New oven build

[Michaelc]

I’m a long time lurker and first time poster. Thank you all for this great resource

I’ve built 4 ovens over the years. 3 old school clay versions with sawdust insulation and 1 using refractory castable and perlite render. I use them mainly for roasting and live fire cooking along with the occasional pizza. I prefer the clay to build and to use but keeping them dry is a challenge. None of my ovens have a chimney.

I’m about to embark on number 5 in a fairly exposed coastal environment so was going to try a home brew version. Any comments or suggestions would be appreciated.

The plan

50- 60mm home brew cast in place on polystyrene mould 3,1,1,1 including stainless steel fibre & AR fibre for strength and polythene fibre for steam spalling.(percentages??)
Fire brick floor on perlite and empty bottle insulated base
50mm insulating blanket
Chicken wire
100mm aluminium flashing on the base with a bead of heatproof silicon to prevent water ingress at base level
50-60mm perlite render

Suggestion for waterproofing would be appreciated

[Michaelc]

Finally ready to begin but with questions.
I'm building a cast in place oven from home brew and I’m wondering what the latest thinking is for a 6cm thick 80cm diameter oven.
1 Should I cast it in one piece or in sections?
2 3:1:1:1 mix of 3:1:1:0.5
3 What proportion of burn out fibres should I add to the mix

Any advice would be very welcome

[david s]

The larger a casting, or fire brick the greater the risk of cracking from a difference in thermal expansion. So the answer for Q1 is a multi sectioned cast oven. My own oven at home at 18 years old is a one piece cast dome and has no visible cracks. By contrast my first mobile one piece cast oven developed a verticle crack directly opposite the oven mouth. I've seen other one piece domes with exactly the same crack. The problem arises because convectional heat which rises creates a huge difference in temperature rise and therefore thermal expansion between the top and the bottom of the oven. However they do say there are two kinds of oven owners, those who say their ovens have cracks and those who lie about their ovens having cracks.
Q2 The original recipe for homebrew is 3:1:1:1 but I've found the high clay proportion leads to excessive shrinkage which can cause problems, particularly if casting over the top of a mould. I've not had problems since reducing the recipe that halves the clay content.
Q3 1% by volume, but because the smaller and thinner the fibres are so the dispersal becomes more difficult and important, so be careful when adding them. If you add them to a dry recipe before adding the water they are more likely to cause agglomeration (balling up into a fur ball). I've found it far better to add the water then pull apart a third of the required fibres, mix, pull apart the second third, mix, then finally mix the last of them and continue the mixing for a lot longer than you would think. I also find it better to mix by hand in a barrow so the texture of the mix can be felt and inspected as you go.

[david s]

I’m a long time lurker and first time poster. Thank you all for this great resource

I’ve built 4 ovens over the years. 3 old school clay versions with sawdust insulation and 1 using refractory castable and perlite render. I use them mainly for roasting and live fire cooking along with the occasional pizza. I prefer the clay to build and to use but keeping them dry is a challenge. None of my ovens have a chimney.

I’m about to embark on number 5 in a fairly exposed coastal environment so was going to try a home brew version. Any comments or suggestions would be appreciated.

The plan

50- 60mm home brew cast in place on polystyrene mould 3,1,1,1 including stainless steel fibre & AR fibre for strength and polythene fibre for steam spalling.(percentages??)
Fire brick floor on perlite and empty bottle insulated base
50mm insulating blanket
Chicken wire
100mm aluminium flashing on the base with a bead of heatproof silicon to prevent water ingress at base level
50-60mm perlite render

Suggestion for waterproofing would be appreciated

1% by volume for the polypropylene fibres
2% by volume for stainless steel needle fibres
2% by volume for the AR glass fibres

You can stick to the 3:1:1:1 recipe, but the high clay proportion is inclined to produce slump cracking because of excessive shrinkage, particularly over a hump mould that it can't shrink away from.
You may want to consider halving the clay content in the recipe for this reason.

IMO A 50-60mm thick outer render that includes perlite is not particularly strong and is quite hygroscopic which creates moisture problems. I find a much thinner layer 12-15mm thick of conventional cement render enriched with AR fibres is a superior solution.
A coating of elastomeric acrylic over that is very good for waterproofing and coping with expansion and bridging fine cracks.
There are plenty of well-documented cast builds on this site that could be helpful to you, but please document whatever you do so we can all learn.