Lost wax casting

[smokytears]

The search tool doesn't show anything, so I'll just pose this question here

Is anyone using a lost wax method to cast one-piece burn chambers?

I have a 3D printer, and just recently found that there's a variety of 'wax' filaments I could use. Filaments designed for burnout with an investment mold material, allows 3D printing metal parts at home

The projects I would use this for go well beyond rocket stoves. For example, I want to cast zirconia powder in order to make my own ceramic valve bodies for custom spool valves. But I do want to build a rocket stove that's small enough to go in a backpack, for boiling water in a cup or cook pot

[Forsythe]

Jun 21, 2022 14:00:31 GMT -8 smokytears said:

Is anyone using a lost wax method to cast one-piece burn chambers?

The casting process itself might work but a 1-piece burn chamber would be ill-advised due to the thermal expansion and contraction of refractory ceramics when fired to vitrify the ceramic and/or when placed into woodburning use. You'll find that if you don't make the burn chambers with expansion joints, the thermally stressed ceramic unibody will form its own "expansion joint(s)" along the weakest sections and those subjected to the greatest thermal gradients... by cracking.

The projects I would use this for go well beyond rocket stoves. For example, I want to cast zirconia powder in order to make my own ceramic valve bodies for custom spool valves. But I do want to build a rocket stove that's small enough to go in a backpack, for boiling water in a cup or cook pot

For the temperature cycling involved in a rocket stove, you'll want to use the zirconia in a blend with alumina, aluminosilicate, or mullite. Zirconia shifts crystalline phase between tetragonal to monoclinic structure when fired to vitrification temps, cooled to ambient temp, and then cycled back above 950°C.

That temperature-dependent tetragonal/monoclinic phase shift corresponds to a roughly 4% volume expansion and contraction, which will completely destroy the ceramic body with widespread fracturing.



However: Using 10-30% zirconia in an alumina or mullite ceramic composition (with minimum 2.5% MgO) will place the zircon under crystal strain by the surrounding aluminosilicate, keeping it from shifting back to the larger monoclinic phase at the lower temperature (until a crack in the ceramic body begins to form, at which time the zirconia is released from crystal strain pressure in the crack location — shifting back to monoclinic phase, expanding, and pushing the crack-tip closed again, preventing propagation of the crack in a phenomena known as "transformation-toughening"...a property which is unique to Zirconia-Toughened-Alumina ("ZTA") composites, and is necessary for vitrified ceramic bodies containing Zirconia which are to be placed under high-temp thermal cycling as refractories.)

[smokytears]

Jun 22, 2022 9:36:22 GMT -8 Forsythe said:

Jun 21, 2022 14:00:31 GMT -8 smokytears said:

Is anyone using a lost wax method to cast one-piece burn chambers?

The casting process itself might work but a 1-piece burn chamber would be ill-advised due to the thermal expansion and contraction of refractory ceramics when fired to vitrify the ceramic and/or when placed into woodburning use. You'll find that if you don't make the burn chambers with expansion joints, the thermally stressed ceramic unibody will form its own "expansion joint(s)" along the weakest sections and those subjected to the greatest thermal gradients... by cracking.

The projects I would use this for go well beyond rocket stoves. For example, I want to cast zirconia powder in order to make my own ceramic valve bodies for custom spool valves. But I do want to build a rocket stove that's small enough to go in a backpack, for boiling water in a cup or cook pot

For the temperature cycling involved in a rocket stove, you'll want to use the zirconia in a blend with alumina, aluminosilicate, or mullite. Zirconia shifts crystalline phase between tetragonal to monoclinic structure when fired to vitrification temps, cooled to ambient temp, and then cycled back above 950°C.

That temperature-dependent tetragonal/monoclinic phase shift corresponds to a roughly 4% volume expansion and contraction, which will completely destroy the ceramic body with widespread fracturing.



