Do I need both insulation and firebrick for the oven base

[daverook]

Hi

I am looking to create a table for my pizza oven but have questions about the protection between the oven base and the table top.

The table will be constructed of red brick (that you'd typically find to build a house) for the legs, and I am going to lay 32mm (1.2 inch) thick concrete slabs on top to form the table top. 

The pizza oven will sit on top (it's a home made oven, using the gym ball and perlite/cement approach which I'm sure everyone knows about)

For the pizza oven floor, I am going to lay fire bricks on the table top (the concrete slabs) and place the oven on top.

If I am right, Firebricks are designed to conduct heat and not insulate! This means the firebrick itself gets very hot.

Do I need a protective layer between the worktop and bottom of the firebricks? Would fireproof cement will be enough to protect the concrete slab worktop, or if I need to buy some fire proof insulation board as a barrier between the 2.

If it matters, I don't imagine the oven going over 450°C (as I'm not sure if the perlite/concrete will handle it) and I expect to use it max once a month. When I do cook, it will be for a meal with friends family, it's a home project (not commercial) so it won't be on all day.

Thank you in advance

[downunderdave]

Apr 13, 2021 7:48:24 GMT daverook said:

Hi

I am looking to create a table for my pizza oven but have questions about the protection between the oven base and the table top.

The table will be constructed of red brick (that you'd typically find to build a house) for the legs, and I am going to lay 32mm (1.2 inch) thick concrete slabs on top to form the table top. 

The pizza oven will sit on top (it's a home made oven, using the gym ball and perlite/cement approach which I'm sure everyone knows about)

For the pizza oven floor, I am going to lay fire bricks on the table top (the concrete slabs) and place the oven on top.

If I am right, Firebricks are designed to conduct heat and not insulate! This means the firebrick itself gets very hot.

Do I need a protective layer between the worktop and bottom of the firebricks? Would fireproof cement will be enough to protect the concrete slab worktop, or if I need to buy some fire proof insulation board as a barrier between the 2.

If it matters, I don't imagine the oven going over 450°C (as I'm not sure if the perlite/concrete will handle it) and I expect to use it max once a month. When I do cook, it will be for a meal with friends family, it's a home project (not commercial) so it won't be on all day.

Thank you in advance

If you don't have insulation between your cooking floor and your supporting slab the heat will easily conduct into the concrete leaving you with a floor that will probably never get to pizza temperature and unable to hold high temperature.
Generally as ovens are heavy a 4" reinforced concrete slab is usually used although depending on the size and weight of your oven, you may be able to get away with making it slightly thinner, particularly if you cantilever the slab over piers.

The perlite/cement mix for the dome is a poor choice for a number of reasons. Standard Portland cement begins to degrade north of 300C which your oven will definitely see if using it to cook pizza. Perlite, while an excellent material to use as an insulator, if used in conjunction with cement to make the mix, results in a casting with very low thermal mass, which you need for retaining heat, as well as very low strength. The low strength results in an oven interior very vulnerable to knocks and abrasions. (See attached table), The perlite itself is good for up to 1100C  If used with cement it provides excellent insulation both under and over the oven, but the hot faces need to be a denser material.
 
Although the construction using a gym ball may sound simple, it requires considerable formwork to cast only the top half. and the final casting has then to be moved into position creating problems of damage and seating. A far simpler method is to cast the dome in situ over a sand castle hemisphere covered in strips of wet newspaper as a slip layer. This eliminates any formwork and having to move the dome. A dense castable is a far more preferable alternative which creates a dome with good thermal mass and strength.

I have not seen any reports of a perlite/cement dome having lasted very long.

[daverook]

A far simpler method is to cast the dome in situ over a sand castle hemisphere covered in strips of wet newspaper as a slip layer. This eliminates any formwork and having to move the dome. A dense castable is a far more preferable alternative which creates a dome with good thermal mass and strength.

You are so right. I wish I had done this now as I am worried it will crack as we move it! 

OK, so it does sound like I need some type of insulation due to my concrete slab (work top) being only 1 inch thick.


In the table you attach, it shows vermiculite, not perlite. Can I assume they are similar enough that the table is still accurate?

[downunderdave]

Yes, perlite and vermiculite are interchangeable. Both have similar characteristics, are good for up to 110C and produce the same strength and insulation values for given cement ratios. Their price, for me at least is the same. When making a mix, particularly if it’s a lean one , is improved if a 50/50 mix of each is used rather than either alone. There is a silicone coated perlite product which reduces the amount of required water addition. It is not available to me so I haven’t tried it, but would if it were.