Fireclay

[njsicilian]

I am reading up on different sites and getting all different types of info regarding what to use to set the bricks. Is the mix on this site of Sand, Portland and Fireclay strong enough to withstand the heat? some sites mention that portland breaks down at 400 degrees. Also some sites mention to add lime to the mixture which takes over when it gets real hot. Does anyone have any info or first hand expierience with using just the sand, portland, fireclay mix.

[dmun]

Originally Posted by njsicilian

I am reading up on different sites and getting all different types of info regarding what to use to set the bricks. Is the mix on this site of Sand, Portland and Fireclay strong enough to withstand the heat? some sites mention that portland breaks down at 400 degrees. Also some sites mention to add lime to the mixture which takes over when it gets real hot. Does anyone have any info or first hand expierience with using just the sand, portland, fireclay mix.

A lot of builders have successfully used just this mixture. It is subject to some cracking, but the inherent stability of the dome structure has prevented any failures. Portland cement is unstable at firing temperatures, but the theory is that only the first inch or so is subject to temperatures above the safe limit, and even in the close to the fire section the fire clay will keep the mortar from disintegrating.

As for me, I wanted to have an oven with close enough joints that I could use the expensive refractory mortar for all joints, and thus I designed the geodesic variant on the pompeii oven, which hasn't been completed yet, but I have great hopes for.

I think the official line is that for home ovens which are fired occasionally, the fireclay mixture is fine, but for commercial ovens which are used continually, only refractory mortar will do.

[njsicilian]

thanks for your answer. Should i build a thick layer of the motar mix over the oven dome or will that make it worse?

[Fio]

Originally Posted by njsicilian

thanks for your answer. Should i build a thick layer of the motar mix over the oven dome or will that make it worse?

That's what I did. When I was done setting the bricks, I smeared mortar over the outside and covered everything. I think that's what a lot of people do.

See the threads on oven curing to find out more about cracking, etc.

[james]

These all sound right to me. The fireclay provides some heat resistance to the mix. It's true that portland based concrete does not withstand heat cycling -- the main problem is the heat cycle, with expansion and contraction, that leads to breakdown. I've talked with a handful of folks who are experienced with concrete, and they say that is more a problem than just high heat itself.

With a home oven, this isn't a huge problem. Your oven won't be fired often enough to where the mortar will be a problem. Also, the oven design is based on tight brick edges, where little mortar is exposed to the inside of the oven, where it is really hot.

Having said all of this, we recommend true refractory mortar for our Forno Bravo ovens, and most of our customers use Refrax -- a true refractory mortar that we make. It's easier to do this with a precast oven, because there are fewer joints compared with a brick oven -- so the cost of the mortar really doesn't add up.

There are definitely shades of grey here, but again, there are many ovens out there (including one of mine) made with the portland, sand, fireclay mix that are working and lasting just fine.

On the oven coating, I think you are looking for a thin layer to provide some structural strength to the dome -- that's all. You definitely do not need more mass than a half brick facing out.

James

[njsicilian]

Thank you all for the info i was getting a little lost a one point but now i can move on.