New 36" Pompeii in Bondi

[Wozza]

Had a double post, so I've replaced it with my curing plan. Did some reading up on this last night and Plan A is:

This weekend, wrap the dome in 3 x layers 25mm blanket, fix with chicken wire mesh and cover with a 5:1 mix of vermiculite. I’d like to think that I’ll have time to do the decorative arch as well. Then leave it all for a week covered in a tarp.

The following Saturday, use the gas burner in the picckie ($49 from Bunnies) to start the curing. Take it to about 100 degrees and hold most of the day at that temp. On Sunday, take it to 150 and Monday to 200. I’ll make a temporary door out of Hebel blocks and fit that door after each burn to keep the heat in.

I will also run the gas in the evenings during the following week, just taking it back up to 200, then gas off and then door back on again. Just to keep it ticking over. I’ll check the temperature with a small oven thermometer I got off eBay for $2.50. I also bought a heat gun for about $25 and I’ll check to see what part of the oven best matches the oven thermometer readings for later reference (the thermometer only goes to 300 degrees).

The Saturday after that, I’ll start a wood fire and carefully take it to 300. I’m not sure how easy it will be tending the fire and keeping it at a specific temperature but I’ll try to do it most of the day. Maybe Saturday’s dinner is cooked in the oven. Then Sunday up to 450 and if there are no problems, Monday let ‘er rip and pizzas in the evening!



I mean, what can possibly go wrong…...

[Wozza]

Yee ha! Finished the dome.

A bag and a half of sand was needed to make the mould. I cut some plasterboard in a cross shape to help shape it. Although, as you can see from the piccie, I?d cut the curve to match the external radius of the dome when it should have obviously been cut to match the internal radius. Dumb, but only a couple of minutes to trim those back.

I wasn?t going to do the final few courses until early next weekend. Then leave it for a day and a half and take the sand out on the Monday. But when I got up yesterday morning I thought ? sod it. Let?s finish the damn thing and take the sand out next weekend. I?d already cut all the bricks a couple of days earlier so it all went pretty quickly. Fart arsing around getting the final couple of keystones in was a bit tricky. The last gap was big enough to fit two bricks in nice and tight ? see the piccie I took on a dry run, then I had to cut a couple of odd shapes to fit snuggly in alongside them. Everything fitted nice and tight, so it should all stay there when I take the sand out. Which is not to say that I won?t be nervous knocking the supports out.

So to beer o?clock and I spent an hour laying out the arch bricks on the floor in various combos seeing what worked best. I think I?ve read that the hole in the arch for the chimney should be 10 ? 15% of the door opening. Mine comes in at 15%. I cut another piece of plasterboard in the shape of what I thought was the best arrangement and it looked OK when I had it resting in place, so it was out with 12mm pieces of ply for the formwork, the circular saw for some rough cuts and then the hand saw to finish. Had them balanced in place and carefully rested the arch bricks on them for a dry run. Looked OK, so had another beer and called it a day.

The dome is covered with a tarp at the moment. There?s just a small chance of rain in my area for the next few days but I didn?t want to risk leaving it open and getting a soaking if there was a shower. I think it should be dry enough next week to fit the 3 layers of 25mm blanket wrapped in chicken wire and then skim a 25mm coat of Vermiculite render on top of that.

Then it?s curing time?and I need to read up a little more on that. Number of fires, duration, whether I cool the oven between fires etc. Is it better to do 2 small ones in one day, letting it cool between fires, or go for one long sustained burn? Is it worth doing 2 or 3 short burns during the week after I get home from work? There seems to be a variety of methods, but all clear on one thing: do it slowly.

One thing worth mentioning which I have just thought about. As I was building the dome I generally found myself with a little Home Brew left after laying a few bricks ? not enough for the next brick but enough to slap onto the dome. Sort of a render. Little extra mass wasn?t going to hurt, anyway. And when I finished yesterday, I had some sand left (and a lot of lime, clay and cement), so I mixed up another couple of batches and spread it over the recently laid bricks. Mainly to make it look a little better. But I think it may have helped in that it slows down the curing of the mortar underneath. Or at least, I?m assuming so. And as far as I know, keeping it slow helps improve the strength. Reckon it might be better to finish with a thin render and keep a tarp over it to slow down evaporation.

And anyone know how to get rid of lime? I?ve got about 10 ? 12 kilos left over (and no fingerprints ? if you know a safe that needs cracking?).

[Steady]

Congratulations on the milestone Wozza - looking good

[Wozza]

Getting close to actually lighting a fire in this thing.

Managed to get half of me inside the dome yesterday. Well, head, shoulders and one arm at a time and it was a couple of hours of slow and painful contortions cleaning the last few courses and doing some pointing between a few of the larger gaps.

I'd just about run out of the refractory mortar so I bought a 5kg bag of high temp mortar from Bunnies for around $25 and that was plenty to finish. Lots left for the outer arch.

Cleaning the bricks was a real pain in the arse. I had to get a small bucket of water in there with me to keep rinsing out a sponge. Twist, rinse, squeeze, wipe, repeat. Can hardly raise a beer today. Stiff as.

