burn out methods for suspenaslurry

Discussion in 'Investment casting Ceramic shell method' started by Ferrisbeu, May 1, 2021 at 12:38 AM.

  1. Ferrisbeu

    Ferrisbeu Copper

    I keep looking for times, temps etc. no dice. please clue me in. wanna try it out tomorrow. have it ready to burn. 9 dips. i have an electric burn out kiln. Im kinda shitfaced right now, but its 1230 so dont judge me.
  2. Jason

    Jason Gold

    I'm sure you don't own an autoclave and I am going to assume you are burning out WAX?? If you are burning out plastic, wood or something other than wax, quit reading now and may the force be with you. David screws with plastic, I do not.

    First keep in mind there is dewax and burnout. Sometimes, it is necessary to just dewax and later burnout. If you are going to immediately pour metal after the material is gone, I'd call that burnout.
    The biggest issue with getting wax out of suspendaslurry is wax expansion. The suspendaslurry is dimensionally VERY stable at all temperatures. This means it resists expanding which is a 2 edge sword. Sure it will hold original dimensions, but it is subject to cracking when wax inside starts to expand. Suspendaslurry has very little green strength (before it's fired) and wax wants to expand it's ass off when you first start heating it. So this means low and slow is not your friend. You must get the wax hot as fast as possible and to start draining quickly. If the wax doesn't have a way to exit, it will expand as it heats up and the shell cracks. Your drunk self didn't say HOW you are planning to get the wax out? I used a weed burner with mixed results at the beginning. Better still would be a preheated hot chamber waiting to receive the invested wax. When the shell is placed into the heat, it would be completely and evenly bathed in that heat. Thus melting the wax closest to the shell, it will start draining immediately and allow an exit for the wax. This can be tricky with a single flame weed burner. And this is why I have been using boiling water to dewax my shells. It provides even heat over the entire shell and the wax floats up and out.

    Another option that I have used with the weed burner is drill 1/8" holes along the sprues, this helps a lot to relieve wax pressure. After dewax, the shell is cooled slowly to room temp and then patched up with slurry mixed with fine silica like a paste. You paste the holes shut with a few layers and then back into the kiln for final burnout, then pour metal. This works great unless you miss patching a hole, then you have a cool metal fountain.

    What ever you do choose, if this is your first time, I highly suggest you do whatever thing you are planning, then cooling it slowly and testing for shell integrity. After the shell has hit around 1700f, it will become more like porcelain or fired pottery. Then tap the shell with your fingernail and listen for the ring. If you tap it and it's more like a thud, you have cracks and need to find them. The fired shell is gas permeable, but you can fill it with water at this point and it should hold it. Cracks and leaks will be pretty obvious. 9layers should be pretty bulletproof, but I've seen it crack with more layers than that. Suspendaslurry does not like thin cross sections or corners. When those come up, wrapping the shell with stainless steel wire around layer 3-4 helps to keep edges from blowing out.

    Let us know how it goes and don't forget the pictures or it didn't happen!
  3. Ferrisbeu

    Ferrisbeu Copper

    Thank you for the info! A day late but it still helps. I got rid of most of the (paraffin) wax with a hot air solderer. Then I put it in my burnout kiln to finish for an hour ramping up to 1k. The pour went fine mostly. The shell had cracked prior to burnout so I recoated it twice which worked fine. BTW you can use SS as glue to add on a broken pour hole. yup. all scrap brass. Original shown. Obviously. Im a believer in this slurry image4 (2).jpeg image5 (2).jpeg image7 (2).jpeg image8 (1).jpeg image9 (1).jpeg image10 (1).jpeg image15.jpeg

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  4. Jason

    Jason Gold

    Yeah, I patch cracks and drilled vent holes with slurry mixed with fine silica.. I've even cracked pouring cups just gently using an old soldering iron picking away at the wax to remove my hooks. The stuff can be a real pain, but it's ability to replicate fine details is worth the trouble of occasional cracks.

    What paraffin wax are you using? Was the skull wax to start with? Understand different waxes expand at different rates. Some are worse than others. Proper casting wax helps with some of this.
    This is what I'm using and I still get cracks. Microcrystalline 2AB-150 https://arizonasculpture.com/wax/
  5. rocco

    rocco Silver

    Slightly off topic, is it possible to form the wax pattern over a substrate that is compressible and also something that could be burnt out after the wax has melted like expanded polystyrene foam? The idea being to give the expanding wax somewhere to go so that it wouldn't crack the shell.
    Last edited: May 2, 2021 at 6:34 PM
  6. Jason

    Jason Gold

    That's a pretty good idea rocco. Depending on the piece, wax over foam would work well. I wouldn't try to recover that wax, but in theory, it should work. Something to consider however, when I cast bronze, I really try not to cast it thicker than say 1/4" thick. You can always tell beginner art castings, it's thick, solid or clunky.

    The real solution is rapid even heating for the entire shell. Barry in lousiana has it pretty close with his burnout kiln. This is why my water bath works pretty good at the moment. I've thought about running 100% antifreeze, but don't know if 235F would do any better than my water boiling at 212f. Maybe a pot of oil at 3-400 would be better?
  7. Ferrisbeu

    Ferrisbeu Copper

    i just use old candles etc. Im not a technical caster. Flyin by the seat of my pants kinda person. I know a little about alot and alot about a little.
  8. Jason

    Jason Gold

    Yeah, ditch your candles. Try the 2ab150. I capture my wax in water, add a little bit of new stuff and strain it through panty hose. The wax costs almost what the metal does so that's why I recycle it. Once you make peace working wax, the world literally is your oyster. But it will take some effort on your part. I hated it at first, but took the time to learn to make basic shapes like slabs, columns and square stock. From there you can assemble or make what ever you want! I did this piece with no silicone mold. I made what I wanted in wax and went straight to metal. Yeah, I roll the dice, but I hate editions and I like one of a kind creations.

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