Oven on wheels

[david s]

Re: Oven on wheels

Gotcha, seems like a fair plan.

[wotavidone]

Re: Oven on wheels

Tried my test mortars today. As mortar between two peices of concrete paver, both performed reasonably well. They grabbed early, seemed fairly strong when set. BUT...concrete pavers are porous, I could see the moisture wicking into the paver, and it was clear to me that the mortar had dried before it had properly set, if that makes sense.
The two test lumps of mortar were about 125mm x 60mm x 20mm, and had been hardening for two days.
Firstly I tried breaking each piece in half. The terracotta clay based mortar was clearly weaker.
They both broke when dropped a piece from waist height, but when I stood on the pieces on the shed floor, the terracotta based mortar was much easier to crush.
With the left over bits, I tried putting each type of mortar in the flame of my butane camping stove.
I got the piece of mortar made with my harvested clay up to 525C according to my IR gun.
It developed a hairline crack, but otherwise looked unchanged.
Then I tried the mortar with terracotta in it. It was sitting in the flame, slowly turning a slight pink colour, when it exploded. No warning at all no noises, no visible steam, nothing. I'd just checked the temp, had about 240C at the time
Conclusions so far - I would not use the terracotta clay for makimg HB mortar, but I might use the home harvested stuff.

[cobblerdave]

Re: Oven on wheels

Gudday
Its not the hardness of the mortar your after its its ability to go throught the heat cycle without cracking. Theres new products around now but traditionally the mix for roof tile caps was basically lime and sand, it was soft cracked easy (try walking on the roof line) but would put up with extemes of heat and cold. It didn't need to be really strong as the weight of the tiles held them down and they are seldom walked on.
With the Home brew the clay provides the slip between the particles not a heatproof glue. Fireclay is fired and doesn't soak up water like unfired clay which if used in to large a quanities or in really wet conditions could contribute to cracking and crumbling.
Sorry I'm not an expert but from what I understand of "homebrew" ...the portland the first setting glue which will burn out over time. The lime the long term glue which becomes stronger in the heat. The sand the main structure of the mortar the particles that all the othe ingredients stick to. And finally the clay which have nataral "slip" betwwen themselves due the the flat shape and provide the movement over the heating cycle .
Your using this "glue" to hold together a dome shape which is self supporting anyway. Like the roof tiles your seldom going to walk on it anyway but will be putting it through the stresses and movement of a heat cycle so there has to be not Hardness but the ability to handle movement and heat.

Regards Dave

[wotavidone]

Re: Oven on wheels

I get your point Dave. I guess the thing that struck me, bearing in mind that my preference is to mould the main part of the dome from the homebrew, is that the terracotta based one was downright weak, and actually exploded on heating, whereas the one made with my white clay merely sustained a hairline crack on heating to 500C.
Could be just a problem with not drying the tests slabs long enough I suppose, though they were an inch or less thick.
I am getting a bit confused wwith the definition of fireclay. The wiki definition of it is a high alumina clay that can be fired to extremely high temperatures, whereas you and several others here on this forum seem to take the definition as being fired clay, which I thought was a whole different beast.
Unfired clay, allegedly (wiki again), consists of platelike particles, which would no doubt do what you say, slide etc. However, fired clay would be fused together, then ground down in the cutting saw or crushed in a mill or whatever, and have a different structure wouldn't it?
Anyway, my clay is currently in the lab getting fused and x-rayed. Soon I shall know what's in it
I have some truly beautiful natural clay pavers that look a million bucks. They are the standard 230 x 112mm, but only 50mm thick. Unfortunately I think this would make them a bit impractical for laying a dome from half bricks. If you put them so the 50mm thick cut end was showing, you'd have a 4.5 inch thick dome, thicker than I wanted, with lots and lots of chains, which would be a battle.
If you laid them with the joins being the 50mm thick faces mortared together, with the 112 x112 face showing, you'd sort of have a standard number of chains of bricks around the dome, but only 50mm face mortar bonds, which I'm assuming would be too weak? I have been wondering what would happen if I laid them with the 50mm faces adjoining, then covered the whole dome in homebrew mortar and chicken wire to a depth of another 25mm.
Decisions, decisions. Did I mention I detest making decisions?

[cobblerdave]

Re: Oven on wheels

Gudday
your going to build a small dome right? Its going to need smaller pieces of brickwork that a larger oven to make the smaller distance around the chains.
Check out any oven in the last chains as the dome gets smaller the bricks get cut smaller and smaller. Recon it will save you heaps of cuts!
Ditch the chook wire you don't need it to support anything the dome will hold up itself.

