Using firebrick instead of a pizza stone
It appears that many have learned the hard way that pizza stones are fragile.
In my area (Seattle) one can purchase fire bricks for less than $2 each (e.g., from Mutual Materials). Firebricks form the hearth of a standard wood-fired oven and are a great cooking surface. Standard U.S. firebricks are 4.5 inches by 9 inches by 2.5 inches thick. "Splits" are half that thick.
You can lay out a pattern of firebricks on your oven rack. It doesn't matter if there are small gaps and irregularities -- that's the way it would be in a wood-fired oven anyway. (A big gap would be a problem, however.)
I've seen speculation regarding the heat-up time. This is a classic heat conduction calculation. Lets say the bricks are initially at 80 F. You place them with edges touching such that the large faces will be the cooking surface, i.e., with the shortest (2.5 inch) dimension vertical. To a good approximation, all heat conduction will occur through the large upper and lower faces.
In the best case (which won't actually happen), the oven temperature quickly reaches the desired value (say 600 F) and the brick faces quickly reach and sustain this temperature. Heat gradually conducts inward until the bricks reach a uniform temperature equal to the oven temperature.
Typical fire bricks have a density of 1860 kg/m^3, thermal conductivity of 1.03 W/m-K, specific heat of 1.05 kJ/kg-K, and thus a thermal diffusivity of 5.3e-7 m^2/s. Using this value and the temperatures given above, within 15 minutes the average temperature of the brick is 469 F. After 1 hour it is essentially at a uniform temperature of 600 F.
In other words, one hour of preheating should be plenty for full-thickness bricks.
For splits, 15 minutes after the oven reaches its operating temperature should be plenty.
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