600mm Castable Build in Australia

[david s]

Much easier to cast in situ.

[allanw]

I've been a bit slack recently and haven't posted any updates. The build is progressing well now. I've welded up and painted the metal stand and fitted the casters. I loaded the stand with bags of sand and my girlfriend and pushed around approx 150kg which was rolled around very well, even on the pavers which are uneven in some places. The calsil board is cut and now I just have to sort out the floor bricks before I can start making the sand mold. I'll post some more photos in the coming days as I continue to progress further.

[allanw]

Things are really happening now. I've finished assembling the stand and loaded it up to the estimated weight of the finished oven and pushed it around outside. The 200mm casters roll quite well on uneven ground. I don't think I will have too many issues on that front.

I've cut the calsil board and the floor bricks and laid them on the stand. I also just finished building the sand mould in preparation for casting the dome. As suggested I mixed in some powdered clay and water with the sand, making up a 9L bucket at a time and building it up by hand. I used a piece of plywood that I cut to the shape of the dome to shape the sand then smoothed the surface with a small trowel.

[david s]

If you lay the castable directly against the sand mould it will pick up some of the sand and leave you with a rougher surface. Better to cover the sand mould with strips of wet newspaper that act as a slip layer. Any that won't peel away wil just burn away leaving you with a nice smooth surface on the inside.
The castable is very thixotropic which means it will become more fluid with agitation, so jiggle it a bit before placing each trowelfull. Also I suggest you only mix half a bag at a time as the stuff goes off very quickly. Calcium aluminate concretes are cured in 24 hrs so you can keep it covered for that long then dig out the sand. No need to cure it for a week like regular concrete. Handle the mix with respect, they don't call them stainless steel needles for nothing.

[allanw]

Thank you for the casting tips. This is the part I am most anxious about so your advice is much appreciated. In regards to putting stainless steel needles into the mix, do you cast a thin layer without them first so that there are no needles exposed and/or pointing out inside the oven dome?

[david s]

No, that's not a problem, there won't be any sticking out from the surface.

[allanw]

I did the casting of the dome today. I laid wet newspaper over the sand before starting and then mixed 10kg of castable at a time. I had a little bit of trouble on some of the vertical sections when the castable drooped a bit and tore the newspaper so there will be some parts that have come in contact with the sand. Just as I did with the sand, I cut a piece of plywood to the shape of the dome so I could check approximate thickness.

[david s]

Good job Allan. You can remove the sand after 24 hrs and inspect the inside. If you have any voids they need to be filled before the casting has dried too much. I find two days after the casting is laid is usually about right. You need to seive a small amount of the castable to remove the coarse aggregate which leaves you with a richer mix. Return the removed aggregate to the bag. Mix in some water and make a paste like peanut butter consistency and rub this hard into any voids. How did the needles go?
Did you design the entry mould so your casting will have a rebate for a door to sit against? If not the placement of a door for retained heating may be difficult.

[allanw]

Thanks David. I dug 99% of the sand out last night and I've managed to get about half of the newspaper off so far. I do have some voids, mostly around the entry so I will mix up some castable without the coarse aggregate and fix those up. If there is a void beneath a section of newspaper that I can't remove what will the concequences be if the void isn't discovered until the newspaper gets burned off?

I used thick rubber gloves to handle the castable so I didn't have any trouble casting the dome. I've discovered half a dozen needles that were sticking out a bit into the dome. They were protruding at a fairly shallow angle but I've unintentionally bent them as I was digging out the sand. I didn't use gloves this time so I have a few cuts on my hands.

I hope I won't regret the decision later but I intentionally went for a no door design because I am planning on exclusively using the oven for cooking pizzas.

[david s]

You could try scrubbing the newspaper off with a scrubbing brush. If there are voids left it will only be cosmetic, but try to fill any before the casting becomes too dry.

[allanw]

I'm happy to report that my oven is almost complete. Since the last update I have had the entry facade laser cut then I painted and installed it. I also built up chimney flue area using left over casil board. A tiny fire has been lit in the oven and it appeared to draw quite well. Without a cap on the chimney the smoke exits nicely and doesn't go anywhere near the house next door. I also did a little bit of re-paving to make the transition between the driveway and courtyard a bit smoother so I can push the oven around without much effort.

Today I installed 2 x 25mm layers of ceramic blanket and then wrapped it with chicken wire. I also did the base render using a mix of 4 parts sand, 1 part lime and 1 part cement. The oven is now covered up in plastic and I will leave like that until next weekend when I will start drying it out with fires. Once I have cooked in it a bunch of times the only thing left to do is add a layer of coloured acrylic render and touch up some of the painted areas.

Thank you David for all of your advice and tips along the way. I am really happy with how the oven has turned out so far and without your help I don't think it would have been as successful.

[david s]

Great, I'm glad it's worked out well.

[UtahBeehiver]

Nice little portable oven. We are lucky to have David S as a casting expert and resource.

[david s]

First post May 3, finished oven July 24, with only two pages of posts, casting rules!

[Jumbalaya]

Hi Allenw do you have an update on this oven,? I'm preparing to build a similar one and was wondering how you like the internal size of yours. I would only cook a hand full of pizzas at a time and had originally planned a 30" internal diameter but quickly finding a smaller one might be a better fit for my first try so I could use a narrower platform.