrate to reduce the melt viscosity, increasing the fining temperature. Please see my textbook for a through discussion of fining of melts.
Hi jim..... Good to see you being active. just wondering if the use of ethanol in the preparation of the batch is counter productive as andy remnants might cause a reduction of the melt.
Bill ... Ethanol can leave a residue on the batch powders. Since both antimony and tin are very easily reduced, it is possible that the glass produced by this method will have a high proportion of the lower valence ions. I've actually used this method to produce glasses with a high stannous (as opposed to stannic) content. (Yep, I'm still alive and occasionally see a question here that I might be able to help with.)
SiO2 70 %mol
Na2O 15 %mol
Al2O3 4 %mol
ZnO 5 %mol
NaF 5 %mol
KBr 1 %mol
CeO2 0,01 %mol
Ag2O 0,01 %mol
SnO2 0,01 %mol
Sb2O3 0,03 %mol
is the batch. Well I feel like it never mixes enough if I don't use a liquid like ethanol. I thought ethanol is completely ok. Thank you very much for the information sir.
Well, it certainly is an interesting composition. Looks like a photosensitive??
While Jim Shelby is the real expert, it seems to me that this composition should be fluid enough at 1500 for it to homogenize fairly rapidly. My only suggestion to improve the melting would be to use a Na-Al-Silicate clay to get the Alumina, if you are having problems of homogeneity. Also, don't know how you know it is NOT homogeneous If it is striations or unequal coloration on exposure, it is likely due to problems with redox.
Yes it is photosensitive. I didn't perform any exposure yet because of the bubbles. I think I can wait at 1500 a little bit more to get rid of them and also try your other suggestions. Thank you very much.
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