Then it was out with the insulating blanket and wrap the thing in three layers. It went better than I thought. The blanket is not exactly stiff, but it's rigid enough so that it says in place when you stand it vertically. With 3 layers around the side I covered that with chicken wire, then cut and shaped three layers for the top. The two boxes of blanket were just about right. Just a few small pieces left over. Then folded the chicken wire over the top of it all, pulled it tight and wired it all together, then on to the vermiculite.

Hell, that stuff is kinda weird to work with. I'd read some guys have done two coats of 25mm. First with up to 10:1 mix with cement and then 6:1 for a final coat. Christ, how does anyone get it hold on a vertical surface at 10:1? After the first attempt with a small mix at 10:1, I decided to try 6:1 and that was a little easier. Though it's still like trying to render with slightly damp porridge. Very delicate stuff.

I wasn't entirely convinced it was going to set at all. Had it all covered with a tarp last night and I just checked during a gap in the rain today and it seems to be OK. Some areas are a little too thin and some small parts are a little crumbly where maybe the cement didn't mix well enough.

My plan is to do another thin coat at maybe 3:1 to make it stiffer still. Do that on Tuesday as long as the weather holds, keep it covered and then start curing on the Saturday.

[david s]

Once the vermicrete has set you want to allow the water in it to escape. If you got the mix right then for every 3 litres of vermiculite you've added one litre of water, tons to get rid of. Covering it with a 3:1 mix will tend to hold the moisture in, not what you want. Don't know what your weather situation is but a week of sun and wind will do lots. Don't wrap the oven up, unless you are protecting it from rain. You can drive out the water before doing the outer render layer.Any vermiculite in the outer render layer will result in a far weaker outer shell, not what you want. To make a lean vermicrete (10:1) I add a handful of powdered clay for every 10 litres to assist in making it more workable.

[Wozza]

Just finished the second coat, Dave. Just skimmed it with a 4:1 mix. I had some Perlite left over from my Tandoori build (see the post a couple of pages back) so it was actually 2:2:1. The perlite is a lot larger particle size than the vermiculite. The grade 3 vermiculite is almost the size of very fine gravel whereas the perlite is kinda Rice Krispie size. I figured the larger size of the perlite will make the last coat more permeable. Anyway, that was the theory.

It's sitting in full sun with a light breeze at the moment. Sun coming over the house and shining straight through the oven door. Very 'temple of Doom'... Hopefully all day if the rain holds off. The weather radar is a godsend in cases like this. We were nearly flooded last night (Bondi) but lots of blue sky today. I'll be in work for the next 3 days so it will have to stay under a tarp in case of showers. But then on Saturday I'll start curing. Still in two minds about that gas ring I mentioned earlier or small fires. maybe stick with small fires.

Three days at low temps and I'll try to keep the fires going all day. Then maybe a couple in the weekday evenings and then three more days the following weekend at medium temps plus a couple during the week. Then one last weekend and if it feels like all the moisture is out...well, see how I go.

Tea break over now. Off to finish the front arch...

[david s]

Heat beads are a good way to keep some gentle heat in the thing. Because they don't flame up you don't get flame impingement on the crown of the dome. They tend to keep the heat more evenly distributed than a wood only fire. I recommend using a little wood and a cup of heat beads for the first fire, two cups and a some wood for the second fire, three cups etc... until you've gone through the whole 10 Kg bag

[Wozza]

Heat beads are a good way to keep some gentle heat in the thing. Because they don't flame up you don't get flame impingement on the crown of the dome. They tend to keep the heat more evenly distributed than a wood only fire. I recommend using a little wood and a cup of heat beads for the first fire, two cups and a some wood for the second fire, three cups etc... until you've gone through the whole 10 Kg bag

I think it's going to be heat beads and wood. The more I thought about that gas burner, the less I liked it. Just too many ways it could go wrong, and your pm was the final nail in the gas burner coffin. Thing was, I was thinking of starting up my home brewing again and I need one of those burners so I thought I'd kill two birds with one stone. Although it might have been more than two birds getting killed if something did go pear shaped.

Finished the arch just in time for beer o'clock - I'll post a piccie tomorrow when I'm in work. Taking a (genuine) sickie today, so the oven gets a bit more sunshine

Thanks for the input.

[Wozza]

And a couple of piccies of the arch...

One thing that I noticed in using the perlite, as it is white, it was easy to see if the cement had been completely mixed through (I mixed it dry, then added water). If you saw any white beads after a couple of minutes churning the stuff around, then keep going until they’ve all disappeared. Just using the vermiculite and I noticed that a couple of small areas were very crumbly – obviously not mixed well enough.

