Cast Refractor Failure?

Here is my first attempt at casting refractory cement. Product is called Meeco's Red Devil Castable Refractory. This is for the firebox roof in a Shorty core, which I split into two parts because one continuous slab would be over 60 lbs, which seemed unreasonable to cure and handle.
I made the form 2.5 inches tall and coated it in wax for form release.
I dry mixed the refractory, and then carefully added the appropriate amount of water and mixed for about 7 minutes (instructions said I should mix for between 3 and 6 minutes but I needed more time.)

From there I packed it tightly into the form. Then I TRIED to vibrate, and this is where things started to feel off. I had 2 massage guns and an oscillating tool on hand and tried them all but the cement did not visibly settle much at all and no water rose to the top as I'd seen in videos of this process. I figured I'd just packed it well enough. I sprayed the top with a squirt bottle and covered it in plastic.

48 hours later I released the form and I'm seeing dry cement around the edges that brushes right off. Also seeing some cavities.

I sprayed down the dry areas and covered it back up, hoping they absorb and firm up.

But... did I totally botch this attempt???

Well Matt;
I agree with you; those do not look like they will hold up in the long run (perhaps even in the short run!).

I used a variable-speed saws- all with no blade. My forms were sitting on plywood with a tire bolted to it.
It worked great.  https://permies.com/t/254174/Casting-Large-refractory-Slabs

I suggest pouring the box slab in one piece to reduce the risk of issues later.

thomas rubino wrote:
I used a variable-speed saws- all with no blade. My forms were sitting on plywood with a tire bolted to it.

I very much appreciated and studied that post before I made this attempt, but I suppose I was unclear on how you used the saw to vibrate the form.
So the form was sitting on a tire which allowed the whole thing to "jiggle" more than it ever could sitting on a solid surface (my first mistake.)

How did you use the saw though? Just pressed the front of the saw against the table and pulling the trigger engaged the piston inside enough to vibrate the table?

The plywood was bolted to the tire and the form sat on top.
Yes, no blade, rest against plywood, and apply throttle.
When you get the speed right you will know,  it will smooth out with no bouncing. The few bubbles there are will quickly rise around the edges.
I was warned, possibly by Fox, who has vast experience vibrating cast items, to be careful "Not" to over-vibrate!

thomas rubino wrote:
I was warned, possibly by Fox, who has vast experience vibrating cast items, to be careful "Not" to over-vibrate!

I took the warnings not to over vibrate or over water too far!

So now I'll put these slabs through a torture test just for fun. And probably end up re-casting (a $100 mistake.)

Or I could sandwich the slab in kiln shelves. I could have used kiln shelf from the get-go but that's more expensive and I wanted the thick cast refractory slab for mass since the top of the firebox will be the bottom of a white oven.

But on the bright side, sharing failures is what helps other succeed!  

Oof, that 100$ is the kinda lose that makes looking for alternatives so appealing.
I think you should use it as is, and then if it fails, put the pieces on top of the kiln shelf and stick them together /parge them with some cheaper, lower temperature refractory.

Well that is unfortunate for you, something obviously went wrong!
The top looks like it has been deprived of ‘water’ or ‘cement’ not something I have seen very often but it looks like the top layer dryed out to quickly however, without actually being able to witness how it was mixed and how it was vibrated, I can only guess at what happened.
The correct amount of water (normally around 4lt to 25 kg depending on manufacturer ingredients )  is really critical for two main reasons.
One is to cause the chemical reaction to cure the cement and the other is to ensure a strong end product that won’t carry too many voids.
So whatever the amount of water added, that volume of water will be replaced by air as the water evaporates forming microscopic voids through out the cured end product. That is why it is mixed so dry, if you double the amount of water you will double the amount of voids!
So the mixing of relatively small amounts of water must be very thorough to ensure every monocle  of cement dust is dampened.
Vibrating offers several benefits, it allows all the grains and cement in the mix to settle down tightly and in turn expel any excess water.
However if you over vibrate all the smaller grain and particles (dust) will fall to the bottom leaving a dry top and dense bottom!
With a proper vibrating table, with a shallow mold it will take about 10 seconds.
With DIY tools you just have to guess and keep your fingers crossed.
With a proper table, the mix is compressed very rapidly forcing the water to the surface, the water on the surface will then partially sink back down but also ensure the mix stays moist and does not dry out to quickly.
However too much water on the surface is a sure sign of, to wet a mix or over vibrating.
Your results look like a mix that has been left to dry in the sun and has dried out too quickly. I not saying you did that but that is what it would look like if somebody did do that.
Refractory cement is not an easy product to work with and the end results are very much based on correct mixing, vibrating and curing.
As it only offers about 20-30 minutes working time you need to be quick and make sure the mold is covered as air tight as possible as quick as possible. IE as soon as it is vibrated, it must be covered to keep the moisture in for the next 8 hours or so.
A big plastic bag would be ideal but anything that absorbs moisture like dry sacking or plywood will have the opposite effect !
The lower part of your cast may be ok, I would scape off the top and test it out.

