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Refractory mortar and wood fired ovens

Mortar making and applications. Refractory heat resistant mortar types for building wood fired ovens from fire clay bricks.

For high quality oven, refractory mortar should be used in only up to ¼" (6mm) thick application. Mortar only assists in forming firing chamber and shouldn't be used or exposed on the inside hot-face.

Refractory mortar should not be used to plug holes or big spaces or gaps between firebricks. Clay body of refractory firebricks face the heat. Thicker mortar applied always shrinks (* more information further down). Shrinkage would create space and increased brick movement / gap between cold state and state expanded by heat.

Every time this gap happens bricks change position, just a little bit but enough to cause rebuilding job in the future.

* Firing linear mortar shrinkage of up to 8%

Before work with bricks and mortar, dip firebricks in water.
Before work with firebricks and mortar
dip bricks in water

Refractory mortar formulas

10 : 3 : 1.5 - Sand, Calcium Aluminate cement, Fireclay.

If you found hard locating refractory cement don't worry, here is one mixing formula with Portland cement plus lime available in ordinary building stores.

There are two Portland cement types, get the common gray in color, the other one is white decorative used for around pools etc. where a white effects are required.

Lime is calcium it takes over the cement in hot conditions (Portland will gradually burn out, it is used only for the mortar to set while working.)

10 : 6 : 2 : 3 – Sand, Fire Clay, Portland cement, and Lime.

This is how to mix the refractory heat resistant mortars.
Mixing heat resistant mortar

Mix first all dry ingredients well, then in small amounts start adding clean water at room temperature while continuing mixing.

Mix into peanut butter consistency. Thin application between firebricks and only where needed e.g. in "V" sections between arched bricks on outside. Inside the arch - these bricks should be in line next to each other touching without gaps.

No need for mortar between floor and wall bricks, in barrel shape dome you will not use too much mortar in arch too - it's very easy. However, if you build a round dome make sure you don’t get spaces in between bricks exposing mortar on inside the dome and big thickness of mortar on outside - start to angle cut bricks as soon as gaps begin to occur.

In any case, on the inside of a dome's hot-face there should not be any unnecessary gaps within the firebrick work. Only a small amount of mortar should always be used for the brick structure to stay strong. When buying firebricks ask your refractory supplier about that too. The mortar could detach in time, peel off, from the inner surfaces. Especially so when it is a fire-clay based mortar. (Fire clay explained.)

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