Fast burnout and no steam dewaxer

by fredsias on February 8, 2009

A few years ago I was talking on the telephone with the retired Director of Research at Ransom and Randolph. I cannot recall the exact topic of discussion but it had something to do with burnout time. He suggested that I could burn out a flask much faster if I put it into a hot oven rather than ramping the temperature up slowly over a period of hours. I remember remarking that I thought that the sudden thermal shock might cause the investment to crack but I tried his suggestion.

Ever since then I seldom put flasks in a cold oven for burnout and I generally don’t use a computer controller to ramp up the kiln temperature. Instead I pre-heat the oven to 1350 F (about 732 C). I have marked my manual oven control to hold that temperature and while teaching the oven remains at that temperature throughout the week.

“This is crazy!” you say, but let me explain what is going on an why I use this unusual burnout procedure.

First, I was cautioned to make sure that the flask was not partially dried. That might cause cracking. If the flask had set for several hours or over the weekend it should be immersed in water until the investment is thoroughly moistened. Then if the flask is placed in a hot oven, the moisture in the pores of the investment will turn to steam and escape. As long as water or steam is present, the flask internal temperature cannot rise above 212 F (100 C) the boiling point of water. In the case of small 2 x 2.5 inch flasks the core temperature remains at 212 F (100 C) for about 20 minutes until all of the water is vaporized. At the same time the wax melts and runs out of the sprue and pouring cup. The investment does not crack and I have done this fast burnout many times.

There is another side benefit of having a very hot oven. We don’t get any significant amount of vaporized wax to smell up the shop. Instead the wax turns to carbon and then combines with oxygen in the air to form carbon dioxide (CO2) a colorless, odorless gas. Be sure to leave the oven door cracked to permit air to enter. I still use a vented exhaust hood over the burnout oven, but there is still little odor to the exhausted gasses. I would suggest this completely eliminates the need for a steam dewaxer.

Once all the water leaves the pores in the investment, the flask rapidly rises to the final burnout temperature where I allow it to remain until all the wax is completely eliminated. This is determined by looking at the sprue opening and the pouring cup in the flask. As long as the surface around the opening is gray we know the burnout is incomplete. The gray coating is incompletely vaporized carbon residue. When all wax or carbon residue is gone the burnout is complete. The opening of the flask is chauky white. Total burnout time is as long as it takes to obtain a  white flask and is not dependent on any particular time-temperature program. Small flasks will burnout rapidly in an hour or two while large flasks (or an oven full of small flasks) will take longer. The test of how long is the white color of the sprue opening and pouring cup.

Try it. You’ll like it.

Do you believe me? Any discussion out there?


{ 7 comments… read them below or add one }

fredsias April 22, 2015 at 7:30 pm

Regarding your question about defining a “partially dried flask”–
Immediately after the investment has “set” in the flask, the pores in the investment are filled with water. During the burnout the water in the pores will turn to steam and will be expelled from the flask. In a 2 x 2.5 inch flask this takes about twenty minutes and the temperature of the flask remains at 212 degrees F. which is the boiling point of water. After the flask has “set” and before burnout one could say that the flask is “saturated” with water. This means that the pores are filled with water.

On the other hand, if one simply leaves the flask sitting on the workbench for several days, the water in the pores near the surface of the investment will start to evaporate leaving the pores in the center of the flask still saturated with water while the pores near the surface are dried out. I have use the term “partially dried” to designate a flask that has some pores that are dried out and some still filled with water.

I must admit I have never partially dried flasks for different lengths of time to see when it will cause the investment to crack during burnout. If I must allow flasks to sit around for several days before burnout, I usually immerse them in water or throw a wet rag over them to keep the flasks saturated.

I hope this clarifies the meaning in my book.

jayson April 20, 2015 at 10:37 pm

has anyone got more information/ finer details of this process. I am new to casting but would like to learn right. What is meant by partially dried flask? how to check? process someone is using that works as a quality control method?

Jimmie May 28, 2014 at 8:53 pm

Hello, I’m getting ready to try the fast burnout method for the first time and was wondering how long to let different size flasks cure before placing in the burnout furnace.
Thank you for your time

Scott Congress October 14, 2013 at 5:09 pm

This is very similar to the process we used to create our sea shell pieces in out Sealife line. It maybe a lost art but its still the best. We recreated actual shells from our own beach home into beautiful keepsakes for ourselves and other Sanibel Beach lovers to cherish for years to come. Check it out.

Thomas Blair April 13, 2013 at 7:42 pm

I too have tried this method of putting a flask in hot oven with good results. Only trouble I have had is putting flask in too soon…the I hear a WUMP! from the kiln. And find half the investment had exploded out. Evidently I placed flask in before investment had enough time to ‘cure’, or dry out . I find no info on times to let various size flask set(?)
I have been able to ‘rescue’ some waxes from the ’embers’ as they haven’t had time to melt, (if I’m quick after WUMP!)
Any info will be appreciated…..via email
Tom Blair

georg October 26, 2012 at 4:44 pm

Thats wonderful if it works like clockwork. Is there any continuation of this experice ?

Michael Johnson February 9, 2009 at 9:24 am

This is very similar to what I have been doing. And, then I read where someone had posted to the forum that the investment wouldn’t achieve a chemical reaction needed, so I tried ramping my temperatures to give it a shot, you know for science :o) After months of perfect castings, using the turn it on and leave it method, I tried to do it the traditional way, and I had all types of problems. Smoke filled the studio, it took forever to achieve a complete burn-out, and then when I casted the bottom blew out.

LOL, so from now on I won’t mess with perfection. I can say that I have tried it the traditional way, but it just doesn’t work for me. But, I just do one-of-a-kind casting occasionally, and I like quick immediate results. I like to be able to go from wax to polish in one day. If I were doing a whole tree of rings or pendants, I might find the ramping to work better, IDK.

I also don’t vacuum my investment. I just tap it three time on the side and let it set. I use steam as my casting method, and it is a violent push downward on the investment, and the steam and air has got to push out into the investment. It was explained to me that as long as I pour my investment in slowly and carefully, I won’t get nodules on the side of my piece, and having the investment being a little porous leaves microscopic escape routes. My results have been very dense castings, similar to centrifugal, with only one incident of nodules. I am not opposed to vacuuming my investment, and if it works for someone else, then that’s great, but once again, if I had a whole tree riding on my results, I would probably play on the safe side of the street :o) But, it seems like leaving the investment a little porous would assist better with vacuum casting as well. I wouldn’t bank on it, but it might be worth an experiment.

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