800 mm homebrew cast

[Linuc]

I've made some progress.

I've drilled a couple of weep holes and managed to find some cheep mosaic tiles. There was some cement at the bottom at the perlcrete slab but it feels strong enough to build on. Other than that I've sawed the CalSil board and been building molds for the casting.

I'm going for 150 mm vertical sides and then 300 mm dome height.

My plan is to do the cast in three steps.
1. The vertical sides
2. The roof
3. Oven opening.

I'm doing the cast directly on the Cal Sil boards since I want to have the biscotto bricks loose so I can change them if they break. So I'm going to use an angle grinder to cut the bricks between step 1 and 2 and make sure they fit and have some room for heat expension.

How long should I wait between the three casting steps?

Does my plan sounds OK? Hopefully I'm starting the cast this weekend.

[david s]

To get a decent bond go to the next step as soon as possible, but definitely before the cast has dried. You may need to wait 24 hrs before removing the mould and proceeding.

[Linuc]

Okey, so this weekend I've casted the walls and the roof. I did the walls on friday, removed the form on saturday and filled some voids and small cracks both on the outside and inside with a runnier consistency of the homebrew. Also on saturday I used an angle grinder to cut the biscotto bricks and I lay it on the floor on a couple of centimeter of sand (no ash and salt as I said before). I thought about using clay but I didn't really have time to fix, so I skipped. I figured sand was enough to leveling the floor.

I then built the sand dome, patched it with newspaper and then I casted the roof today on sunday. I did ran into some big problem as I ran out of fire clay - on a sunday! I was sure it should have been enough for the roof and I needed to buy some more tomorrow for the opening, but I ran out in the middle of the casting. Fortunately, my bricklayer whom I bought the fire clay from said I just could drop by his workplace and get some more. Not always, but sometimes I like people!

However. It was really nice to work with the homebrew but also a bit hard since I used double layers of gloves and the SS needles made it a biiiiit harder. I did use 2% SS needles and about a handful of PP fibres for every ten litres of dry homebrew. Hopefully I got it right.

It was very, very time consuming so the outside of the oven is not smooth at all. Probably the ugliest looking homebrew oven on Forno Bravo to date. But I honestly didn't have time to work on the smoothiness and I figured that's not so important since insulation and render will be on later. The last layer is most important, right? Also it's not so easy working with a one and a half year old child running around and we have a ton of other projects going on - like painting the house!!

My plan is to remove the sand tomorrow night. That's about 30 hours after the casting. I will then cast the opening and then leave it for a week before putting on the ceramic fire blanket and perlcrete. Sounds right?

[david s]

Yep, sounds about right. When you demould it tomorrow, make sure you cover it to keep the moisture in for a week.

[Linuc]

Last casting step done! It was very time consuming and tricky getting the sand and newspaper out of the oven, as well as filling all the voids and small cracks. Regetting making an low dome oven at that moment!

It all went fine I think. Starting to look like a proper oven. Quite happy I made it this far. Excited to look at the result tomorrow! Then I will leave it under a tarpaulin for a week.

[Linuc]

Okey, so I'm seriously no mastermind when it comes to building things - I am well aware of that. But I might just have done such a stupid mistake even I'm impressed by my stupidness.

So I removed my opening form and discovered that the chimney hole was way too big. I did use a casting pipe (unsure of the translation here) with the same dimensions as the chimney and wrapped it with som card board to make some space for the heat expansion. My plan was the to use the famous tab solution for the chimney. But the hole was way, way too big as you can see on the pictures.

So after a discussion with my swedish friend Petter I decided to make some heels at the bottom that the chimney can stand on. See pictures. This was a bit tricky.

I also decided to make the homebrew casting a bit higher for the chimney so it will have some support if it's windy. Here comes the stupid part. I then used the casting pipe (again, unsure of the translation but look at the pictures) but without the cardboard. So now it's narrower at the top and I have no idea if I can put the chimney in the hole or if it has room for heat expansion. Sometimes the brain is just not working.

Well, I guess we'll see tomorrow. Maybe it's possible to grind the inside in a couple of days if it's too narrow?

[david s]

yes, try and ream it out a bit. You don't need much. If you can rotate the pipe in the hole, the seam does a pretty good job of reaming it.

[Kris S]

I'm also interested to see if the heels would be enough to support the weight of a 90 cm single wall pipe.

Like you, I also ended up with a chimney hole that's too big (it's like 17-18 cm diameter for my 15cm pipe), and because I must have moved something while casting the flue, I also ended up with having a 'natural heel' at the bottom about around half of the circumference... (see picture)

I've used that heel or rim to support a short piece of tube, but fear it might crack over time, so I plan to rest the pipe on the rim but also cut a few tabs to rest it at the top of the flue at the same time...

[david s]

I' ve built a few ovens with removable flues and have successfully made sleeves for the pipe to insert into. You could cut some tabs in your pipe similar to the ones I cut in the sleeve.

[Linuc]

No problem to ream it out. Now it fits just fine, and hopefully it's enough room for heat expansion. It feels like the heels is strong enough to hold the flue. It's pretty light.

David, my plan was to cut some tabs but I figured it needed more support if it's windy. So I made the chimney a bit taller for more support and made the heels. Hopefully it will work.

[Linuc]

Today I started the insulation. I wrapped it with two layers of ceramic fire blanket (2,5+2,5 cm) and then applied perlcrete 10:1. I had the same problem as with the slab, the water started to pool at the bottom. This time I noticed that right away so no problem. I found that the best mix was 10 parts perlite, 1 part cement and only 1,8 parts water.

The blanket was nice to work with but damn, I hate the perlcrete.

I only did a couple of batches tonight so I have a lot of work left. Figure it will take a couple of nights. Damn, I just realized I have some clay left!! Will use some when I continue tomorrow night.

Should I keep covering the oven with a tarpaulin or only if It's risk for rain?

[david s]

ing.That’s not a lot of water, it’s possible that you have the silicone coated stuff. The finer the grade the greater the uptake of water. Also try adding a little powdered clay to the mix. It imparts stickiness making the mix more workable. As you want to get rid of excess water, only cover it if it looks like raining.

[Linuc]

ing.That’s not a lot of water, it’s possible that you have the silicone coated stuff. The finer the grade the greater the uptake of water. Also try adding a little powdered clay to the mix. It imparts stickiness making the mix more workable. As you want to get rid of excess water, only cover it if it looks like raining.

Hm, it might be that way. I bought perlite for ground insulation. It does say hydrophobic(?) with silicate. Is that a problem?

[Kris S]

I'm afraid David S is right, hydrophobic = water repellent...

[Linuc]

I'm afraid David S is right, hydrophobic = water repellent...

The slab is fine, even though I had the same problem with the water then. And the perlite decreases when I add water.