Fire clay is an essential component of all high-heat resistant fire-clay mortars or fire brick insulation beds. When using fire clay you need to make sure that it is free from impurities. Any impurities or pebbles in fire clay when creating an oven hearth will prevent the surface from being level and smooth. For the sand bed under the fire bricks fire clay may commonly be added to improve the insulation properties. The clay may be mixed with mortar to create a mix more resistant to heat and capable of having adherence properties. This may be necessary for areas which are exposed to the greatest amount of heat but which need to have high structural strength.
Fire clay is available at local markets or refractory building suppliers. Fire clay may be collected from natural sources however it may need to be tested by a laboratory to determine the quality.Generally the clay will appear as a light colours mud in a river bed, bank, or where soil erosion is occurring. You can also find fire clay where excavation is in progress. If it is wet the mud will be sticky and soft along with no organic matter. It is important to be aware that fire clay will shrink nearly 10-15% when it dries. As a result the potential for cracks to occur is increased.
Fire clay contains higher alumina content of about 24-34%. Silica content in fire clay ranges from 50-60%. After the water dries out fire clay tends to shrink around 10-15% so you may need to take some extra fire clay if calculating amounts based on dry volumes.