The hearth will consist of a level area of clean flat fire bricks, placed atop a level bed of sand and fire clay. It will be the final true cooking surface of the oven and so it is imperative that the bricks used in its construction be of high quality, flat and clean. The purpose of the fire clay in the sand mixture is to improve stickiness, insulation, and heat retaining ability. Cement, however, should be avoided in the under-hearth mixture.
Before constructing the hearth be sure to measure and mark out the location of the oven center and walls on the foundation surface. You can do this by using a tape measure and chalk to locate the center of the oven; but be mindful of the thickness of the oven walls (generally 8" or 20 cm), the vent landing (minimum 6"or 15cm), and the oven landing (minimum 6" or 15cm). Don’t forget to mark out the following:
Additionally, the measurements will be affected by the shape of the oven you choose.
Depending on the type of oven built, the fire bricks laid for the hearth may either be placed within the oven in either a circular or rectangular shape. The bricks may also be placed as an entire layer over the total foundation surface.
Building an igloo oven can be challenging in this respect as multiple small fire bricks will need to be cut to create the circular shape. As an alternative, the dome oven may be constructed, which requires less cutting, and the dome may be built as a circle over the flat hearth leaving fire bricks exposed around the perimeter. The disadvantage of this approach is the increased expense of utilizing extra fire bricks, and the implications of heat absorption and dissipation through the hearth under the oven wall insulation. Generally, either option is acceptable, and depends on the builder’s skill and aesthetic goals. It is recommended that fire bricks be laid dry on the hearth as a test, before the underside sand and clay mortar is applied.
Full herringbone pattern
Full offset pattern
Partial herringbone pattern
Partial offset pattern
The sand bed may be made from either plain sand, a dry sand and fire clay mix, or a wet sand and fire clay paste. If using sand and fire clay the mix should be created in a 1 : 1 ratio.
To construct the hearth using dry mixture, you will need to begin by placing refractory fire bricks on a thin layer of fire clay and sand (sieved) to remove any particles/pebbles.
A few things to keep in mind for this part of construction:
1. This layer should be about 1 inch (2.5 cm) deep.
2. At least half a bucket of pure sand without impurities is needed.
3. Impurities/pebbles in the sand, if any, will prevent proper leveling of the hearth.
4. Begin by screening the sand to a flat surface with a builders level, or a long flat piece of wood.
5. Depending upon the foundation, you may need to create a box out of wood specifically for the purpose of helping create a flat surface with the sand.
If using a wet paste, it should be applied with a 1/4"(1 cm) square notched trowel. Here are a few things to note during this process:
1. These trowels are commonly used by tillers when laying ceramic tiles.
2. Mix the sand and fire clay thoroughly and add water to make a workable, sticky mortar type mix.
3. This mix should not contain cement.
4. Spread the paste on the concrete slab with the plastering trowel in a zigzag motion.
5. Trowel the sand and clay mix onto the makeshift oven hearth, and then lay the firebricks from the center.
6. As with other processes, this should be done without material placed in between the fire bricks.
Ideally, bricks for the hearth should be laid in a diagonal offset, or herringbone pattern, so as to reduce the number of straight joins created. These joins create the potential to catch on tools such as oven rakes and peels. If these joins are placed diagonally to the entrance of the oven, this will reduce the likelihood of catches. Mortar or grout is undesirable between the laid fire bricks, as it will increase the size of gaps, creating problems. In addition, larger fire bricks can be used to improve the hearth condition: they will further reduce the number of hearth joins and catches.
Once the fire bricks have been laid for the surface of the hearth, it is now time to place a small amount of fire clay mortar around the perimeter of the bricks to keep them in place with the foundation below. The mortar only needs to be applied onto the outermost brick. Once it has set, the hearth is now complete, and construction of the brick or cob dome may begin.
It is strongly recommended that you don’t. The hearth of the oven will experience extreme temperatures, and will need to be both structurally and chemically reliable, as it will come in direct contact with cooked food. Fire bricks are constructed specifically for the purposes of tolerating high heat, has good heat absorption properties, in addition to low porosity. For these reasons, bricks will survive the heat fluctuations, and resulting expansion and contraction experienced. Red builder’s bricks, however, are not built to tolerate high heats, and would gradually fail over time as the oven is used: cracks may form and bricks would even begin to flake off into the pizza base. Eventually, the heath will become uneven and unusable. In such a case, the entire oven would need to be rebuilt.
The answer is yes and no. Concrete, if made of refractory products, such as refractory cement, fire clay, or crushed fire brick grog, may be used for building both the oven hearth and dome. If a suitably quality mixture is being utilized, the concrete created may be able to handle the high temperatures and resulting fluctuations without any issue. This type of hearth, however, is not generally recommended: the potential for cracks and flakes is high due to fluctuations and the low tolerance to structural movement. While fire bricks which are laid with natural space, may allow for minor movement, concrete does not. When laying the concrete, care should be taken to utilize the correct amount of water, and permit the minimum curing duration.
Mortar or grout should never be used in the construction of the hearth. The fire bricks should be butted up against each other, without any adhesive. The join created is thus called a butt join. And although it may seem unstable, the remaining oven dome and construction developed will prevent movement of the bricks. If grout or mortar were used: this would prevent a stable, flat surface from being prepared. The high temperature will cause the adhesive material to crack and flake, leading to an uneven surface, and allowing foreign material into the raw food place into the oven. This will lead to an uneven surface, causing snags. Only clean fire bricks should be placed directly upon a level base of sifted sand and fire clay to create the hearth.
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