In building high quality oven, refractory mortar should be used in only up to ¼” (6mm) thick application. That’s the general rule but there are 2 or 3 exceptions. Mortar only assists in forming firing chamber and logically shouldn’t be used or left exposed on the hot-face.
Refractory mortar may not be applied for plugging holes or big spaces or gaps between firebricks. Clay body of refractory firebricks does face the heat. Higher mortar thickness applied always shrinks (*more information further down). Shrinkage would create space and increased brick movement or gap difference between cold state and the state expanded by heat on material reheating. Every time this gap happens bricks change position, just a little bit but enough to cause rebuilding job in the future.
Linear shrinkage in re-fired mortar is of up to 8%. There may be a thicker mortar application possible if such a place is located further away from the hot face. For instance plugging a cavity. In such a situation, the advice would be mixing a firebrick grog into the mortar. It would be an approach which causes lesser shrinkage. However, if the place requires structural stability and strength under some heat, then wedging with firebrick off-cut is obviously much more appropriate and professional.
To elaborate even further, the next option would be using a refractory concrete or coarse castable. These can be either mixed at home or purchased in a form of readily available pre-mixed products – they consist of firebrick grog as the aggregate volume and some common heat resistant bonding agents such as refractory cement.
Conclusion
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 and stable. When buying firebricks ask your refractory supplier about that too. You might even be able to buy a quality premix airset heat resistant mortar in a bucket for joining firebricks, they are good (these products aren’t dear, plus not too much will be needed for one oven job, so of course go for it and buy the bucket. Most commercial mortars do not require soaking firebricks in water before work starts, so read the instruction on its packaging.) Exposed mortar in the dome could detach in time, peel off, from the inner surfaces. Especially so when it is a fire-clay based mortar – the fire clay explained.
Disclaimer : This article expresses the author’s opinion and the accuracy of the information in this post is not guaranteed. We hope you find the refractory heat resistant mortar information useful.
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Source: Rado Hand (Traditional Oven .com)