Casting Refractory - This is NOT a drill!

[david s]

Re: Casting Refractory - This is NOT a drill!

Unlike portland cement concrete, calcium aluminate mixes do not need an extended curing time. ie kept moist for a week or so. They rely on a different chemical reaction and set without need to cure in 24 Hrs. Some time should be given though, about a week, to allow the moisture to dry out before firing.
Dave

"Kindled with Zeal and Fired with Passion"

[altamont]

Re: Casting Refractory - This is NOT a drill!

I am not associated with or have seen the oven in question. But I am a Ceramic Engineer that worked in the field for a long time, and worked for AP Green who developed Mizzou Castable. APG merged with others when manufacturing plumeted in the USA.

Unless discussing something different, the short fibers mentioned disappear when heated - sublime/sublimation is the term meaning they go from a solid to a gaseous state - leaving extremely small tubes/pores behind. These reduce cracking by stopping crack propagation (cracks travel; when a crack hits a pore the stresses at the tip of the crack can no longer stay concentrated as there is no crack point anymore; the crack then stops there). Great technology! Additionally, these tiny tubes/pores left behind create extremely tiny 'vents' for steam to travel in. This reduces the pressure of steam during the initial heatups!

Ultimately, IF anyone is going to cast an oven using a castable refractory you really, really should request that these fibers be included.

But do NOT confuse these fibers with the stainless steel needles that might be supplied by your supplier. Those are sharp, remain in the castable, eventually will rust which expands their size risking the eventual creation of cracks from the inside out (i.e. ceramics are strong in compression but not tension - such inside pressure puts the material in tension). I got cut up a few times by leaning on a cast shape that used those (you can't help but have some needles exposed on the surface).

[altamont]

Re: Casting Refractory - This is NOT a drill!

Another note on castable refractories: By nature they all use water in the mix. And there is always going to be more water than is required by the chemistry within - all of the binder/cement will have sufficient water if you add enough water to be able to pour the material. All of the excess water (that is not tied up in the bonding) will turn to steam when mixing and that needs to be allowed to be driven off during the initial heats until it is all gone. Those fibers mentioned above assist in that.

But another side affect of excess moisture is that, if too much water is added, it can actually weaken the strength of the final cast shape.

BUT THERE IS AN EASY TEST FOR THIS when mixing. In the industry it is called the "ball in hand test". As you are mixing and adding water, stop any mixing and grab a baseball sized sample or slightly smaller (if you use a mixer be sure it is OFF! Safety first!). Form a ball with it. Rest it in your hand, palm up and fingers slightly splayed apart. Jiggle your hand.
- If the ball of material falls apart easily it is too dry. Add a little water.
- If the material slightly slumps between the fingers but the ball stays
together you have a great mix! Do not add any more water. Mix a bit
more and confirm by doing another test.
- If the material flows down between your fingers and easily drips it is TOO WET.
Kind of late as it is impossible to remove water. And adding more
castable is touchy - you don't want to grab a lot of binder out of
the next bag changing its proportion of materials.

Ideally, the castable can be mixed 'slightly' on the dry side. This means it is slightly harder to work with - it will not pour and will require a lot more vibrations to settle it and remove voids. But it will be stronger because it will have less excess water and be closer to having just the water needed for the binder. Again, it will be harder to work with and if you do not have a good mechanical vibrator used for pouring concrete you run the chance of having voids - "honeycomb" in the final product.

Have fun with this information!

[Bakehus]

Re: Casting Refractory - This is NOT a drill!

Thanks Altamont for this in depth information!
My only worry/question i have:
can a castable refractory replace the properties of a firebrick?
im always worried that a cast oven wont hold the heat as long as an oven out of firebricks would . and when you say the fibers leave tiny holes behind, that sounds like its more insulative due to the airpockets.
I hope enz is still around to report back on this issue or anyone else with long term experience with that mix?

by the way do you get a discount for that stuff just saw some shocking prices!
All the best
hendrik

[enz]

Re: Casting Refractory - This is NOT a drill!

I am indeed around, but have not been checking the forum.

After four years this is what I have to report.

1) Mizzou castable is an awesome product. It has exactly the thermal properties of firebrick (it's not insulative in the least) and is easy to cast. That being said, it is EXPENSIVE. I spend just under $1000 to cast a 42" volto basso, not including the stand, insulation, flu or stucco.

2) Cast in one piece, or smaller pieces that will be assembled like a Forno Bravo kit. I cast my oven it two pieces, the front including the arch and the back. The oven cracked right along that line. That has not effected the performance one bit though. I patched with a bit more mizzou, insulated with pearlcrete and stucco'd over a 1" ceramic blanket decoupling layer. After 4 years the oven still works almost flawlessly (see 3, not an issue with mizzou).

3) Manifold your flu properly. In other words, make a nice lead in from the arch to the flu for better flow. I just had a square corner and have a little smoke come out of the front of the oven. To solve this, I made a stainless steel door that is open at the bottom and forces the smoke up the flu. As a bonus, the air that gets sucked in the bottom is blasted straight into the fire and helps the heat up immensely. I will make one for the new oven that I am building at our new house.

Feel free to send me a PM if you have further questions. I don't always get e-mail alerts when people post to this thread.

Enz

[fxpose]

Re: Casting Refractory - This is NOT a drill!

3) Manifold your flu properly. In other words, make a nice lead in from the arch to the flu for better flow. I just had a square corner and have a little smoke come out of the front of the oven. To solve this, I made a stainless steel door that is open at the bottom and forces the smoke up the flu. As a bonus, the air that gets sucked in the bottom is blasted straight into the fire and helps the heat up immensely. I will make one for the new oven that I am building at our new house.

