600mm Castable Build in Australia

[allanw]

This is my in progress, 600mm moveable, refractory castable oven build. I've spent a fair bit of time ordering and collecting the materials I need and have recently started work on it. So far I have built the steel plates for the castors, cut all the steel for the stand and started cutting plywood for parts of the dome form work.

I've attached a copy of the oven plans, which are accurate to what I am building with the exceptions of it not showing the castors and the hole in one of the floor tiles is a 'Plan B' in case it ever has to be converted to gas because of issues with smoke.

I'm working pretty slowly because I don't want to stuff anything up and it's my first time taking on a project like this so don't expect to see rapid progress here.

[david s]

Hi Alan,

Looking at your plans there are a couple of points I'd raise.
1. Your planned dome thickness @90 mm is pretty thick. Most castable ovens have a 50 mm wall thickness. Unless you want to bake multiple batches of bread a 50 mm wall has plenty of thermal mass to do roasts and a single batch of bread and provides sufficient thermal mass for pizzas with a live fire.
2. Because the flue gallery is not required to provide the thermal mass for cooking there is no need for it to be as thick as the oven walls. If fact a lighter weight gallery is an advantage because a heavy one acts as a bigger heat sink drawing heat from the floor and dome.
3. You do not have to make the burner port so big. I made mine 70mm diam and have since reduced it by making a collar so it's now 50 mm (see my kiln build here)
https://community.fornobravo.com/for...kiln#post14564
4. Not clear on your plans about how thick you intend the floor to be or how thick you intend the insulation under it. Normally a floor thickness of 50 mm is considered a good balance between sufficient thermal mass and heat up time.
5. Consider melt extract fibres (stainless needles) for castable reinforcing. (Antec Eng.)
6. A few options on castable refractory, use a proprietary mix or make your own.

[allanw]

Hi David, thanks for your comments. Please see my replies below:

1. The dome is 40mm thick with 50mm insulation. I do however have enough castable to make the dome 50mm thick although I'd like to keep the weight down if possible.
2. Thanks I will have a look at that and alter it.
3. Awesome!
4. I have 50mm calsil and 300 x 300 x 40 refractory floor bricks.
5. I have melt extract fibres that I will be adding to the mix.
6. I have proprietary mix ready to go once the form is done.

[david s]

3. If you make the port slightly conical, then you can line it with plastic and cast a flush fitting plug into it. You'll end up with a floor that's flat until you need to remove the plug for your burner.
6. Make sure you have dense castable not insulating castable.

[allanw]

Cool that is a good idea. I definitely have dense castable so all good there. I do have a few questions though now that I am heading towards the stage of casting the dome.

1. I've read (I think on this forum) that someone mixed sand with wood glue to make their form set. Is this the approach you use when moulding the form for a dome?
2. If I remember correctly (the documents are at work) my refractory castable requires approx 14% water by weight when mixing it. Is it possible to do a drier mix to help the castable stay in place on the near vertical parts of the dome wall without external support while not compromising the finished product?
3. The datasheet for my refractory castable doesn't indicate if it contains the plastic fibres that melt at a low temperature. Are these easy to identify if I open a bag and have a look at the castable?

[david s]

1. I use damp sand ie dry sand with 10% water by volume, with some powdered clay rather than glue, added to it. It is quite easy to get a perfect form by tapping the outer surface with the flat of a trowel. I also use a stick of the chosen height to place in the centre of the sandcastle so it is the correct height. Then cover the sand with strips of wet newspaper, avoiding folds. This prevents any sand getting stuck to the castable. Any newspaper that you don't remove later will just burn away.

2.Using the recommended amount of water from the manufacturer usually always results in too dry a mix from my experience. Better to just use your own judgement., but it does need to be a fairly stiff mix especially at the base of the dome or slumping will occur. Make small batches to start with (10 kgs) because the stuff goes off quickly. I use chilled water in hot weather for increased working time. It is also very thixotropic ( .
becomes more fluid with agitation or vibration).

3. Sieve a small amount of the castable and you should see the fine fibres if they are present. If not try to buy some fine polypropylene fibres used in concrete reinforcing. I buy Novamesh 950 concrete reinforcing fibres which have two types of plastic fibres, long 50 mm thick fibres and short 12 mm very fine polypropylene ones, these are type that are suitable. You need only a small amount of these, but mix for double the time to ensure proper dispersal.If you want to use random fibres for reinforcing add stainless steel needles.

[allanw]

Awesome thanks. I am not feeling quite as nervous about casting the dome now. In regards to the stick in the middle of the dome I'm actually building a tool to shape the dome. I have a threaded rod which will have a piece of dome shaped ply attached to it that can spin freely to shape the sand. I will then remove it before casting.

