My first WFO build thread... I'm torn clay/cob or the much discussed homebrew cast

[Robarb]

Thanks. I'll remove the sand tomorrow then (don't have time tonight) and make a peanut butter texture cast mix to fill in any cracks.

[Robarb]

I'll get some pics uploaded too

[Robarb]

To anyone that can help...

I'm not ready yet but i want to make sure i'm prepped for the waterproof render coat. This won't be my final coat (i'm planning on mosaic tiles in a bespoke pattern) but i'm pretty sure from what i've read some form of waterproofing is needed for ovens without a cover.

So... what mix should i use for the waterproof lime render? Are there any additives i should use? I assume this is directly applied to the vermiculite insulation layer?

Thanks.

(i assure you all pics are coming, i'm just mad busy at work too!)

[david s]

Most commercial cement renders are designed to be partially waterproof to keep out most water but to be still breathable. You can tell this when mixing water with it because it’s difficult to get the water to penetrate the dry mix and you need to work at it to mix it in. If you want it fully waterproof then seal over it with something else like acrylic render, elastomeric coating like bathroom membrane or paint. Bear in mind making it fully waterproof also prevents moisture getting out. Another alternative is a porous mix of 4:1:1 sand lime, cement that you can choose to either waterproof over or not. This mix is pretty good in that it’s high lime content imparts some elasticity and crack self healing properties.

[Robarb]

Thanks david s

So... I've been a little busy over the past few weeks. Not only have i been trying to build the oven, but new decking and a pergola have gone up too. It's not quite finished yet and i'm only just starting to air dry it out (damn weather) so i reckon i'll be ready for small fires in about 2/3 weeks.

[Robarb]

I went with a brick base build with lintels and thick concrete flags instead of a concrete base. This will eventually be rendered / tiled so you won't see it.

The bricks were free

[Robarb]

The decking was nearly rotten due to an error i made a few years back (long story). The timber frame underneath was fine as drainage is fairly good in that bit of the garden.

The oven base was built directly onto the flagged patio as this itself sits on a fairly substantial solid aggregate sub base and it hasn't moved in years

[Robarb]

I've stood on the flag/lintel surface and i'm not the lightest....

In my rush to get everything ordered i didn't order any additional wood for form work and given the current COVID situation that still going on, a lot of suppliers had 3-4 weeks wait for timber and aggregate...

So i used additional bricks to make the form for the vermicrete base. This was about 60-70mm thick

[Robarb]

The CaSi board i got a discount for as it was all uniform offcuts. It was also easy to install like this as i'd made the vermicrete layer level.

The firebricks were OK to cut in the end. I used a second pair of hands to spray the dust coming off and i borrowed a pretty big angle grinder off my dad. The bricks are 74mm thick. The only issue i had was with the last few cuts, the blade on the grinder had slightly moved and it was vibrating too much, hence why it looks 'unfinished' on one side. The tool for the blade was at a different location (don;t ask) so the cutting had to wait till the day after! i didn't take a pic of the finished base before the sand mould went on it.

The kiln dried sand was used to make any small level change needed in the brick work to make it super level

[Robarb]

Going back to the formwork issue and the wait for wood. I was pretty confident of my 'sandcastle making skills' so went for an all-in-one mould made of sand and bricks.

I used builders sand as it remains pretty damp and is easy to mould. Only down side is that is has currently died my bricks a slight rustic orange colour but i'm hoping that will eventually burn off anyway.

I had some spare sheets of thin ply (luckily), so made a rainbow piece of wood and used that for the dome. The entrance gallery was done by hand and measured accordingly so all the sides and the arch entrance was the same in mirror parallel.

I was a bit unsure on the corners of the mould not being as solid as wooden form work but mixed the entrance sand mould with a bit of fireclay to make it hold

[Robarb]

The newspaper was a pain of a job! i had a slight breezy day when all other days had been still and the first layer was not playing ball. In the end i used a mix of water with a small amount of cornflour and PVZ glue to make a sort of very thin paper mache. It eventually held and actually made a very smooth surface with not many crinkles. it also helped hold the corners of the entrance quite well too.

[Robarb]

Next was the actual cast using the tried and tested homebrew as posted here many times. I used sharp sand / fire clay / hydrated lime / portland cement in a 3:1:1:1 mix. I also added the polypropolene fibres and stainless steel needles at just under 3% of the mix (old scale did the trick here).

The first small tester batch i made was too wet so i doubled up with dry material and did the ball test David talks about (make a ball, if it splats it's too wet, if it cracks it's too dry). I pushed this into the mould all around making it 50mm thick (some places ended up being slightly thicker, but only by 5-10mm as this stage took ages and it was getting late in the day. I had to get it covered and keep it damp as it really was warm (we were mid heatwave). I used the other bit of spare ply and more free bricks to ensure the sides of the gallery remained straight. I had my concerns about this bit but it worked so i was chuffed with the outcome of using a complete sand mould.

I'm aware there is no 'thermal break' as such reading the pros v cons i decided a one piece mould was the way to go for my situation and materials / tools available.


NB. I didn't get a picture but it is worth noting that a made more of a chimney entrance that what is shown around the chimney hole to make a better support for the flue.

[Robarb]

When i took the sand out after 2 full days i inspected the oven. A couple of small creases in the newspaper meant i had to make a small mix of peanut butter type consistency of the cast mix to infill these. However, there wasn't many and there was no cracks (so far!)

I kept this cast damp (we really have had some warm weather) with an old curtain kept damp, covered loosely in a plastic tarp for the next 6 days. I kept checking the tarp and each evening you could see the condensation build up on the tarp. There really is a lot of water in this thing, as people keep saying

[Robarb]

Next job was the ceramic superwool. Oh! just forgot... PPE is a must through all of these different stages. The firebrick dust is nasty, the lime is nasty, the blanket is nasty, the cement is nasty... full mask, goggles and long sleeves in full on 30 degrees C is not fun, but better than the stuff coming off the oven build!

The blanket was easy to work with and quite pliable. I was careful not to compress it though. I put 2 layers on, each 25mm thick, so 50mm in total. Then wrapped it in chicken wire. I also added the twin wall flue (150mm internal) and made sure it was wired / mortared in.

[Robarb]

Last stage i've just completed (yesterday) was the vermicrete insulation layer (8-10:1 mix), this was a pain to use as it's much crumblier than the base but i got there in the end. Used about 225L of Mica vermiculite in the end. I also made a 5:1 mix for further support around the flue.

I considered a concrete cast arch but i just like the brickwork entrance so made a brick arch. Again used sand / wood for the form work and seemed to work well. I haven't moved this form work yet, that's my next job.

And whilst all that was going on, as you can see the decking and pergola has also gone up!

It's not quite finished yet but it is now pouring down with rain.