An update: I opened the mold and sprayed the tops and edges with water. Then re-covered in plastic again for another 24 hrs. The top edges that got squirted from both exposed top and sides seemed to have firmed up a bit. The bottom edges were still awful soft (dry/powdery) so when I took a wire brush to all the edges, the bottom edges gave up more and exposed some aggregate. A metal scraper knocked off any loose aggregate.

No idea why those bottom edges were so dry. The soy wax I used was plenty thick and souldn't have robbed moisture. But the bottom face is nice and smooth at least.

Now they're suspended over a dehumidifier to begin the drying process. I think I'll keep playing with these and not count them as a 100% loss just yet. I will do the gradual heating ramp up curing and see how they look and feel after all that.

The slabs spent Sunday in the oven, wife barely approved. I read that the free moisture in refractory is the most dangerous and that's what requires a careful and slow ramp-up. So they spent 9 hours slowly ramping up to 500F degrees.

After that I brushed off more dry/loose material and the edges look pretty rough. But the slabs have a nice ceramic "ring" to them now that they are partially sintered.

I took them out to the shop to mock up the core. The upright bricks on top are the oven walls, which still need a kiln shelf top. More details on the core and Shorty riser will come in another post.

Now you can see why I chose refractory slabs over kiln shelves. I designed the build to support the oven above, and I'm afraid of the direct flame on just the center of the kiln shelf would cause uneven heating compared to the sides that are resting on brick. Uneven heating of tile is what makes it crack so I have the same fear with kiln shelf. I'm much less afraid of this with a 2 & 1/2 inch thick refractory slab. But still undecided if THIS slab is good enough. Next up: I'll heat the slab pieces with a powerful yard torch to see how they fare.  

Matt,

Actually the thinner kiln shelf is exposed to less breaking forces.
The breaking torque at the heated element is expressed by formula:

M = (deltaT * E * W^2) / 12
delatT: temperature difference between two sides of the heated element
E: elasticity coefficient
W: element thickness

So the M is proportional to the power of thickness.
I have a ceramic gas kiln and use 5/8" shelves and so far only one developed a hairline crack just above the propane torch where temperatures reach 1200 C for few hours. I also fire heavy objects so the shelves are under more stress.
Your slab will store more heat. Are you planning to make it a black or white oven?

Cristobal Cristo wrote:Matt,
Actually the thinner kiln shelf is exposed to less breaking forces.

Your slab will store more heat. Are you planning to make it a black or white oven?

Good to know!  I was majorly concerned about both the uneven heat and heavy items on top of a suspended kiln shelf (however heavy something like a full casserole dish could be.) But if you're successfully blasting the bottom of a kiln shelf with a higher temperature than a firebox would ever see, that's encouraging. I did consider double stacking 5/8th" kiln shelves to be even more robust.

It will be a white oven topped with kiln shelf. I had no reservations about putting on on top where there is less heat and no weight load.  

So far I'm hoping to stick with the cast slab precisely because it will hold more heat, but I'm open to folks here trying to convince me to go kiln shelf instead.

I think a thick solid mass is a good choice for the oven floor as it will hold and stabilize the heat inside the oven.
Kiln shelves are amazing and certainly have there uses although for me they are an expense option for me.
A lot of refractory materials seem to be very regionally priced, for me 25kg of casting cement would be around £25 a 12” x 18” kiln shelf would be around £75.

One 5/8 shelf supported by 4 posts holds around 14 kg of pieces that I fire, so 30 lbs. The span between posts that supports 15 lbs is about 8" wide.

Regarding the white oven, please be aware that it may not work as you wish. Usually white ovens are made from steel or cast iron and are used while burning.

I have built a two chamber oven. I use the bottom chamber (being also the firebox, 200 mm BBR equivalent) for quick baking and when cooler, for slow baking and cooking soups/stews. It gets up to 450 C.
The top chamber is installed on the path of the riser exit and gets heated to 230 C. So already the black oven is twice colder despite the hot exhaust blasting it for 3 hours. It's much more massive than yours, but also more powerful and insulated quite extremely.
In baking, the most important is the floor, but if the walls are cold, nothing will be baked. I remember when I was baking in my improvised oven. The floor was hot - still covered with hot coals, but too cold walls resulted in failure.
If the interior of a brick oven will not get penetrated by hot exhaust it may just get warm and not more. The exhaust gases will freely travel in the bell and not so much energy will be left to heat the oven walls from the outside and even if they got heated, the inside will be too cold. When my temporary oven got heated by direct fire (not expanded and colder exhaust gases) to pizza baking temperature, the outside was just hot to the touch. Imagine to have it in reverse - outside quite hot and inside just hot to the touch but too cold for any baking.
If such an oven was constructed from steel with the slab floor then there would be a chance that you can be happy with it.

I do not want to be negative, just want to share my findings. It would be great if you could try it outside first - dry stacked. I know it would be an effort to dry stack the bell, but it would be worth. If you found that it does not work you could try to fabricate or purchase a metal oven and then see how it performs.
Even if it fails I can see that your firebox will be quite massive with wall bricks laid as stretchers and additional lining. If you insulate under the firebox floor with insulating bricks or at least 4" of perlite clay, insulate the walls and the slab, you will get a wonderful oven.

