First Build & Casting – Homebrew Vs. Dense Castable

[david s]

Filling cracks is invariably unsuccessful. If you go down the homebrew route I'd strongly suggest you use the sandcastle single casting route, for the dome, remove the door mould when set, then build another sand form in front of the door for the flue gallery and cast it in place. Making complex multi piece moulds is very time consuming and not worth the extra effort for a single casting IMHO.
By the way the oven in that link appears to lack any underfloor insulation, a pretty basic error.There's no need for a flue damper if you have a door rebate which isolates the flue gallery from the dome for retained heat cooking (baking and roasting).

[Chicharron]

Hi Stoox,

I've just completed my final curing fire on my oven. I was on the same train of thought you are in. I just wanted to save some money and experiment.

I ended up using Homebrew with the addition of Stainless Steel needles although I might have used a bit too many. The mix feels very solid and strong and after the 3rd or 4th curing fire I started seeing hairline cracks. After the 6th and 7th I ended up with quite a few of them they don't seem to be growing and are only noticeable when the oven is hot. As soon as the oven cools down they are barely noticeable, if at all.

I'm not sure how it's going to affect it in the future but I can tell I haven't seen them grow after 8 fires (two really hot ones!). Also smoke doesn't come out of them at all, I'm guessing that's a positive but only time will tell.

My Oven is pretty small too (600mm) so I don't know about a bigger one.

BTW, cracks are only visible from the outside.

[david s]

If you have visible cracks on the outside then you are presumably firing the oven uninsulated. This then means there is a huge difference between the inside and outside faces of the refractory and a major cause of cracking. If it's the vermicrete layer cracking it's because you've not dried it slowly enough. It will swell and crack with steam if fired too fast and too soon. Don't over-worry though because the dome can't fall down and the cracks are unlikely to get bigger. Plug up vermicrete cracks with more of the same mix.

[Stoox]

Chicharron Good to see people attempting the Homebrew method (with some success). I've made the decision to go Homebrew and I finished the entrance arch yesterday so the dome is the next job. What thickness did you make it? and how wet did you make the mix? I've heard too wet or too dry can cause problems.

[david s]

Too wet and it doesn't stand up on its own and tends to slump. Too dry and you'll get more voids. Just start with what you think is about right. It's easy enough to add more water to make it wetter or more dry material to make it more dry. Start from the bottom and build all the way around the base leaving a flat top onto which you can sit the next row. Once you get about half way up the dome it starts to lean in and gets much easier. If it slumps a little at the base you can punch it where it appears too thick.

[Chicharron]

david s yes I've been curing without insulation and it's quite cold here in Melb already so I'd say that's the case. I just wasn't sure if it was worth completing the oven as I wasn't completely happy with my concrete pouring (ended up with some voids) so I just wanted to test if it wouldn't just collapse altogether. I got my dome completely white on my last firing so it's looking goo so far. If after a couple of pizza nights it still hold I'm planing on completing my CFB + percrete insulation layers.

Stoox I did what David mentioned. I started with a bit of a dry mixture and added water until it "felt right". First couple of trowels I thought it was too wet and hard to work with, It slumped a lot. However at the end I worked with a mix that was too dry and left quite a few voids internally. You just have to be patient and prepared to push it up a few times as the walls start to get heavy.

I made mine 50mm, which also found difficult to keep, specially when you are getting to the top as you dont have much reference. I used a stick with a mark and poked a few times to see where I was at. most of the time I was well below the mark and had to adjust. I made the entry 30-35 mm thick

Also if you are using the needles I recommend you to stick to the recommended 2-3% I used about 4% and I reckon that's a bit too much but what do I know? apparently there's a thesis somewhere somebody presented at a university and determined 5% was ideal.