I left a 12mm air gap between the inner and outer arch to cut down on heat loss. Then I realised that maybe I should have taken the vermiculite render completely over the inner arch and rounded everything up to the outer arch. That will mean that it would cover the air gap (which is the reason I didn’t do it in the first place) but I can’t now see that being a problem. The heat won’t transfer across any vermiculite mix that gets in there. The obvious solution will be to jam in some…what would you call it…not asbestos rope as I doubt if they still make that, but some sort of heat resistant fibre. Don’t think I’ve seen anything like that mentioned in the posts I’ve read. If anyone has any ideas..?

Anyway, when I get the first curing fires going this weekend I’ll check to see how warm the uncovered portion of the inner arch gets. If it’s too warm then I’ll mix up some more render during the week and finish it off before the curing gets too advanced.

I’ve got 4 weekends to go before the pizza party and still not sure if everything will be dry enough to stucco the whole thing. See how we go. Softly, softly , catchee monkey.

[Steady]

I don't know anything about thermal gaps - plenty of very knowledgable people on this forum who can advise. The modern version of asbestos rope is ceramic fibre rope commonly used for wood heater door seals and would be be readily available at a wood heater shop, maybe even Bunnys - I'm sure it would be used in other refractory installations and available from fire brick suppliers as well.

Is it is of any use in your build - no idea

[Gulf]

Wozza,
I think that you will want to bring the insulation and render out over the outer arch. If not, I'm afraid that water will be funneled into your oven.

[UtahBeehiver]

I used a ceramic rope stuffed in the gap and covered with a high temp caulk to isolate fibers from any food sources.

[Wozza]

Thanks for the input, guys.

No ceramic rope at Bunnies but there are places I can get it. About $5 a metre for 12mm diameter. Google is my friend…

And insulating the inner arch? Yeah, I hadn’t thought about water making its way into the oven. I was worried about heat getting out, but I guess that rainwater would wick into the dome via the arch if it wasn’t covered. I think I have a few small pieces of ceramic blanket left over and still plenty of vermiculite. So maybe that’s a job for the weekend.

And thinking out aloud…the outer arch is going to get wet. I don’t intend rendering that. It looks OK and I want to leave it as is. That gap I have will, at the moment, prevent water wicking to the inner arch. But if I jam some ceramic rope in the gap, will that then allow water to cross over?

And the chimney: I may post this question in the relevant section later on, but I was thinking of a brick chimney. The disadvantage of which would be the weight. But then I thought – hang on, I’ve quite a few Hebel blocks left over. If I cut those up into brick sized pieces and form a chimney with them, the size is such that if I line up the hebel ‘bricks’ with the inside of the opening, then it leaves 25mm on the outside which I could cladr with either tiles (thinking of those slate type ones) or maybe 25mm slices of brick so it would look like a proper brick chimney.

I think the cladding would give it enough structural stability and it would be a lot lighter. And effectively insulated into the bargain. Any thoughts..?

[Gulf]

.............And thinking out aloud?the outer arch is going to get wet. I don?t intend rendering that. It looks OK and I want to leave it as is. That gap I have will, at the moment, prevent water wicking to the inner arch. But if I jam some ceramic rope in the gap, will that then allow water to cross over?.................

Not unless you seal and add a water proof render over it. But, remember one fact of roof construction. It is much easier to waterproof a ridge than it is a valley.

............And the chimney: I may post this question in the relevant section later on, but I was thinking of a brick chimney. The disadvantage of which would be the weight. But then I thought ? hang on, I?ve quite a few Hebel blocks left over. If I cut those up into brick sized pieces and form a chimney with them, the size is such that if I line up the hebel ?bricks? with the inside of the opening, then it leaves 25mm on the outside which I could cladr with either tiles (thinking of those slate type ones) or maybe 25mm slices of brick so it would look like a proper brick chimney.

I think the cladding would give it enough structural stability and it would be a lot lighter. And effectively insulated into the bargain. Any thoughts..?

I think that you could keep the chimney question here. It will be easier for future oven builders to understand how you arrived at your final decision. I don't know much about Hebel. But, I understand that it is a portland cement based aerated concrete. Some parts of your chimney (at the bottom and on the oven side) will be exposed to some extreme temperatures. Portland cement will degrade at those temps. You may want to consider at least forming the lower portion of your chimney out of firebrick or a true clay brick. They can be placed on there side, as a liner, which will be about half the weight.

just sayin' .

[Wozza]

Regarding the ceramic rope, I was lying in bed on Saturday morning and I remembered that the glass door I've got from the old wood heater has a ceramic rope gasket running around the inside which sealed it against the fire box. It won't be needed on the door now so would it be OK for sealing the air gap between arches? Well, what were the chances that it would remove easily, be the right thickness and the right length.
Well, wouldn't you know, it came out all in one piece and was exactly the width I needed and exactly the length as well. Now that's a good start to the day.

I jammed it into the gap and then used some left over ceramic blanket to fill the gaps in the top and to insulate the side of the inner arch. Mixed up some 4:1 vermiculite/cement and skimmed over that. See the before and after pictures. I don?t think I?ll do another layer. Just let this one firm up and then I?ll render the whole thing when I finish drying the oven out.

There's a piccie of the rope in situ, then a couple showing the top of the arch before and after.