Cristobal wrote:
Regarding the white oven, please be aware that it may not work as you wish. Usually white ovens are made from steel or cast iron and are used while burning.

You're absolutely right. Last night I did a test on a 1 inch thick slab that I'd made from extra refractory. I used a very powerful propane torch to blast the bottom up to 1000F degrees (538C) but the top never got above 190F (99C.)

Extrapolating that into the firebox, the slab bottom would never get anywhere near 1000F from woodfire alone and with the actual slab at 2.5 inches thick... it would never truly heat up to anything close to a usable oven temperature. And like you said, the brick side walls would not pick up enough heat from exhaust gasses alone.  Perhaps it would reach an operating temperature if I continuously burned wood all day long but that's not desirable to anyone.

So now I'm thinking I should replace the refractory slab with 5/8ths kiln shelf (since the slab is TOO massive to conduct heat) and top that kiln shelf with a steel box oven. This would give me heat conducting up from the firebox and also heat from the sides/top via the exhaust gasses.  Quick heat response is good, I think. If I'm cooking something that needs more oven time, I could just add a log or two. Does that sound like a more reasonable design?

Cristobal wrote:
I do not want to be negative, just want to share my findings.

Not at all. This is why we post here, so others can provide feedback and save us from mistakes!

I think you have a point however I am not so sure, it depends on how you intend to use your stove.
If it is one load in the morning and one in the evening then the slab wont take in enough heat.
If your stove is more dedicated to cooking then you could burn it for longer and saturate the slab, then it might be a good option.
There are millions of pizza ovens in use that use thick mass for prolonged cooking times but they do take several hours to saturate with heat. You can expect the face side of a 3” thick pizza oven to reach around 500c and the reverse side around 300c after 2-3 hours of heating. The good news is it takes two days to return to ambient temp.

I really dont know how effective your idea will be as you will have the opposite issue with a thin kiln shelf as it will only work while the fire is running, maybe it will still stay warm once the fire dies down to coals but not for long?

White ovens are one of those things we see regularly on this forum but rarely find out how or if they work!
It can certainly be done with a J tube that is used for continuous radiant heat but a batch box does offer some unknown challenges.

Matt,

Traditionally white ovens were not used for baking, but for lower temperature operations. "Baking" traditionally means bread.
In case you use the shelf you have a choice of using 5/8, 3/4" or 1" version. I would recommend 3/4 as they definitely feel more robust. I would recommend to design (or purchase) your heater/oven doors in such a way that you will be able to remove and replace the shelf if needed.
If even the steel white oven with kiln shelf floor will not be satisfactory you will still have an awesome oven of the firebox. Please insulate the firebox floor really well and then the side walls. You will be able to cover the shelf with removable insulation material to improve the firebox oven performance. If this case you could put another thick shelf/slab on the 3/4 kiln shelf and then put some insulation to increase the firebox ceiling thermal capacity for the following baking session.

Taking all your comments into consideration, I'm going to go with a silicone carbide kiln shelf. Thinner and stronger than the standard cordierite shelves with better thermal conductivity (only con seems to be a higher price!)

I will surround this with a steel box for the oven. When I'm not using the oven, I will leave a piece of superwool blanket on the oven floor for better firebox efficiency.

Hopefully this will give me the best of both worlds: a quickly heated oven that is powered by both the firebox below and the hot gasses in the top of the bell. The Shorty core will allow for adding wood if I need a longer cook time and the thermal mass of the bell should ensure longer useful cook time.

I acknowledge that this is all experimental and unproven but I'll give it a shot and report back!

Sounds good apart from putting super wool in your oven, that is not a good idea due to the risk of microscopic fibres being ingested!

Fox James wrote:Sounds good apart from putting super wool in your oven, that is not a good idea due to the risk of microscopic fibres being ingested!

Super wool is the mineral based safe alternative to Kaowool and Ceramic wool. I would still probably rigidize it though! Or perhaps use vermiculite board.  

Matt Todd wrote:

Fox James wrote:Sounds good apart from putting super wool in your oven, that is not a good idea due to the risk of microscopic fibres being ingested!

Super wool is the mineral based safe alternative to Kaowool and Ceramic wool. I would still probably rigidize it though! Or perhaps use vermiculite board.  

Mineral wool or not, I will agree with Fox James 100% about Microscopic fibers, I have used all kinds of Super wool, and if you still don't believe, just have some strips by the heating area ( and when they have turned a slight tan/brown in looks, hold in the sunshine and very gently rub them slightly together) And that should cure you of any thoughts of using it as a seal in any food bearing areas.

As we get older, we unfortually get to learn what cronic things  micro fibers can  provide for you.  Think of it as a persistent present for the rest of your life.

I relate that to a lable that reads 1 part per million is safe, and 20 parts per millon is not safe,   Says WHO?