Hello from the Philippines

[Manolo54]

Hi to everyone out there! Can someone tell me whether it is possible to use a mixture of perlite and portland cement as inner dome material for an igloo oven? I have searched the Forum and seen it being used as the outer dome over insulating blanket, as well as hearth flooring underneath refractory bricks but not as inner dome...

[Ken524]

Hi Manolo,

Perlite/portland is an insulation material that goes on the outside of the oven to keep heat inside the oven. It is definitely NOT appropriate to put an insulation material inside the oven.

You want to build the oven with firebrick (or perhaps a cast refractory material) then insulate the outside of it with blanket, perlite/portland or both.

[Manolo54]

Thanks, Ken...but aren't firebrick and castable insulating materials as well?

[Ken524]

There is a BIG difference between insulation and thermal mass. Firebricks are thermal mass. They absorb and store heat. This makes them perfect for an oven. When you fire the oven, the bricks absorb the heat and keep the oven hot.

Insulation is the ability of a material to slow down the transfer of heat energy. Insulation doesn't absorb or store heat. Think of a Space Shuttle insulation tile: you can put one in a 2000F kiln for several minutes, pull it out and pick it up by the edges with your BARE HAND. This is because the tiles are excellent insulators and don't hold heat. This would be a horrible building material for an oven. An oven made out of space shuttle tiles (or perlite/vermiculite) would never get hot.
Space shuttle thermal tile test - YouTube

Once you heat up your bricks, you want to keep that heat from transferring to the atmosphere and wasting all your energy (firewood). We use insulating products like ceramic blankets and vermiculite/perlite (or space shuttle tiles!). These insulators resist the transfer of heat.

For an efficient oven you need some thermal mass for storage and insulation to slow heat dissipation.

Here's a good discussion on the difference between Thermal Mass and Insulation:
Thermal Mass vs Insulation | Tiny House Design

Now... Be careful when shopping for firebricks. There are "INSULATING Firebricks". These are very lightweight firebricks - they have a lot of air pockets in them to give them insulation qualities. These are NOT appropriate for building the dome of your oven.

Bottom line: Use bricks for the dome, use ceramic blankets and/or vermiculite/perlite and portland cement for your insulation.

Here ends today's physics lesson. If I've screwed up any information, I invite the pros to jump in and correct me!

[Ken524]

By the way, what part of the Philippines are you in? I was at Clark Air Base for a couple of hours last night.

[Manolo54]

Thanks, Ken, I am in Quezon City, about 90 kms south of Clark Air Base...What brought you to Clark?

[Laurentius]

He's a CIA+M-I6, operative and if he tells you he would have to kill you.

[Ken524]

Originally Posted by Laurentius

He's a CIA+M-I6, operative and if he tells you he would have to kill you.

Close, really close...
I fly for a package delivery company. I'm hopping around Asia this week. Back home in a few days to fire up the oven!

[Laurentius]

See what I mean. He's what they call a "Cleaner", hops around dropping body parts all around Asia, and we know why he has to fire up the oven, right?

[Manolo54]

Wow, so is this site a cover for some clandestine organization? Seriously, I work for a multinational with HQ in Boston, so Ken might have flown a "package" or two for us...