Refractory Mortar Poll Tools, Tips and Techniques Discuss Refractory Mortar Poll in the Pizza Oven Design and Installation forums; I called the manufacturer for advice and they said that the mortar needed to be cured slowly at a temperature ...

[philiph4@ameritech.net]

I called the manufacturer for advice and they said that the mortar needed to be cured slowly at a temperature below 200 deg. for at least 10 hours,


Doug,

I lit my first fire yesterday with just newspaper. Are they saying to keep a low grade fire going for 10 hours or can I go several fires over several days? Anyway, the mortar that I used feels hard to the touch but it is definately not waterproof. Maybe I need a space heater to control the heat. Anyway thanks for the post it seems to make sense.

Phil

[Lars]

Phil,
I am sure that your mortar has 'set', since you have been working on it for weeks, right?

Even though it has 'dried', it is still vulnerable to any moisture. Did they tell you at what temperature it would vitrify? ( become waterproof), harden?

Lars.

[Les]

Originally Posted by Mudologist

2000 years ago they used dirt and dung because thats all they had.

Dude - thats where James got the idea. If it worked then , why not now (not the dirt and dung, but the roll your own)? There are a couple of areas that I used the good stuff, for most of the build I don't think it's required. Hell - you could keep the brick to level with a river rock.

For what it's worth.

Les..

[Lars]

If Mudologist is thinking of using the particular mortar that Phil used in this thread, then hopefully he is building his oven indoors.

The plibrico mortar may be good for kilns, but the last choice for wood oven.

My 2 cents...

[ThisOldGarageNJ]

Phil, If thats the case, low heat, then I agree you should use a small heater and close the door, Im sure if you go more than 10 hours you wont hurt any thing,,, you can always put refmix over the dome for and additional safety margin, though remember the strucure alone is self supporting I believe..

Good Luck and dont worry
Mark

[Beagle]

Hi Phil,

Your mortar has dried on the outside and feels solid, that is normal but it is not cured, that only happens when it is fired at a high temp. The high temp part comes AFTER the 10 hours of low temp. I was told that my mortar would never be waterproof. That is not a problem for me because the oven is enclosed.

They seemed to think that keeping the temperature at or just a little below 200 deg. for 10 hrs. would allow the moisture to be driven from the mortar. I don’t think the temp inside the oven has to stay exactly one temp; it can vary because there is so much mass in the bricks that the temp of the oven structure will change slowly. After that, I went with the oven-curing schedule that James recommends.

Doug

[Dutchoven]

Originally Posted by Beagle

Hi Phil,

I was told that my mortar would never be waterproof.
Doug

Doug is very likely correct because most of the airset mortars and not cementitious mortars but rather use sodium silicate or waterglass as the binder. It will not be water proof in the sense that if rainwater falls on it it will wash out and is therefore the reason why it should never be used on flue tile. In the oven it should be OK but I would fire low and slow for a few days. You could possibly burn an out side fire to coals and then shovel the coals into your oven and shut the door. That would reduce some of the shock and give you the low and slow heat you want.
Good luck!
Dutch

[cynon767]

So I've been doing more research on this lately... If I were actually smart, I would have done the more thorough research before starting the oven, but better late than never.

There has been some debate as to the benefit of mixing a homebrew mortar with a higher clay content with regard to the portland. At issue is the fact that portland cement breaks down at high temps, which has important implications for the long-term stability of the dome. There has also been speculation as to the amount of lime required, and its effects on both wet and cured mortar.

Most of what I've been able to find out has led me to think that the lime may be a key component with regard to the effect on both the clay and the cement. This paper Quotes earlier studies showing that mortar with lime in high concentrations, equaling or exceeding that of portland cement, had increased flexibility and resilience to movement and cyclical changes like those of the heating cycles we put our ovens through. It mentioned brick industrial chimneys, which are subject to a great deal of flexing from wind stresses, of which many of the longest lasting used a mix of 5:2:1 of sand, lime, and cement. It also mentioned that a high lime content can help seal minor cracks, through a chemical reaction in the lime with airborne carbon dioxide which causes a crystalline limestone deposit to form.

(Heat-dry mortar made of refactory clay and refactory sand (and nothing else))
Frances' mortar took the clay use to the extreme: just clay and sand. She basically counted on the heat of the oven to fire the mortar into little mortar-pocket shaped bricks. The outer surface of the bricks is not exposed to the direct heat of the fire, so they aren't going to be quite as hard as true brick; but the inner surface will likely fire quite solid. As long as the outer surface is weatherproofed, the oven should last quite a while. This is true of cob ovens as well, which are built entirely of a clay mixture.

Here's some more information about Fireclay... and here's the wikipedia entry on fireclay. What I'm reading is that in all likelihood, the firing of a clay-based mortar will indeed produce a relatively strong, temperature-resilient gap-filling wedge, helping prevent the large mortar gaps we have between rings from breaking down. It won't quite turn into brick, but will be fairly stable through the heat cycles of the oven.

The real kicker here, though, is that the harder and more solid we make the wedge-shaped clay "brick" between the rows, the less plastic the mortar joint. That's where I see a good relationship between clay and lime coming into play. The lime increases plasticity, not just in the wet mortar where it makes things stickier and more spreadable, but also in the flexibility of the cured mortar. Portland cement hardens up and sticks the bricks together, and provides excellent compressive strength, but breaks down when heated... especially when repeatedly, cyclically heated and cooled as in our ovens. Fire clay, which has high vitrification temperature, will never fully "fire", as the temps we're working with won't reach the threshold needed to turn the clay into ceramic; but the fact that the clay will remain porous and slightly plastic is actually beneficial in this case... the low temperature of our ovens (relative to pottery kilns, anyway) prevents it from becoming brittle. It will, in essence, remain a part of the mortar. It will still be susceptible to cracking during firing due to moisture inclusion if it gets wet; but when taken together with the lime's plasticity, the clay in the mortar will (theoretically, at least) take the place of the portland to provide compression resistance and give the dome long-term stability, while the lime will help prevent shrinkage cracking in the clay and will even help "heal" hairline cracks which do form.

Such is the hope, at least. The homebrew might not be as good as a truly refractory mortar; but all in all, it seems to be a pretty good balance of the properties we need: temperature-stable, good in compression, and flexible enough to withstand heating and cooling cycles. Whether or not you add extra clay or extra lime, it will likely perform quite well, and is certainly better than the airset sodium-silicate mortars previously mentioned.

[ThisOldGarageNJ]

Wow Jamie...
Thats quite a statement.. Reading through it though does seem to make quite a bit of sense... Nice job on your research.. I did use the heat stop 50, the main reason being it removed so many variables.. I also buy pancake mix that is also Just Add Water...

Cheers
Mark

[kebwi]

Yeah, thanks. Those were some interesting references actually.

Cheers!