However: Using 10-30% zirconia in an alumina or mullite ceramic composition (with minimum 2.5% MgO) will place the zircon under crystal strain by the surrounding aluminosilicate, keeping it from shifting back to the larger monoclinic phase at the lower temperature (until a crack in the ceramic body begins to form, at which time the zirconia is released from crystal strain pressure in the crack location — shifting back to monoclinic phase, expanding, and pushing the crack-tip closed again, preventing propagation of the crack in a phenomena known as "transformation-toughening"...a property which is unique to Zirconia-Toughened-Alumina ("ZTA") composites, and is necessary for vitrified ceramic bodies containing Zirconia which are to be placed under high-temp thermal cycling as refractories.)

Ok, I'll just need to define some temperature zones in the shape & divide it up accordingly. The bit about using zirconia for refractory was unexpected, but makes perfect sense. Another reason to have some on hand, thanks! The mix ratio is by weight, right?

[Forsythe]

Jun 24, 2022 7:41:23 GMT -8 smokytears said:

Ok, I'll just need to define some temperature zones in the shape & divide it up accordingly. The bit about using zirconia for refractory was unexpected, but makes perfect sense. Another reason to have some on hand, thanks! The mix ratio is by weight, right?

I realized that I didn't answer your question here, sorry for the radio silence. I've been experimenting in this area with producing Zirconia-Mullite blocks for fireboxes & burn chambers via the two methods below, and hoped to have a solid this-has-proven-doable answer for you sooner than now.

The mix ratio is by weight, right?

Yes, by weight...BUT... it depends on a couple of factors. The biggest factor of concern for most of us doing this kind of DIY ceramic chemistry at small scale in home kilns is going to be the firing temperature you're able to achieve — and that's intimately related to other biggest factor: whether we're talking about using Zircon vs using Zirconia

If you haven't seen this post in the Refractory / Firebricks durability thread, I'd recommend reading it:
donkey32.proboards.com/post/37770/thread

From that post:

Some clarification of terms might be helpful here, because "zircon" and "zirconia" are very often used interchangeably in common speech, but they are not technically the same thing from a chemical / ceramic / refractory perspective.

"Zircon" *technically* refers to Zirconium-Silicate (ZrSiO4) — which is the naturally-occurring form of the element — you always find Zircon in nature as a crystalline combination with Silica, most frequently as "zircon sand" which is then processed and purified to varying degrees. The Zirconium-silicate your ceramic supplier sells is only purified insofar as to remove the very small traces of radioactive hafnium that often occurs in zircon sand deposits, and then milled into [relatively large] particle sizes to be used as a glaze opacifier. In *that* glaze-additive use, multiple-micron sized Zirconium-Silicate doesn't crack away because of the fluxes used to make the zircon stay dispersed in a glaze melt ... but those glaze fluxes pretty much defeat any "refractory" characteristics of Zirconia... since it's suspended in a glass specifically designed to melt at relatively low ceramic temperatures, (thus acting as a "glaze") and something that melts at low temperature is, by definition, not "refractory." (Zirconium-Silicate has become a very common glaze additive used on things like commercial porcelain floor tiles, porcelain toilet bowls, and food preparation pottery, etc. and that's why it's in stock at the ceramic supplier.)

"Zirconium" (Zr) is the un-oxidized, metallic, pure form of the element ...much the same way as Aluminum (Al) is the pure, un-oxidixed, metallic form of "Alumina" (Al2O3). [This Zirconium metal is not commonly used, except in very specific metalurgical applications, as it oxidizes on its surface extremely rapidly, just like aluminum does.]

"Zirconia" (ZrO2) is the purified oxide of Zirconium, **which has been dissociated from Silica (SiO2).** (it's sometimes also referred to as "Zirconium Oxide.") It is what is used in technical ceramics, dental implants, and (I think) is what is used in these refractory coatings.

