Line a perlcrete cast oven with refractory?

[Ronstarch]

Hi All,
I built a cast Pompeii-shape oven using just 4:1 perlite/Portland cement with 3% by weight stainless steel needles. It isn't pretty but works quite well, heats up to 800F in about 40 mins and holds its temperature surprisingly well for not having much thermal mass. I do have split firebricks covering the entire oven floor.

My question is, would it be possible to add a thin-ish layer of refractory cement to the inside of the oven? Idea being to enhance the longevity of the oven (assuming the Portland cement will start to break down eventually), and possibly improve the performance slightly. The interior surface of the oven is quite rough, so maybe a castable refractory would adhere well? Furnace cement? Is this trying to put lipstick on a pig?

Any thoughts are appreciated.

[SableSprings]

Welcome to the forum Ron. I suspect that this oven will not last for a long time (the portland cement does degrade at these temps). I think trying to coat the inside of the oven is going to be along the "lipstick on a pig" line. David S is our forum expert on casting but he's using and advising refractory casting mixes...not perlcrete. Many of us have used perlcrete as our insulatiing base or dome insulation. Four to six inches of 5:1 perlcrete for the base is "normal" and five to eight inches of 10:1 for the dome is common. (Ceramic board and batting are more expensive, but considered superior to either vermicrete or perlcrete by a lot of us.)

The great thing here is that you have a great opportunity to see how much you like using the oven and now have the experience to much more easily move to a new and better cast oven using refractory materials. My personal feeling is that this oven isn't going to last very long, especially when you get wet & cold weather working on it this winter. The perlcrete will absorb water and freezing will start breaking it up pretty quickly. Start researching on the forum for your next build/casting...you'll be pleased with the performance and look of version 2.0 in your back yard next year

My real concern right now is that (by the photo), you are using a wood pallet underneath the oven floor. The 4:1 perlcrete does NOT have great insulating properties at the apparent thickness seen. That pallet wood is going to get pretty hot during every firing and pizza party Many of us have "lost" a firing door or a load of drying wood well after a pizza party...it doesn't take much to ignite hot, dry wood. Please try to put your oven on something that isn't flammable ASAP!

[david s]

Hi Ron,

The problem with perlcrete or vermicrete is that it's not particularly strong at 4:1 even with stainless needles and like Mike said, being made with Portland cement will have a limited life.Another problem is that it lacks thermal mass so won't store much heat and will cool rapidly if the fire is not maintained. However a coating of calcium aluminate based material on the inside would both increase strength and add thermal mass although being a thin layer could tend not to bond too well. In order to allow the material time to hydrate you will have to get the whole dome wet (but neither too wet or too dry), so it will retain moisture for the 24 hrs during the time the coating can cure. I'd try thinning down castable refractory or mortar, but make sure it's calcium aluminate based, with water to paint consistency, so it can be painted on in successive layers. You may need to sieve out the more coarse aggregate. You will then have to allow the whole thing to dry out for around a week before beginning any drying fires. Not sure how successful it would be but worth a try. Some insulation over the dome, after it's all dry would also help.
Good luck.

Dave

[Ronstarch]

Thanks Mike and Dave for the detailed responses, much appreciated. Mike, I hear you loud and clear on the pallet base, I'll make that change. And, when I have my own place I would like to build a properly designed oven that is more "permanent". Dave, I may try the layer just for curiosity sake, if I can source the calcium aluminate material inexpensively. Any particular product you'd suggest? Is there anything I could do (other than the wetting you mention) to help the refractory layer adhere to the perlcrete?

Thanks again for the replies; this forum is an incredible resource.

[BenKeith]

I'm sure these guys know more than me because this oven I'm building now with an industrial grade refractory is the first time I've messed with the stuff in over 40 years, but with this I'm using, they warn very much about letting lime get anywhere near it. They have in big, bold letters to make sure equipment and tools are clean because even the smallest amount of lime will cause adverse affects. Even from previously mixed concrete so personally, I don't think I would be trying to lay a refractory over a Portland mix with lime.