I picked up a bag of powdered clay today so I can mix that with the sand and I will sieve the castable tomorrow to check for the plastic fibres.

I built the form work for the entry today. I've put it all together in such a way that it should be easy to pull apart once the castable sets. The front piece unscrews revealing the sand behind it and the cylinder for the chimney flue will get unscrewed from the main entry form work by undoing the bolt on the threaded rod, which should then let me remove it easily. That's the plan anyway.

[UtahBeehiver]

Why don't you post where you god you're powdered clay. This is one of the ingredients that the Aussie builders seem to have a harder time finding.

I am interested in how your shaping tool will work on a cast oven. Both Gulf and I used something similar for p-crete.

[david s]

Awesome thanks. I am not feeling quite as nervous about casting the dome now. In regards to the stick in the middle of the dome I'm actually building a tool to shape the dome. I have a threaded rod which will have a piece of dome shaped ply attached to it that can spin freely to shape the sand. I will then remove it before casting.

I picked up a bag of powdered clay today so I can mix that with the sand and I will sieve the castable tomorrow to check for the plastic fibres.

I built the form work for the entry today. I've put it all together in such a way that it should be easy to pull apart once the castable sets. The front piece unscrews revealing the sand behind it and the cylinder for the chimney flue will get unscrewed from the main entry form work by undoing the bolt on the threaded rod, which should then let me remove it easily. That's the plan anyway.

Allan,
Is there a reason the form work for the entry is so deep? A deep entry makes the oven much harder to work. Also the thing will draw a lot better if you can taper it to the base of the flue pipe, giving the smoke some volume to collect in before it enters the pipe. Also if you cover your cylindrical former (a conical form would be even better) with a couple of layers of plastic it makes removal much easier.

[allanw]

Why don't you post where you god you're powdered clay. This is one of the ingredients that the Aussie builders seem to have a harder time finding.

I purchased the powdered clay from Pottery Supplies Online. They also have a store front in Brisbane, which is how I purchased the powdered clay.

[allanw]

Allan,
Is there a reason the form work for the entry is so deep? A deep entry makes the oven much harder to work. Also the thing will draw a lot better if you can taper it to the base of the flue pipe, giving the smoke some volume to collect in before it enters the pipe. Also if you cover your cylindrical former (a conical form would be even better) with a couple of layers of plastic it makes removal much easier.

The form work is deeper than the entry. I figured it didn't really matter if it was deeper than required so I didn't worry about making it exactly the right depth. Hopefully that won't come back to bite me later. Thanks for the advice about tapering to the base of the flue pipe. I will do this with the sand and I will make sure I wrap the chimney flue former in plastic before hand. It doesn't look it in the photo but it's actually very slightly conical (approx 2mm smaller in diamater at the bottom).

[david s]

I purchased the powdered clay from http://www.potterysuppliesonline.com.au/"]Pottery Supplies Online[/URL]. They also have a store front in Brisbane, which is how I purchased the powdered clay.

Too late now because you've got it, but a much cheaper alternative is Cement Australia's Bricklayers clay, around $1.00 / kg

[allanw]

I'd appreciate a bit of guidance in regards to the order things get done before I continue any further. I originally thought it might be a good idea to cast the dome away from the stand and then lift it onto the stand once it was ready. Being a small dome the weight would be manageable. The first reason I considered this was out of concern for all the extra weight the stand would have to hold up while I made the mould and casted the dome. The second reason was that if I casted around the floor tiles that the inside edge of the dome would exactly match the shape of the floor tiles, meaning it could be very difficult to get them out and replace them if required.

I'd like to validate these ideas with you guys, the experts, before I follow through with potentially stupid decisions that could have unknown consequences. Do you think I should abandon this idea and stick to the standard method of construction where the under floor insulation and floor tiles get laid first, then the sand mould and the dome casted over the top in situ?

[david s]

You can do it either way. The advantage of having the dome outside the floor is that you can replace any floor bricks. However damage is only likely to occur with the bricks in the centre of the floor. If casting a dome around the floor just place a couple of layers of corrugated cardboard to create the gap. It is simple to remove and the gap will fill with wood ash. Doing it this way means you have to cut the floor bricks at the perimeter of the floor with more precision. If building on top of the floor the outer perimeter of the floor can be cut a bit rougher.

[allanw]

Thank you David, that makes sense. Given that my floor tiles are 300 x 300, meaning they all touch the perimeter with none just in the center, I think I should cut precisely and build the dome around them.

In regards to casting the dome and then lifting/moving it into position once it's done, do you see any benefit to this approach in comparison to casting in the finished position?