...and Zirconia (Zirconium Oxide; ZrO2) is also what's used sometimes in the optics industry as a glass lens polishing powder — usually this form is in sub-micron-sized particles in the 0.2-0.5 μm size range. Zirconia; ZrO2 in sub-micron-sized powder is what you'll want to use if your home kiln is only capable of reaching 1350-1450ºC. This is important because that temperature range is the upper limit for even the hottest-firing fuel-burning pottery studio kilns for producing porcelain and stoneware. (and even *that* lower 1350-1450ºC temp range is waaayyyy beyond the reach of electric kilns powered by resistance heating elements.)

IF (big if) your kiln is capable of reaching 1600ºC without destroying itself:

...Then it's possible to use the cheaper and more widely-available, glaze-opacifier type of Zircon (Zirconium-Silicate; ZrSiO4). ...bear in mind that 1600ºC is insanely, stupidly, fires-of-Mordor hot. It's a temperature at which even dense, high-fired, super-duty aluminosilicate firebrick begins losing structural integrity and can warp, and it is 175ºC above the temperature rating of even zirconia-aluminosilicate ceramic fiber (which is usually only rated for 1426ºC at the highest)... so a kiln of this type absolutely has to be internally lined with a zirconia-based refractory hard-coating before attempting this 1600ºC temperature range. ...And you'll be going through an insane amount of fuel. ...And you'll need either a special pyrometer that can take readings this high — most of them max out at 1500ºC, tops — or (more affordably) you'll probably just need to buy a set of pyrometric cones to place in the kiln to gauge the temp / firing schedule, based on which ones have slumped [or liquified.] ...And you'll need some welding goggles to avoid burning your retinas when taking internal kiln temp readings or checking pyrometric cones for melt stages. Yes, seriously. The UV and IR radiation coming off the utterly incandescent surfaces of everything in the kiln is no joke at these temps. A full welding helmet is not a bad idea to avoid "sunburns" on your face when peering through a spyhole to check the pyrometric cones.

For context, most porcelain (the hottest-fired pottery ware) is only fired to cone 10 or cone 11. To use (and dissociate) Zircon (Zirconium-Silicate; ZrSiO4)... we've got to push to the face-meltingly volcano-esque range of cone 27 - cone 28. It's just shy of 3000ºF.

This ultra high-temp (1600ºC) processing route is the one used by most refractory companies to produce Mullite-Zirconia refractory brick... because it's a lot cheaper for them to start with bulk zircon (ZrSiO4) than it is to start with already-dissociated Zirconia (ZrO2). The brick type they make is called "Zircon-bonded" Mullite-Zirconia brick, because it utilizes the extra Silica (SiO2) [which is ejected during the dissociation of Zirconium-Silicate (ZrSiO4) into ZrO2 + SiO2 at 1600ºC] to form additional Mullite as it reacts with the Alumina fraction (Al2O3). This leaves behind very small ZrO2 grains (usually ≤1 μm) interspersed amongst the Mullite... which is the end goal of either processing route.




Mullite (2Al2O3-SiO2) is the crystalline matrix of interlocking aluminosillicate crystals which bonds the refractory body together, provides good thermal shock resistance, and good corrosion resistance.

Adding Zirconia to Mullite improves all of those positive properties of Mullite by severalfold:

• Adding 10+% ZrO2 starts to impart that transformation-toughening effect I mentioned earlier.
• Improved corrosion resistance starts at 16% ZrO2...and continues to improve with increasing Zirconia content.
• Optimal thermal shock resistance is around 20-24% ZrO2... but begins to decline again somewhere around 24+% Zirconia.
• The ability of the Mullite matrix to constrain the Zirconia grains and avoid destruction by their tetragonal-monoclinic phase shifting ends at around 30-35% ZrO2... meaning that the transformation-toughening effect is rendered moot above this 30-35 percentage range, because thermal cycling will crack the composite apart.

