70cm Homebrew Pizza oven build, Manchester, UK

[loumw]

After around 36 overall hours we took apart the flue gallery framework, which came apart fairly easily with a bit of patience, then dug out all the sand. Really pleased to find no internal cracks at all. There were a few small gaps in the mix around the arch opening which we patched up. Dome is now back under a damp cloth and tarp while the warm weather continues.

Next up is wrapping the dome in the insulating blanket, held down with chicken wire, followed by a Vermicrete layer at a 5:1 ratio. If anyone’s got tips for either of these stages we’d love to hear them!

[loumw]

Next up was wrapping the dome in the ceramic fibre blanket. The key insulation layer that’ll hold heat in for those long cooking sessions. We initially secured it with chicken wire but found it was stopping the vermicrete from sticking properly, so took it off.

On top went a 5:1 vermiculite/cement mix which went over the whole dome, kept deliberately dry and crumbly rather than wet. Very different to working with normal cement. We then left for a week, which ended up almost three, to dry.

[loumw]

Finally the flue went in. We snipped three cuts into the base to allow for expansion when it heats up, then packed vermicrete around it to hold it in place. Next up, curing fires and then the render.

[loumw]

I hope you made your plywood mould easy to dismantle and remove because it will probably require pulling apart piece by piece and being sacrificial.

We’re onto the curing fires now and just quoting this comment to see if you had any advice with where we are at! Quick question: we put the vermicrete layer on around 3 weeks ago and since then the weather has been record-breakingly hot and dry. Would that have had any effect on the curing process?

On the fires themselves, the weather has turned and it’s been in and out of rain, but we’re finding windows in the day to get them done. We’re on day 3 and reaching around 270°C. We’ve seen people recommend very different temperatures for each stage so we’re not 100% sure we’re on track - any advice welcome.

No visible steam coming out of the dome, but when we took the tarp off this morning there was condensation on the inside of it after leaving it on overnight. Assuming that’s a good sign?

thanks!

[SableSprings]

Slow down on the firing! This drying process is really important to GRADUALLY force the internal water out. The generation/expansion of steam internally within the build materials is a major contributor to cracking in new builds. What also happens is you can quickly build up a big difference in temps at the dome top vs the lower sides...not good. Seeing moisture under the tarp is telling you there's still plenty of moisture being driven out. I'd make a smaller fire and move/push it more around the sides throughout the day for a couple more days. Keep putting the tarp over the oven for the night and wait until you don't see much condensation the next morning before you start ramping up the temps again. Going too fast for this drying period of firings is a pretty common mistake but don't stress out...your oven may develop a few more cracks but will still work just fine in the future. It is truly an exceptionally rare oven that doesn't have some cracking, after all the entire oven physically expands and contracts with every firing cycle.

Relax, just slow down a bit

[UtahBeehiver]

Mike gives sage advice. We see many builders go to fast, too hot, and damage all their hard work. The turtle wins the race in curing. Vcrete contains a lot of water, the surface may look dry but underneath could still have a lot of water as indicated by moisture on the tarp. David S suggests using a cheap plant soil moisture meter to check vcrete moisture content.

[david s]

Plus one on Mike’s advice. If you’ve got to 270C then you probably noticed the carbon burning off at the top of the dome, leaving a white patch. This will probably grow larger on the next firing, but is also a vulnerable time because the lower dome will be way cooler.
you don’t say how thick your vermicrete layer is, but just as a pile of sand will remain damp for months although the surface is dry, so does a thick vermicrete layer. Three weeks of hot dry weather should be sufficient as long as the layer is not too thick. I use around 35mm thick layer of 10:1, over the blanket layer (no chicken wire), which is a better insulator than 5:1, pretty loose and dries readily.

Thanks for reporting back, it greatly helps other builders. A cheap garden moisture meter should hep you gauge its drying. it will obviously be more damp towards the bottom of the dome. Long low and slow is now the go.

I don't like the term curing fires because at the service temperatures of a WFO there are no chemical changes, it is only mechanical water removal. Curing fires makes it confusing because it is nothing like the damp curing which refers to the chemical changes of the hydration process. A better term is drying fires.

Attached is the drying fires schedule I use that might help you, but bear in mind that this schedule is for a cast oven that has wall that are half the thickness to that of a brick oven which will take longer to purge itself of moisture. In Australia "heat beads are BBQ fuel and are mini briquettes about 50g each which produce a glowing heat rather than a lot of flame impingement. It is important to allow the oven to return to a more ambient temperature so the temperature evens out, then you can start again.