Hi from Blackpool

[blackpoolbaker]

I’m still in material gathering stage but poor weather restricts an outdoor build until weather dries up. I’ll be using exercise ball to form dome approx 90cm diameter. I’ll be casting with refractory mortar 70mm thick then fire blanket 2” then a 100 mm perlite cement coating. A 6”x1meter flue. Base will be 4” reinforced concrete with a 4” layer of perlite motar then cooking floor of fire tiles

[downunderdave]

Mar 2, 2024 17:01:41 GMT blackpoolbaker said:

I’m still in material gathering stage but poor weather restricts an outdoor build until weather dries up. I’ll be using exercise ball to form dome approx 90cm diameter. I’ll be casting with refractory mortar 70mm thick then fire blanket 2” then a 100 mm perlite cement coating. A 6”x1meter flue. Base will be 4” reinforced concrete with a 4” layer of perlite motar then cooking floor of fire tiles

Using an exercise ball to cast over seems to be a good solution because you can simply deflate the ball for removal. However, because you only require  a hemisphere, considerable formwork is required to to cast the top half only and it needs to be strong enough to support the weight of the dome. A 7" thick 90cm dome will weigh a lot. You then need to lift it free of the formwork and reokace it in position over the under-floor insulation. This will probably require lifting equipment.

A far easier solution that requires no lifting and relocation or extensive formwork, is to cast in situ using a sand mould (covered with strips of wet newspaper which act as a slip layer).

Most cast ovens are made with 2" thick walls which provides sufficient strength as well as adequate thermal mass for retained heat cooking and stable temperature retention. Large ovens over 1.5m in internal diameter usually go for 3" thick walls. I built one last year that had a 7 piece cast dome that took three men to lift each section in place.
Was there a specific reason you wanted to go to 7" thick? The extra thermal mass requires far more fuel to heat as well as far longer to reach temperature.
My calculations say a hemisphere of those dimensions and thickness is a volume of 320litres which will weigh around 600kg. A 2" thick casting of the same internal diameter would be 71 L weighing 133kg with plenty of thermal mass for extended heat retention cooking, but even that would be unsuitable to cook three pizzas on a Friday night.