Regards Dave

[wotavidone]

Re: Oven on wheels

Reckon it'd be all right, even though the mating faces would only be 50mm? Most ovens have mating faces of 100mm

[wotavidone]

Re: Oven on wheels

Got my assay results for my "clay".

11% Al2O3
67%SiO2

Doesn't match any clay assay I've ever heard of.

It looks like clay, you can mold it like clay, you can dry it and fire it to a nice terracotta red colour, when you make homebrew mortar it comes out strong and doesn't crack when you heat it to 500 degrees centigrade, but it doesn't assay like clay. (Too low in alumina, too high in silica.)
So what the heck is it?
I guess the next step is I use up next year's once a year favour from the labs and get them to assay my commercially obtained terracotta clay.

[david s]

Re: Oven on wheels

It's clay, it's just low in alumina. Fire clays start at about 24% alumina. The higher the alumina content the more refractory is the clay. Also the deeper you dig the more refractory the clay usually is. Clay is just decomposed rock.

[wotavidone]

Re: Oven on wheels

Was talking to my consultant hydrogeologists at lunch today. They said I should speak to their colleague, who has a doctorate in geology and is also a keen potter.
He sent me an assay of one of the white stoneware clays he likes to use.
It was extremely close to the same assay for the major components. He said mine sounded like an pretty clean clay, the test firing result was encouraging, and suggested reasons why the alumina content was on the low side. He suggested maybe it was silty clay, skewing the silica result a bit. Next time he's over I'll show it to him.
He reckons I could give it a go. Which I reckon I will. I was always planning to give the oven a few test fires before I insulate it, so I might try it and if the mortar fails I can clean things of and start again.

[wotavidone]

Re: Oven on wheels

I am at the stage where I could cut bricks if I wanted to. The plan was to build the entry arch first, then build the dome off that.
I want to bevel the inside (inside the oven) face of the arch bricks so they mate to the dome at a nice angle. I'm struggling with this.
I have enough brick to build the whole dome. Original plan was to stand the 9 inch bricks vertically to do the initial wall, then pour a shallow dome (wide radius) for the upper section from some sort of castable, probably homebrew mortar.
Now I'm thinking I can do something like this picture from the plans.
But, I worry about several things.
1) potential for the low dome to push the tall walls over
2) the radius doesn't have the same centre as the radius of the entry arch so the bevelling inside is not the same for each brick.

If i went low side walls it'd be a lot more bricks to cut.
Decisions, decisions.

[cobblerdave]

Re: Oven on wheels

Gudday
Thats a photo of a really old oven right? Its still standing even though the mortar between the bricks is eroded and the bricks are battered and chipped!
Don't be too worried about the union between the dome and the arch , I married a dome to a square entrance the angles blew my tiny brain at first but I pulled it off, you will too. I think too many of us think we have to cut ever brick perfectly to have a workable oven when you can pull of a perfectly good workable oven ....that you can be proud off without perfect cuts.

Regards Dave

Ps wack some hebel under that hearth now.....you'll not want to go back in the future to disturb you hearth and dome to fix anything

[wotavidone]

Re: Oven on wheels

Hebel might just as well be called unobtanium.
A mate was going to Adelaide on Saturday arvo. He gave 20 minutes notice. I jumped in the truck with him. He was looking at caravans, he hires them out, but consented to stop at the Bunnings at Munno Para. So I whipped in and had a look for 50mm hebel panels. "sorry, only have the big blocks, we can order it for you".
2-3 weeks wait to order it up here.
On the other hand, there is a guy selling 20 packs of vermiculite bricks on Ebay. About $3 each by the time you add the freight. 230 x 110x 38 mm. Tempting.

[david s]

Re: Oven on wheels

Try CSR they are the manufacturers of Hebel.

[wotavidone]

Re: Oven on wheels

At last, some progress to post. Got some bricks laid before a long standing social engagement got in the way. Cut the floor bricks too.
Home brew mortar works well, but if you don't want the clay component to shrink and crack the mortar then you need to keep it moist until the Portland cement has set.
Currently covered by a sheet satureted with water.
Laying bricks seems very rewarding but fiddly. Hats off to anyone who has ever made a living from it.

[cobblerdave]

Re: Oven on wheels

Gudday
Neat work.... looking good
Question what sort of under hearth insulation did you use in the end ?

Regards Dave