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The first firing of my investment molds produces a considerable amount of smoke. Enough to set off the smoke detectors in my studio. I vent the kiln (till 1100deg.) and I also have two exhaust fans that can't seem to keep up. One of the fans is 400cfm the other 150cfm. Of course I could get a more powerful exhaust system and that might be where I'm headed. However, I have been using the same investment, R & R 910, for years now and I don't recall having this much smoke in the past. I have narrowed it down to the refractory because on the second or third firing of the same molds, I do not get any smoke so it is not the kiln wash or the glass. There is nothing else in my kiln, no glue, no inclusions, no forgotten tools.

Besides making the studio unusable during a firing, it also effects the quality of the castings. The glass seems to get an iridescent haze on the surface that is not in contact with the mold (not devit) and seems not to flow as well. I use both Bullseye and Sys96 transparents but notice this haze more on the darker 96 glasses. When I have tried Prefiring the molds ( to 1000deg.) they turn a greyish color as well as turning my kiln shelf grey. Once the molds are fired to 1480, they revert to pure white, as does the shelf.

So here are my questions...
What is burning out of the refractory? And how is it that it effects my shelf as well?
How hazardous are the fumes? It smells pretty toxic.
Why now, when in the past I have not had this problem, at least to this degree?
Prefiring without glass seems like the obvious answer, however the refractory breaks down so it really shortens the mold's lifespan for any usable casting. By Prefiring, I'm solving one problem only to introduce another. (for which the only solution is to make a new mold.)
Is the Orton Vent system better for venting than leaving the lid open?

Would appreciate any insights.
Cheers, Jen

Jen, for some reason I just saw this (don't ask me why)...you've probably solved it by now, seeing as how it's August and you asked it in January, but it's an important question. If you don't mind and still want to discuss, I have some questions in return:

--When you say "smoke," do you mean actual, visible smoke? What color is it?
--Is the odor typical burnoff refractory odor, or is it more acrid? Or some other smell--"pretty toxic" doesn't sound like just refractory plaster. I do a lot of R&R910 and that's usually a strong plaster burnoff smell but not much more than that, so I wonder what else is in there.
--How are you making your molds? By this I mean, what models are you investing? Wax? Clay? Silicone? or just what?
--Where are you placing your exhaust fans and how are they positioned to the outside? Can you stand outside and still smell the smoke?
--When does the smell/smoke stop? (at what temp)

Unless you've got contaminated mold mix it sounds as though the model is leaving residue in the mold that's burning out with the first firing. The most usual is when inorganic wax (such as "victory brown") seeps into the mold material during steamout, or isn't completely removed, and burns out during the ramp-up.

IF that's what is happening (and I have no idea if that's the case), the kiln will start stinking around 500-700 degreesF, then greasy, heavy, dark smoke will start rolling out of the kiln around 500-600F and keep coming until around 900-1000F, depending on how much wax is in there. It is toxic to breathe these fumes ( http://www.freemansupply.com/MSDS/scann ... ownwax.pdf ). The solution is to get all (or the vast majority of) the wax out of the mold before firing.

I had the same problem with the R&R 910. Might be they started putting polyester fibers in there or something.

me too.

R&R910 generates a stronger smell than it used to.

Pete

I stopped using it. Stinks worse than the stuff I'm trying to burn out. You can make something pretty close with perlite, chopped fiberglass, and grog.

Hmmmm. If that's the case, call Ransom & Randolph and ask if they've changed the formulation or something. The cases of it I've used haven't done that, but I haven't used any new 910 in about a year. They're pretty forthcoming with technical information, so you should have an answer quickly.