Solid Investment Burn-Out


I use an investment mix of brick dust, plaster, and ludo made from the same. I brush on several coats of a plaster/silica flour mix, reinforced with chopped glass strand, first. It has excellent gas permeability, adequate strength, and renders perfect detail—I am more than pleased with it.
When I follow recommended burn-out heats and schedules I get gas eruptions when I pour. I have found that I need to burn-out a small investment (9 inch dia. X 12 inches tall) for 48 hours at between 900 and 1000 F in order to get a good bronze pour. And a medium size investment (15 inches dia. X 24 inches tall) that I just poured I burned out for four days at between 700 and 800 F and it was not enough—I got serious gas. It had a bottom wax drain which provided a passage through the center of the investment for the passage of air.
Other people tell me they burn out medium size molds at 1000 F for 36 hours with success. I read that in books as well. When I try it, I get serious gas eruptions. Am I missing something?

Rich Waugh's picture

It sounds as though your

It sounds as though your investment mix is working fine. That eaves temperature, time and/or wax as the culprit(s). My guess would be the wax has some component that needs either higher temps or more time to fully combust.

Do you start the burnout process with the investment still slightly damp? There are those who maintain that doing so prevent so much wax from permeating the investment and causing outgassing.

Also, that 700-800 temp for the burnout is much too low, in my experience. You really need a sustained core temp of 1000F or higher for at least couple of hours to completely oxidize and consume all the wax residue. That means getting the outside up to that temp about 24 hours earlier, to allow time for the temp to penetrate to the core

I have only marginal experience with molds the size you're doing so I may be missing things here.

Raspero's picture

I normally allow the

I normally allow the investments to cure overnight, covered in a plastic bag to prevent drying. Then I place them in the kiln wet. That doesn't really seem to do much to prevent the wax from soaking into the investment. I allow the wax to drain out through a hole in the floor of the kiln into a pan of water. I barely capture half of the wax, if that.

bpfink's picture

 This may sound odd but

 This may sound odd but here goes. Can only say I have been doing this for over 50 years now and my final results adhered to are based on the successes and failures during this period. I actually soak the investments to go into the kiln to be as wet as possible.  That keeps the wax from impregnating the plaster body during its melting to get out.  I plan drains for as much as possible and since it is upside down while doing that when poured later and turned back over it gives great air and gas releases for the pour.  I go to about 900 F max though higher is not  a problem, just not necessary.  Also give it time to sit there awhile.  My molds are much larger (600 to 2000 lbs. each) so that means about 4 to 7 days time depending.The water in the investment is also a great heat conductor so final time spent is less than if it had been a dry investment and needed more time to re burn out any wax of residues that would have soaked into it if it had been dry to start.  I can not see what your problem is except that the chemical water of the plaster needs to be dried out as well as the mechanical water.  Now this is where it may sound odd.  I use temperature gauges,  but seldom rely on them since my nose tells me much more.  I can smell any left over wax or media gases better than the gauges can tell me.  Kind of like putting your finger into the soil of a house plant can tell you more about the moisture content.  Got a small nose but it works really well.  bpfink

Rich Waugh's picture

I'll vigorously second what

I'll vigorously second what Bruce said about smelling the presence of wax residues. I always trusted my nose more than anything else when it came to determining if the mold was ready to pour, and even for determining when it was time to ramp up the kiln temp after driving off the chemical water. If you make a habit of sniffing the exhaust from your kiln, you'll quickly see (well, smell) that there is a difference in the odor of wet heat, dry heat with wax vapors, dry heat with wax residue vapors, and dry heat with no residues at all.

I also think that Bruce has a point about introducing the mold to the kiln in a wet state, though I never actually did this myself. My work was on a much smaller scale than either of you two, so my times were much shorter and I had little need to experiment to discover ways to speed up the cycle or decrease wax soak-in to the investment. I can certainly see how starting with investment that was saturated with free water would speed up the transmission of heat tot he core up to the point where the free water was all driven off. I also think that the presence of the steam from that water would retard the migration of wax into the investment until more of it had melted and run out, particularly if you do it as Bruce does using multiple vents to act as drains when the mold is burned out upside down.

I'd say, try it Bruce's way and let us know how it works for you. Bruce certainly has the most experience of any of here and you can rely on his knowledge.

Raspero's picture

Thank you for your well

Thank you for your well thought replies.

I have posted below a photograph of the piece as broken out of the investment. I burn out my larger investments right side up with a bottom drain. I am a one man shop and use a lift off kiln. I pour it in place on the kiln floor.

As you can see I had one wax drain at the bottom. There is also a drain from the lowest point of the crest of the helmet down to the runner. All of the gas was located in the black ellipse. The rest of the casting is almost perfect. That box thing there on the vent was a Styrofoam riser. The upper two thirds of the riser is hollow from the gas.

Fortunately, given the nature of the piece, I can use the gas holes to a good effect.

Thanks again for the help.



Gas Problem

Gas Problem

Raspero's picture

After removing the gating

After removing the gating and chasing it, here is what he looks like:

Da King (composite)Da King (composite)

Raspero's picture

And the finished

And the finished product:

King DoneKing Done