Hi all, about to start a pizza oven build and been reading lots of useful advice on this forum. One thing I can't find is the best fire brick thickness for the hearth? Not decided whether to lay on top of cal-sil board or a 5:1 verimcrete layer, but just wondered if the thickness of the fire brick for the cooking surface matters?
I'm planning on using 64mm bricks cut in half for the dome, but wondered if I could get away with cheaper 25mm thick bricks for the hearth..?
Thanks
Rob
Hi Rob,
First up the floor insulation: if using calcium silicate board, its thickness is usually 50mm or 40mm depending on the manufacturer. One layer is the minimum, two is better, but the stuff is expensive so many builders go for a 5:1 vermicrete mix as an alternative. It is not quite as good an insulator as the cal oil. You can make it leaner to improve its insulating capacity, but unfortunately that weakens it too much as a support for a heavy oven sitting on it. The problem is that the mix is wet and it should be dried before building over it. The cal oil board already being dry does not suffer from this problem. See my attached study on drying a vermicrete slab.
Regarding the firebrick floor thickness, it should be similar to the thickness of the dome walls. For cast ovens both the walls and floor range between 50-75mm thick. For brick ovens, because the mortar courses require 100mm wide for adequate strength, both walls and floor thickness are usually 100mm. Some builders have attempted to create brick ovens with thin walls and floors, but have found the structural integrity of the dome is compromised. Although a hemisphere is a self supporting structure the continual thermal expansion and contraction places enormous stress on the structure especially when the chamber is heated unevenly and at a very high temperature increase rate. To explain another way, there is a reason house brick walls are not laid with the bricks on edge (3") and that is because experience demonstrates that 3" is insufficient width for a mortar joint.
Vermicrete insulating slab
The purpose of this experiment was to calculate the water loss from a vermicrete
insulating slab after casting and placement.
Materials: Mass (Kg), Vol (L)
Vermiculite 1.82, 27.00
Cement 7.25, 5.40
Water 8.10, 8.10
Total 17.17, 21.6
For the aggregate in this mixture a 50% vermiculite 50% perlite proportion was
used.
The mix is generally considered a 5:1 vermicrete and comprised of vermiculite,
perlite, Portland cement, water 2.5, 2.5, 1, 1.5 by volume.
The form was filled with dry aggregate to obtain required volume, but on mixing
and placing in the form, has reduced approx. 20% as some of the grains break
down from abrasion during mixing and compaction on placing also reduces
volume. The resulting volume reduction is an estimate based on the finished slab
thickness.
Mixing was done gently by hand in a barrow.
The compacted slab was covered for one week to allow good hydration.(no
additional water was added)
The weather was fine, cloudless, low humidity and the temp in the range of 12-
24 C. The slab was exposed to approx 3 hrs sun/day top surface only.
Weighing of materials was done using digital scales, but weighing of finished slab
was done using bathroom spring scales as the weight was beyond my digital
scales range.
Although the top of the slab appeared quite dry the bottom was still wet so I
lifted it up, stood it on edge to assist drying after Day14
(Kg)
Day 7 16.0 Day
8 15.0
Day 9 14.5 Day 10 14.0 Day 11 14.0
Day 12 13.5 Day 13 13.5 Day 14 13.5 Day 21 12.0 Day
28 11.0
Conclusion:
Given the ideal drying conditions, relative thinness of the slab and that it was
uncovered, the experiment still demonstrates the large quantity of water present
and the slow rate of removal. For a thick covered slab expect the water reduction
rate to be way lower. After 3 weeks of drying more than 50% of the water had
been removed by the weather from the aprox. 2” thick slab. After 4 weeks
approx. 75% of the water added had been removed. Given the 0.4 W/C ratio that
consumes the water in the hydration process, that consumes 1.34 litres leaving
6.76 litres of free water. So after 28 days of drying 5 litres lost is pretty much
most of it. This excess free water ensures that plenty is available for the
hydration process so therefore no wetting of the slab is required during curing.
In hindsight I probably should repeat the experiment and cover it with firebrick,
then compare the results. Given that the water under the floor and the base of
the dome is the last to be eliminated and the resulting problems and damage it
can cause, the drying of a vermicrete slab before building over it is advisable.
Apologies for the format, the forum does not accept the pdf version.
