My first WFO design, any comments before I build?

[Mark1986]

david s

Sorry I'm a bit confused now. What did you mean with:

The floor bricks are better laid loose IMO, to allow for thermal expansion of individual bricks as well as easier replacement down the track if needed. The gaps will simply fill with ash.

Is this top-down spacing between the perlite concrete slab and firebricks or between the fire bricks?

[SableSprings]

Use a little (thin layer) of damp sand/clay on top of the Perlcrete insulation base (for leveling). Lay your cooking floor bricks directly on this leveling layer & tap lightly as you lay the bricks to make sure to keep the bricks level without any raised edges between bricks. We recommend the cooking floor bricks laid in the herringbone pattern to further reduce the possibility of catching a peel edge while working the oven.

Using a notched trowel first on the sand/clay layer is very helpful...just like laying a tile floor. Do not use mortar to set these bricks in place or to fill the seams between bricks...ash will fill these tiny seams & the bricks need a little "wiggle room" for expansion/contraction during firing cycles. Hope that clears up any confusion about laying the cooking floor.

[MarkJerling]

Ok perfect I won't worry about the cracks then!

About the spacing, would you advise about a 2mm gap? I'm asking because I want to use spacers

And the setting/drying time? Can I put to the dome on it after three days?

Cheers!

Floor bricks should be laid tight, no gaps.

[Mark1986]

Ok thanks for the information on the fire bricks all!

Update on progress:
First on a positive note, the cast seems to be set properly and looks like it is going to tield a good end result. See video: casted dome.

Today was a bit of a set back. I bought new polypropylene fibers from a different brand, these were not a lot of individual strands, but patches of strands 1cm by 1cm. The result of the homebrew mix looked different, and I didn't trust the end result. Here is a photo of how it looked: homebrew mix. Maybe I overreacted by not using it, but I didn't want to take the risk. I re-ordered the first fibers I used. Was this the right call?

The next thing was that the dome was way thicker around the base of the oven. Which was to be expected from the downward pressure and vibrating the homebrew. This is a problem because the dimensions now don't fit on the stand I build. Is there a way to trim the dome? I got an angle grinder I could use. Any ideas?

Cheers!

[david s]

Unless the fibres are mixed for quite a long time they will clump together and not disperse throughout the mix. I’m not sure what you have , they should be finer than human hair.

Regarding the slumping, normally solid inner and outer moulds are made and the mix can be vibrated to eliminate air. This is a lot of work and not worth it for a one off casting. It does mean that you can’t vibrate the mix against the sand mould or you risk moving the sand at worst and slumping at least. Most of the air can be eliminated by carefully wriggling the mix against the mould which results in a pretty smooth inner surface requiring only minimal void filling on the inside.

[Mark1986]

Update

david s
I had a little break from the project as I needed help with lifting the cast on and off the stand and I went on vacation. I did finish the oven floor which turned out good!

Today I started putting on the insulation and chicken wire. It was quite a job, but it wasn't that hard. I did ran into a problem that I hope someone can help me with. I started putting on the perlite render with a 6:1 ratio, but when I used a different bag of cement, the mix turned out really wet. I started panicking and mixed more cement and perlite in, as I thought I miscalculated the amount of water needed. The mix turned out lumpy and wet, but I did put it on. I finished about half of the dome. Below you can find the photos.

I'm thinking about a couple of options to fix the problem:

- See how the render turns out, buy new cement and finish the rest of the render.
- Repair the render with a layer of a good mix of perlite.
- Remove the render and start over.

I'm thinking about the second option, as the layor may not isolate at all of crack easily. What would you do?

Cheers Mark

[david s]

I’m not quite clear on what you mean, but a 6:1 perlcrete is not a very good insulation layer (see table). You don’t say how thick the blanket layer is, but if you are hoping for decent insulation value from 6:1 perlcrete it is about half as good as ceramic fibre blanket. A 10:1 mix is far better, over which you can apply a dense cement render. But you need to do the drying fires to eliminate the moisture before the outer rende
r is applied.

[Mark1986]

I’m not quite clear on what you mean, but a 6:1 perlcrete is not a very good insulation layer (see table). You don’t say how thick the blanket layer is, but if you are hoping for decent insulation value from 6:1 perlcrete it is about half as good as ceramic fibre blanket. A 10:1 mix is far better, over which you can apply a dense cement render. But you need to do the drying fires to eliminate the moisture before the outer rende
r is applied.

david s
Sorry for the confusion..

I have about 6cm of insulation blanket around the dome. I could remove the perlite layer and reapply a 10:1 mix of perlite concrete. Would you advice that? Is it possible to remove the render layer without damaging the insulation blanket too much

Also I have left over ceramic fibre blanket I can add, but this can only go on the top part, as I don't have any space left to expand in the width.

Cheers

[david s]

6 cm of blanket is quite a lot of insulation so you are not relying so much on the perlcrete as additional insulation so if it were mine I’d be leaving it rather than removing it. Just allow the perlcrete to dry out really well before doing any drying fires and before applying any outer render layers over the perlcrete.

[Mark1986]

david s That's great to hear! I will finish the rest of the dome with the 6:1 mix today, I'll send an update later!

[Mark1986]

Update

I finished rendering the dome with the 6:1 mix, I did have some inconsistencies in the mix but I think it will turn out fine. I added a small steam vent as I live in a wet area.




david s My girlfriend thought it would be nice to put mosaic tiles on the render as a finish. Would I need to flatten the surface with some kind of cement layer and then use tile glue? Or can I work with a thick layer of tile glue to flatten it out?

[david s]

The 6:1 mix is not strong enough as an external coat or as a suitable substrate for tiling against IMO. If it were mine I’d be doing the drying fires and finally with the inside white, apply a cement render over the vermicrete.

[Mark1986]

david s

The 6:1 mix is not strong enough as an external coat or as a suitable substrate for tiling against IMO. If it were mine I’d be doing the drying fires and finally with the inside white, apply a cement render over the vermicrete.

Ok, and if I apply a cement layer over the prelitecrete, can I then put tiles on it?

What kind of cement should I use? And how thick should this layer be?

Cheers!

[Mark1986]

david s



Ok, and if I apply a cement layer over the prelitecrete, can I then put tiles on it?

What kind of cement should I use? And how thick should this layer be?

Cheers!

david s

I found a page where someone describes a cement render consisting of: "... 4 parts plastering sand with 1 part lime and 1 part cement.."




https://gardengeek.net/pizza-oven-build-part-9/

Would this be a suitable render to use before putting tiles on the oven?

If so, can I just use a small layer of about half an inch over the perlitecrete?

Cheers!

[david s]

Yes, That's what I do, but use an acrylic sealer over the render before tiling it. The render layer can be strengthened with the addition of AR glass fibre random mixed reinforcing.