Plans for a 70 cm/28 inch hombrew oven

[spoton]

Hi,

Have been reading this forum for some time, and time to put the theory into practice.

Planning to build a 70 cm ID oven using the homebrew mix.

A few questions that I got so far:

1. Size of the base. Want to make it compact on the sides, so I guess to leave room for around 8 cm of wall thickness + some perlcrete insulation on each side I shouldn't go below 110 cm width.
Lenghtwise I want to leave room for the flue gallery + some open space, so around 120cm?

2. I that see some are people casting their flue gallery separately from the sphere. Are there any pros/cons of doing that?

3. Materials:
- Fireclay is an undefined word here. But I can get kaolin/china-clay. Having done a search I see that others has used it with success. But open for any comments.
- As for reinforcement I see that the main concrete supplier here has both SS-needles and basalt fibers in stock. They claim the latter has better refractory properties. They also look a bit finer and maybe easier to use than SS-needles. Has anyone tried it?

Thanks in advance for any advice

[david s]

1. Draw a plan, starting with internal diameter. 8cm is very thick for a small over. The usual inner dome cast thickness is 5cm. Any thinner starts to reduce strength too much.
Plan depth of flue gallery (depth needs to be big enough to accomodate flue pipe fitment and gather smoke), but shallow makes working the oven easier.

2. Casting the flue gallery separately (but in situ), cast domes first then cast gallery in front of it, allows some relief from a difference in thermal expansion between the hotter dome and the cooler gallery. Look at the build threads on cast domes in the Other Oven Types Section.
Creating the door rebate in a combined, one piece dome/gallery is difficult.

3. Kaolin or China Clay is more refractory, but expensive and not required for our service temperatures. Bricklayers Clay from building or concrete suppliers is a better choice.
Basalt fibres (at least the ones I have) comprise a range of length and thickness and the finer fibres do not disperse well. They tend to clump, as all really fine fibres do, which means much more extended mixing. I prefer AR fibres which have a consistent 0.5 x 19mm size and disperse far more readily with similar strength and cost characteristics.
SS needles make placement more difficult are expensive and small quantities difficult to obtain.
I also use nano reinforcing products but also expensive and hard to obtain small quantities. Do not use steel rebar because it’s too thick to withstand the thermal conductivity differences. Chicken wire is another alternative, but hard to apply over a compound curve. It can be done with smaller overlapping pieces, but must be embedded in the middle of the casting which is way harder than random mixed fibres.

It is well worth the trouble of deep planning and research before forging ahead.