Smashing up fire bricks

[bearsparko]

Hi all,

I have been building on and off for about 3yrs a pizza oven......let’s not dwell on that.

The base structure is done, and I’m planning on casting vermiculite or perlite over a Swiss ball type construction.

At some point in the past I found on eBay 60x fire bricks
230x114x76 type JM23 Morgan Thermal Ceramics. There were cheap and I thought would come in useful.

I have 2 questions;
1. They are really dusty to touch as they are a high alumina type brick. Are they safe/nice to cook pizza directly on?
2. Could I smash them up and mix them in with the perlite/vermiculite to bulk out the mix for the main dome.

Sorry if these are daft questions, or they have been asked before, but I just want to get this done and I am struggling with the amount of conflicting information I find on the net.

Thanks!

[downunderdave]

The addition of perlite or vermiculite to any mix reduces the density drastically. It turns the mix from a dense castable to an insulating one. The resulting reduction in thermal mass creates an inability to store heat because it becomes low in thermal mass. (see attached table) This results in the oven being unable to to be used for retained heat cooking as well as temperature dropping quickly if the fire dies down. A much better set up is for the inner dome to be dense and then well insulated. The second detraction for the perlite mix is that it also weakens the mix leaving it subject to abrasion and knocks and bumps. A dense castable is around 10x stronger than a 5:1 vermicrete mix (see table again)
Building over a gym ball requires formwork under the bottom half of the ball, removal and replacement of the cast. A far simpler method is to cast over the floor in situation over a mould of sand covered in wet newspaper to prevent sand sticking to the castable.
Crushing firebrick is really hard work, not sure if yours are dense firebrick or insulating firebrick.