Casting Refractory - This is NOT a drill!

[david s]

"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".[/QUOTE]

I used mineral wool for two ovens, but won't use it again. It is quite a bit cheaper than ceramic fibre blanket (by about 25% from memory) and only slightly less insulating, but using and applying it was another story. It doesn't cut nearly as well as CFB, is far fluffier and compresses too easily. Covering it with a layer of vermicrete was also quite problematic. The vermicrete wouldn't stand up vertically against it easily the way it does with CFB. If building a dog kennel design I would consider it, but not for an igloo.

[UtahBeehiver]

If you do a search on "hawaii" you will see several builds and maybe some info on supply sources. You may want to reconsider the use of a yoga ball and do a in place sand mold. I know that this is the preferred method by DavidS for a myriad of reasons. Area of a 41" circle (37" id 2 x 2" cast wall) = 9.2 sq ft each 4" wide by 2.5" thick by 9" long fire brick is 0.25 sq ft (4 x 9)so you need a min of 40 bricks plus a few extra. The 6 per sq foot is assuming laid on the 2.5" edge (not necessary). Mineral wool insulation is not your best dome insulation option, ceramic fiber is best with a p or v crete or possibly a pumice/concrete mix. There have been several good cast ovens done this year. Do a seach on these, Nick's from the UK did a very good job. PS family roots from Oahu and Maui.

[Boogie-D]

Aloha David S and Utah beaver... thanks for the tips... I have looked at all the Hawaii builds and am using them for reference on material locations... I new something was up with my math.. lol... on edge... looks like away to get you to buy more brick... anyways I feel like I am hijacking enz’s mizzou thread so propabaly should start my own build thread now... ttyl

[david s]

To achieve its full strength castable refractory requires sintering. This is the process that converts the chemical bond of the calcium aluminate cement to one which makes the clay content permanent. This process begins at 573C and continues up to around 650C and the material getting harder north of those temps. In the normal situation the material is fired in place with burners in a very controlled manner up to service temperature well over 1000C. If you have access to a kiln, sections or bricks made from castable refractory can be kiln fired and the resulting products are equivalent to firebrick. The use of castable refractory for ovens does not sinter the material, so the bond relies on the chemistry of the calcium aluminate cement. This actually leaves it vulnerable if continually fired up to the 500-650C range where differing expansion rates of the materials contained in the mix make it vulnerable. As this is generally beyond our cooking range, as long as the oven is not fired up too hot it will be fine. It is a mistake to think you can fire it up to sintering temperatures in order to strengthen the material. When firing with wood it is not possible to control the temperature rise in a slow controlled way. The material simply results in micro cracking.
For this reason, because of the vast difference in cost between castable refractory and homebrew, for a one off build the homebrew is a better choice.
Firing an oven quickly can also damage the mortar either homebrew or calcium aluminate based mortars as they have not been sintered either. Much better to take it slow, allow the temperature to rise gently <200C/hr and be kind to your refractory. Unfortunately we always tend to be in a rush to get the temp up as fast as possible.

[david s]

This reminds me of a very instructive incident some 40 years ago when I was biscuit firing a small wood fired kiln at the same time as teaching classes. Consequently because of the wood firing and not being able to constantly monitor, I was unable to control the temperature rise slowly and the kiln contents had lots of micro cracking, those cracks generally getting worse when the wares were glaze fired. Thereafter I always biscuit fired in an electric kiln.

[Boogie-D]

Great knowledge David s... I am getting excited.. I’ll start my own build thread soon

[Jubilado]

"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".

I used mineral wool for two ovens, but won't use it again. It is quite a bit cheaper than ceramic fibre blanket (by about 25% from memory) and only slightly less insulating, but using and applying it was another story. It doesn't cut nearly as well as CFB, is far fluffier and compresses too easily. Covering it with a layer of vermicrete was also quite problematic. The vermicrete wouldn't stand up vertically against it easily the way it does with CFB. If building a dog kennel design I would consider it, but not for an igloo.[/QUOTE]

I was considering mineral wool layers on top of an initial layer of ceramic fiber blanket, topping with an inch or so of vermicrete. The mineral wool shows better insulation characteristics with lower cost and it doesn't need to be cured like vermicrete. I prefer having the much higher temperature rated ceramic fiber right next to the fire brick. I would use mineral wool be in roll form.

I am wondering if the issues David S (Pizzaiolo) had were due to using bats, which are stiff, rather than roll form material.

[david s]

The characteristics of the rock wool may vary from different manufacturers, but the stuff I got was too floppy which created problems with getting the vermicrete to stand up vertically against the sides at the base of the dome as well as compression when applying even a really lean vermicrete over the top. The same problem occurs with fibreglass insulation. If the insulation compresses then its capacity to insulate is compromised, apart from the requirement of a vermicrete layer needing a firm substrate on which to apply. In addition to this problem the stuff was hard to cut neatly the way you can with ceramic fibre. You may find a product that does not have these characteristics and will work as well as ceramic fibre blanket, I only offer my experience with using it instead of ceramic fibre blanket for two ovens. If you are not building an igloo and creating a hard outer rendered shell, but building an enclosure then a layer of ceramic fibre blanket followed by rock wool or fibreglass insulation would be a good solution.

The ceramic fibre blanket I use (Superwool) also comes in two different densities 96 or 128 kg/m3. I use the 96, slightly cheaper and a higher rated insulator. It is quite firm enough to build over without compressing.