...So with that in mind, the target ZrO2 content of Mullite-Zirconia bricks [for our purpose: biomass-burning chambers] would be around 21-22% ZrO2. That's the sweet spot between 1) excellent corrosion resistance and 2) excellent thermal shock resistance... for an intermittent woodburning device. (The higher ZrO2 content around 30-35% is better for the always-on glass melting furnaces used in the glassmaking industry, which needs the higher corrosion resistance — and doesn't have such extremes of thermal cycling & temperature shock that come with our intermittent-firing use.)

...if you're going this ultra high-temp route with the cheaper zircon (glaze opacifier):
1) find the Zircon manufacturer's listed ZrO2:SiO2 ratio content for their particular Zirconium-Silicate. It's usually going to be around 59-65% ZrO2.
2) Then find what the ratio of Al2O3:SiO2 is in your particular kaolin clay.
3) Then calculate how much Al2O3 you'll need to add to maintain a 2.55:1 ratio of Al2O3:SiO2... accounting for the amount of Al2O3 already present in the kaolin, as well as the combined SiO2 from BOTH the kaolin clay and the soon-to-be-dissociated Zirconium-Silicate.

The forming and firing would then be approximately the same as the alternate (easier) recipe below, except that you'll need to take the kiln to 1600ºC to dissociate the Zircon (ZrSiO4) into Zirconia (ZrO2) and Silica (SiO2). After a soaking hold at 1600ºC for 1 hour, you'll want to cool the kiln to 1300ºC with a 1-hour dwell at that temp to re-evolve acicular mullite from the copious glassy phase which formed at the earlier 1600ºC threshold.

...If your kiln is (as typical) only capable of reaching 1350-1450ºC:

...Then you'll want to start with the sub-micron sized ZrO2 —which is already dissociated from Silica— and make a slip-casting mix with kaolin clay and extra, added Alumina. Keep the Al2O3:SiO2 ratio at least 2.55:1 for mullite stoichiometry. The approximate mix proportions of constituent oxides should be roughly 56% Al2O3, 22% SiO2, and 22% ZrO2.

With a kaolin clay around 38% Al2O3 content, that would be roughly:
35% kaolin clay,
43% Al2O3 (Alumina) addition
22% ZrO2

(...if you're starting with un-oxidized aluminum for your Al2O3 addition, that would be 22.75% metallic aluminum, which will gain molecular weight from the additional O2 to become 1.89 times its original (metallic) mass upon oxidation to its eventual alpha-aluminum oxide, Al2O3. (During wet ball-milling with the other ingredients, it will partially form Aluminum Hydroxide (AlOH3) which later converts to Alumina Al2O3 under heat of calcination or kiln firing.)

Sidenote:

Another processing route to convert aluminum metal to reactive Al2O3 is by simmering granulated or shredded aluminum (cans, foil, etc.) in aqueous solution with a hefty dose of organic carboxylate acids (like citric acid, lactic acid, etc.) to form aluminum carboxylate sol-gel ceramic precursor. This produces nanometer-sized Al2O3 powder upon calcination to burn off the organic acid fraction. It's much, much safer (although quite a bit slower) than using strong bases and acids like sodium hydroxide and hydrochloric acid. It also doesn't produce toxic chlorine gas like hydrochloric acid can, as all the organic carboxylate acids are comprised simply of Carbon, Hydrogen, and Oxygen. And it doesn't contaminate your Alumina with sodium like sodium hydroxide does. The reaction happens slowly [but is accelerated by heat] as the water oxidizes the surface-most layer of the aluminum, which the carboxylate acids then chelate away from the metallic surface, allowing water to attack to the continually-exposed metal. This cycle continues until the aluminum is consumed (or until you need to add more acid,) slowly releasing tiny hydrogen gas bubbles at the aluminum surfaces from the water, which is donating its oxygen atoms to the process of aluminum oxidation.

