Perlite/Mizzou castable

[shuboyje]

Re: Perlite/Mizzou castable

I personally wouldn't add anything to a commercial refractory, but if you need to I think coarsely crushed firebrick would be your best economical easy to find bet. Perlite is the absolute last thing you would want to use, it is a great high temperature insulator which you are adding to an area of the oven that you want to conduct heat half way decent so it can first store heat, and eventually release it. Perlite is going to drastically reduce the conductivity of the oven dome.

You are so close to a small but totally functional oven I would really do what it needs to get there. Add a couple inches of 6:1 perlcrete on top of your structural hearth. Don't put the perlite in your dome, and finally insulate well over top of the oven. If you do that you will essentially have a miniature version of what this site is all about.

[tinkerric]

Re: Perlite/Mizzou castable

Hi,
Your slab is a little bit insulated. I think a 3 to 4" thick perlcrete (6 to1) is what is generally used for hearth insulation.
You could cut your hearth bricks to the diameter of the oven and then cover it all with a 10 to 1 mix of perlcrete.
I used all of a 6 cubic foot bag of course agricultural perlite ($33) and a bit of an 80lb bag of portland cement ($10) to cast a 4" pad and cover the entire oven.
The perlcrete is fun and easy to work with.

I had planned to cast an arch with a chimney, but since this oven is experimental I made an entry out of a bit of copper flashing I had.
It worked well for 3 firings so I left it in place when I insulated.
The oven works fine without a chimney. It draws fine, though I wonder if it might burn a little cleaner with a chimney.

I'll attach some pictures so you can see what I'm talking about
Have Fun Rich

[mekdigital]

Re: Perlite/Mizzou castable

Dome Day part one just ended with a twist. Just a few minutes before starting the Mizzou with plans of setting that on top of the weber I decided it was a bad idea. Trying to pop the cement out and sitting it on top of the bricks may have called for troubles, so I decided to go with sand. I filled a couple of bags of packing_peanuts to save a few dozen pounds of sand, taped them to the cooking floor and covered with sand. Added more sand to shape the dome. The situation is now calm and solid.
My question is now simple: can I lay the cement on the sand or will it create problem as it's not water tight? Should I cover it with a thin plastic cover?

Peace!

[tinkerric]

Re: Perlite/Mizzou castable

Builders of clay/mud ovens usually put newspaper over the sand form. But..
I think with castable refractory you don't want water drawn out of the cement.
So maybe cover the form with cling wrap and then newspaper?
I'm just speculating , others here have cast over sand and might chime in.

As long as you are making a sand form, why don't you also cast the entrance arch at the same time?

Btw, I would have to agree with Shuboyje about just casting with straight Mizzou if you can handle the weight.
I think a thinner dome, insulated on the outside is the way to go. The perlite that I mixed with dense castable is just short circuiting heat transfer.
Using perlite came from my trying to reverse engineer Pizzahacker.
But as I said earlier, I don't think that he was adding perlite to dense castable, he has made something much less dense than even that.

Good luck and keep us posted

[mekdigital]

Re: Perlite/Mizzou castable

Great!

I will do a first layer of pure Mizzou and save the perite for the outer layer. Wrap and newspaper sound good. The two bricks at the entrance make me think it will be kind of stronger and less likely to get accidentally chipped.. They are also easier on the eyes!

Should the little chimney be in between the two entrance bricks?

Getting Ready!

[david s]

Re: Perlite/Mizzou castable

I found tearing the newspaper into strips and then wetting it, was then able to overlap them and have it conform to the shape of the compound curve without getting creases.

[tinkerric]

Re: Perlite/Mizzou castable

I think that now that you have a sand form it would be very simple to shape a nice entrance arch. You can get the door height, 63%, just right and then the oven is ready for insulation. With the brick entrance as pictured, it looks like you have a lot of fussing to do to get the height right and arch bricks to stay in place.
The sand form would also make it easy to include a chimney in the cast if you want one.
I was going to make an entrance with a chimney. but after firing the oven with the copper entrance arch I didn't find the smoke or heat to be a problem.
You won't be burning as much wood as the larger ovens.

The other suggestion I would make is to cut your hearth bricks to the diameter of the dome. This way, you have less to insulate and less hearth to heat.
Rich

[mekdigital]

Re: Perlite/Mizzou castable

Dome is up!

I will soon post some pictures... I am sure I messed things up, but so far everything seems to be okay... Not enough cement to cast the entrance arc, I'll have to stick to the bricks or live with the short pre-arc...

How can I determine the setting time before I cover all with a insulation layer? We're in SD and it's freaking hot these days, after two hours the surface is hard and dry already...

[tinkerric]

Re: Perlite/Mizzou castable

I think you don't want it to dry too fast. I kept mine covered with plastic for a few days, it was also cooler here in May
How many pounds of Mizzou did you have.

[mekdigital]

Re: Perlite/Mizzou castable

This is much less attractive than the send dome!

[mekdigital]

Re: Perlite/Mizzou castable

Hello!

the dome is nice and hard as a rock this morning! The sand mold - although being hollow inside - held up the 60+lb well and did not move at all.

Of course I will not know if the structure will stay up when I remove the sand and I am a little bit concerned about that: I guess there is no way to find out if any part is too thin and ready to collapse. The instructions have a big warning about steam and explosion. As far as I understand the steam is produced both by air curing and fire curing, but especially with the second. Does this mean I need to do the fire curing, litte by little, before I pour the insulation layer?

I'm sure I can find all these answer in the forum, but my structure looks so atypical that I'd better be sure...

THANKS!

[david s]

Re: Perlite/Mizzou castable

If your castable contained fine fibres they are designed to burn away at low temp and leave minature pipes through which steam can escape, which assists the elimination of water. The castable does not need to be kept moist for days like normal concrete. It is a different chemical process and 24hrs covered after casting is all that's required. Your dome will not collapse, you can remove the sand. It will not hurt to wait at least a week to let the weather dry the casting for you. In fact the more sun and wind you can get on it the better because a moist casting is a bitch to get a fire going in.

[david s]

Re: Perlite/Mizzou castable

If you are using perlite or vermiculite mixed with water and cement as an insulating layer, you will also need to let this dry for a couple of weeks as it contains tons of water.

[mekdigital]

Re: Perlite/Mizzou castable

thanks David! so should I burn the fibers with low fires and put the insulation (perlite, water, cement) only after that?

[tinkerric]

Re: Perlite/Mizzou castable

I followed the curing schedule in the attached pdf.

I let my dome sit for a week before I started to cure it.

I insulated 9 days ago,4" of perlcrete, and left plastic on the dome for 5 days.
I fired the oven a few hours ago. The insulation really works. The surface temp of the dome didn't change as the inside heated up.
After about 45 minutes the dome began to steam. At this point the dome got a bit warmer, maybe 125 F.

The insulation is keeping the heat in the oven, but I imagine the water in the perlcrete is still robbing my dome of heat.

How long does it take to dry out 4" of perlcrete on the dome.

Here's my question for those with more experience, which is preferable, curing a cast dome before it is insulated with perlcrete or curing it after the perlcrete is put on?

Thanks Rich