Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

[Frances]

Yep, another thread about my mortar... Because it seems to be a bit different from what most of you are using.

Apparently it cosists just of refactory clay and refactory sand. And nothing else. It doesn't air-dry at all - I've just been on holiday for two weeks, and I can still push my fingernail into the joints of the first course. So I thought, oh well, it'll probably dry up really well when its heated up. Stuck two bits of brick together and put them on my grill. It dries up ok, but I could pull the bricks appart again without much effort and the dried mortar turns back into clay when wet... Maybe it dries harder when it gets hotter for longer?


I do have several good reasons for continuing with it:

I got this stuff from a local place that produces firbricks, on the recommendation of the guy who's in charge of their products, i.e. someone who seemd to know what he was talking about. It is made for mortaring ovens.

He said the dome would be less likely to crack, as the mortar would expand and contract with the bricks.

Its a dream to work with, really sticky for putting bricks exactly where you want them, but cleans off clothes, tools and the inner face of the dome days later.

Its a natural product, which doesn't hurt your skin (or the environment), so I can work without gloves. And I don't throw a fit every time my four year-old twins start dabbling their hands in it.

Mixed mortar can be stired up again and used the next day. So it doesn't matter if I have to stop an oven building session to deal with a family crisis.

If it turns out not to work, I will be able to wash it off the bricks and start over... BUT I HOPE NOT!!

Has anyone here built an oven with anything similar? I ask myself if it matters if the joints can still absorb water after the oven is cured - after all the inside of the oven will be protected from the elements.

Any thoughts or experiences would be appreciated

Frances

[dmun]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

I think the temporary ovens made at the Masonry Heater Association meetings use this mortar, so that they can be disassembled and the bricks re-used.

A famous example is this one, with the interior light:



These being one weekend events, they fire these ovens while they are still wet. I don't quite get how that works.

I don't know how well a fireclay-sand oven would hold up over time. You would have to keep it ABSOLUTELY dry, of course, not even wind-blown rain through the door. We eagerly await hearing about how your oven works over time.

[Frances]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

"We eagerly await hearing about how your oven works over time."

Yeah. Me too.
Made me laugh!

[Frances]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

Slept on it (again).

So what you're saying is no one has used this mortar except on temporary ovens. Well appart from the guy who sold it to me I guess (who is not on the end of a phone, else I'd be bothering him instead of you lot).

What about the original Pompeii ovens, what mortar did they use?

[phoenixzorn]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

Slept on it (again).

So what you're saying is no one has used this mortar except on temporary ovens. Well appart from the guy who sold it to me I guess (who is not on the end of a phone, else I'd be bothering him instead of you lot).

What about the original Pompeii ovens, what mortar did they use?

Just clay I think.... heh... Not really sure.

On your non-drying mortar issue, if you skim coat the whole dome with something waterproof when you're done, you shouldn't have any issues with it coming apart if it rains. Also, firing it might dry it out enough that you don't have to worry about it at all. My friend has an all-clay oven in his barn that we bake in, and we've been modifying it. Last weekend we removed and started rebuilding the opening/door area, and when I went there today to bow hunt, the clay was still pliable, though not wet to the touch..... more like a very hard sand castle. I'm sure once we fire it again, it will dry it all out very quickly, but with the recent rains and cooler weather, it's not looking like it will dry on its own any time soon.

[dmun]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

I wrote a response to this yesterday, and it seems to have vanished into the ether.

What about the original Pompeii ovens, what mortar did they use?

You mean the Romans? They had something very much like portland cement. From Wikipedia:

In the Roman Empire, concrete made from quicklime, pozzolanic ash/pozzolana and an aggregate made from pumice was very similar to modern Portland cement concrete. The secret of concrete was lost for 13 centuries.

Now, whether they used it in ovens is another question.

There is a long tradition of unfired (or unvitrified) clay ovens. There were cobb and mud ovens in Europe, and India is full of tandoors which are never fired above wood fire temperatures. I don't doubt that your oven will be successful if you keep it really dry.

[Frances]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

Thank you, that makes me feel a lot better.

I think the reason I'm continuing with this mortar (rather than adding cement to the mixture) is that I really like it as a material. IF it works I think it will be really cool to have an oven with clay mortar.

The sandcastle description is good. It certainly "feels" solid enough - but then, what do I know, with no experience in oven building to fall back on? Come to think of it though, I fell onto the oven the other day (better than falling off the hearth I suppose...) and yeah, it feels very solid :-)

[phoenixzorn]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

The sandcastle description is good. It certainly "feels" solid enough - but then, what do I know, with no experience in oven building to fall back on? Come to think of it though, I fell onto the oven the other day (better than falling off the hearth I suppose...) and yeah, it feels very solid :-)

Heh... that's a good sign... if it stays up when you fall on it, it's self supporting forever.... just don't let that clay get wet...

Someone said something about a clay oven earlier and not having gotten it past wood fire temperatures.... how hot does it have to get to melt the sand in the mortar and make a permanent mortar?? I.E. How hot is a kiln?

[edschmidt]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

Having oven, and ceramic experience I will handle that question. Typical clays vetrify (basically turn to glass) at between 1500 and 1900 degrees. I assume what you are using is fireclay which is closer to 2000 degrees. Also dont underestimate the temps attainable with wood. Pretty much all ancient kilns were and many kilns today are fired with wood. I think you could easily vetrify the internal mortar with wood a small blower would help. The exterior would be trickier so I would cover with a waterproof layer.
Hope this helps

[phoenixzorn]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

exactly what I was looking for, thanks Ed.

[stormy]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

I have fired local raw clays in kilns and open fires. We dry the projects in the sun till they are bone dry, put them in cast iron dutch oven and build a big fire over the oven. Those that survive are a basic brisk, the color of a flower pot. We have things the kids made 20 years ago sitting in the garden beds, out in the rain and snow that look like the day they came out of the fire, well except for the green stuff that has taken up living on their surface. To get into the range of heat to change clay, I would use charcoal and a fan. You will have some cracks but if you used fire bricks, the oven will hold together. As to the local clays: I found that mud from the bottom of ponds, mixed with crushed up failed projects (frit), about 5 to 1 ratio held together best. The pond mud had some organic matter that burned off in the firing so it had the most shrinkage but it was also the hardest and the smoothest in the end.

[Keepemlo]

Re: Heat-dry mortar made of refactory clay and refactory sand (and nothing else)

Man there are so many recipes for refract mortar on this site ...I wish I knew the standard tested tried and true formula...I can't see how your formula wouldn't work...I've seen em build with mud