Ball-milling the materials together is highly advised, either way. Calcining 60% of the ball-milled clay+aluminum powder-to-alumina mix is a good idea, too, to ensure conversion of aluminum hydroxide to Al2O3, and will help mitigate firing shrinkage.

Adding about 3-4% MgO before ball milling helps as a sintering aid, as does 1-2% TiO2. (The kaolin clay I use already has 1.3% TiO2 in it... and small fractions of natural impurities like this in the kaolin are actually really helpful in sintering the composite.)
EDIT: The addition of 2-3% Lanthanum Oxide (La2O3) shows vastly superior resistance to alkali corrosion (as in: Sodium/Potassium/Calcium/Iron vapor & ash residue) and La2O3 makes the glassy phase far more resistant to carbon monoxide (CO) reduction in pyrolysis zones, AND comes with further improvement to sintering densification and thermal shock performance — versus relying solely on MgO. It is an expensive additive, but if the data from recent research is to be believed, Lanthanum is probably the very best sintering aid and corrosion inhibitor studied to date in Alumina-Silica-Ziconia Mullite composites.

To ensure all the ingredients stay suspended in the slip, it's best to use a small (like 1-2%) addition of bentonite, preferably "veegum-T" or "Bentone"...or one of the purified and ultrafine magnesium-aluminum-silicates which doesn't contain sodium, potassium, or calcium. This helps to deflocculate all the particles and hold them in a colloidal suspension for slip-casting into plaster molds.

The plaster mold will quickly help to de-water the slip — the brick can then be removed and slowly air-dried. Get it as absolutely bone-dry as you can, but give it 2 weeks to evaporite the moisture slowly, such that it doesn't develop drying cracks.

(This type of brick doesn't contain grog, which would normally help with the evaporating water to escape evenly and avoid cracking during this drying stage. Adding grog to a mullite-zirconia brick is extremely tricky, because it will cause decohesion between the grog aggregate and mullite-zirconia matrix if the grog's composition is not perfectly matched to the mullite fraction's expansion upon crystallization. That weakens the brick structure by several mechanisms. If you do choose to use grog, tabular alumina or white fused alumina sandblasting grit would probably be the best choices...but they are expensive options and still kinda chancy.)




Bricks/slabs made from this type of ZrO2 + Alumina + Kaolin clay composition can be fired to between 1350-1400ºC and fully convert to Mullite with the pre-sub-micronized, already-dissociated ZrO2 interspersed between the Mullite crystals. The MgO not only helps in sintering, it also helps in densification to further improve corrosion resistance, promotes Mullite formation, and helps to pin the ZrO2 grains within the crystal matrix.




Propane, natural gas, diesel, or waste vegetable oil are your best bets to reach temp without impregnating the green bricks with wood ash/flux/slag during firing while they'll be susceptible to (and absorptive of) those impurities. Be sure your kiln has plenty of excess O2 during firing. We don't want to go into reduction atmosphere at any point during this process. Forced air is a good idea here to ensure the fuel is completely burned within the kiln with an excess of oxygen to keep the brick's constituent oxides from de-oxidizing (AKA "reducing.")

Take the firing schedule from warmup to 700ºC slowly to allow the molecularly-bound water to escape, and for the organic impurities to burn out of the brick. Going too fast will cause steam to become trapped in the brick and spall — and/or will seal the microscopic brick pores before the carbonaceous organic material can oxidize to CO2 and escape the ceramic body. That causes boating and black-coring, which is doubleplus ungood.

If you happen to go above 1400ºC, then just slowly cool back down with a dwell at 1300ºC to help re-crystalize acicular (needle-like) Mullite from the glassy phase that will have evolved.




Cheers

[martyn]

I am not sure about lost wax but a lot of folk use hard, home roof insulating foam, it can be shaped to a fine detail and scraped out of the mold,
I have used this method for dozens of projects, getting the foam out involves breaking it down with screw drivers etc but it will always come out ….though be it in a hundred pieces