Glad to hear your oven has been performing flawlessly and to expectations over the last several years.

Are you able to post a photo of your manifold and the mods you made? If not, no problem...

Thanks a bunch,

George

[enz]

Re: Casting Refractory - This is NOT a drill!

George,
We sold the house and so can't take a pic, but I'll try to describe it in more detail.

The door is made from one sheet of .032" 304 Stainless Steel. The door is the shape of the arch at the front except the bottom two inches where it bends 90 degrees inward and goes all the way to the dome entrance. It has 2" aluminum feet to support it. The back left leg is 4 inches to the right to clear the thermocouple stand that goes in the doorway too.

I made a quick CAD model and attached a jpeg of it. At least I think I attached it.

Enz

Glad to hear your oven has been performing flawlessly and to expectations over the last several years.

Are you able to post a photo of your manifold and the mods you made? If not, no problem...

Thanks a bunch,

George

[fxpose]

Re: Casting Refractory - This is NOT a drill!

Thanks Enz! I see exactly what you did.....that is very clever.

George

[altamont]

Re: Casting Refractory - This is NOT a drill!

The fibers are of such a small volume that the porosity they add has an insulating affect approaching zero. The porosity left over from excess water contributes more.

Mizzou Castable is a relatively dense, high alumina castable refractory that has very high strength and can take temperatures far exceeding what we would experience in a normal wood-fired oven. Heck, it would make a great lining for firing pottery and hold up for years.

If there are newer castable refractories out there with densities approaching a dry-pressed fire brick (low, high or super duty) and can withstand @ 2200 F. check them out - they would probably do very well as the hot face material in a wood-fired oven. I recall that AP Green had a series of castables that were referred to as flux vibrating castables that I would love to get my hands on! These were mixed with minimal amounts of water, would appear to be damp (not wet or dry) and would fail the 'ball in hand' test above (the handful would just fall apart and never stay together when properly mixed). The only way to cast anything with them was to use very high frequency vibrators immersed in the castable - and if the vibrator lost power when in the castable you'd rarely be able to get it out. Heck, right after vibrating you could walk on top of it and not sink in. VERY dense. But VERY susceptible to exploding on the first heat up. The material was so dense steam from any excess water had trouble getting out, building up pressure. A very slow initial heatup was required. But it would give you an extremely hard and dense lining. Heat storage capabilities were right there with brick! And the cost was about 1/3 lower than Mizzou Castable.

I would suggest that home owners look into adding Mineral Wool to the list of insulating materials. At our temperatures, the insulating properties are fantastic!! Even when compared to ceramic blanket. I would still put a layer of ceramic fiber (paper or blanket) against the dome as extra insurance for where ever a dome cracks (they all will but not cause any structure issue). But right after that, a good 2 - 3 or 4 inches of mineral wool! That would help keep heat loss through the dome down a lot, keeping more inside the hot-face lining and inside the oven.

[altamont]

Re: Casting Refractory - This is NOT a drill!

I referred to a group of castables AP Green had manufactured. These were also referred to as "low cement" castable refractories. Do not let the "low cement" term scare you - these were extremely strong materials! Their impact and abrasion resistance were phenomenal too.

[arvinyl]

Re: Casting Refractory - This is NOT a drill!

Enz,

I am reading with interest your post on casting your own refractory bricks. I am in desperate need of refractory bricks for a 1956 oven I use. This oven uses cast ceramic elements attached to cast iron air/natural gas premix burners to heat laminating materials. Would you or someone you know consider trying to make these bricks for me. Know this is a strange request, but I am up thee proverbial creek without these bricks. Thanks.

Mike
mike@arvinyl.com

[Boogie-D]

I know this is a very old post but when I search Mizzou Castable this pops up... this build is very close to what I will do... Mizzou sounds like a great expensive product but much better than casting the dome with perlicrete and at the cost of fire brick ($5 a peice) in Hawaii this may be my option... I am looking into pacific refractories in Honolulu who claim to have Mizzou refractory cement.... we will see... in Honolulu we have $5 dollar fire brick and Mizzou refractory... we also have white Portland cement and perlite..
1) question to enz or who over can help...
in this build you use perlicrete over the concrete base... then covered that with the Mizzou refractory cement as your pizza oven floor...
a) is this an acceptable product for food contact application...???
b) how did this mizzou product work out for you as your pizza oven floor???

thank you

[david s]

Not a good solution. For the floor you really need dense firebrick. A floor made in castable refractory really needs to be kiln fires. It’s ok for the dome but not for the cooking floor. An alternative to using a proprietary dense castable refractory for the dome is to use the homebrew which is much cheaper. 3:1:1:1 sand, cement, hydrated lime, powdered clay.

[Boogie-D]

Thank you David for this advice.... at $5 a fire brick local Honolulu ... I will need about 12 square feet... local web site says 6 bricks for each square foot... so 72 bricks.. ($360 just for the oven floor bricks and I am not even sure of the quality of these fire bricks.. we don’t have many options in Honolulu... maybe I am doing something wrong with the math here.. these are standard clay brick size... 72 bricks for oven floor just don’t seem right... I am using 95 cm yogas ball 37 inch...

as far as the Mizzou I here it’s very expensive... and still waiting for pacific refractory to respond to my call and to get pricing... we do have all the ingredients in Honolulu to make home brew... I will search the forum for more info about using home brew for the dome... I am all in if it works well for absorbing and refracting heat and is cheaper... I am assuming you follow the protocol of insulating with some kind of mineral blanket and a fat layer of perlicrete...

thanks for the quick response David.. I am excited to begin and know my strategy for building this in Hawaii... aloha

[david s]

If you're near the beach you may be able to access some free pumice stone to use as insulating aggregate instead of perlite or vermiculite.