As per an independent report about the Fischer chamotte clay core:
“A chamotte brick is essentially composed of the oxide components silicon dioxide and aluminium oxide. Chamotte bricks are classified according to their aluminium oxide content. With an outstanding thermal conductivity of 1.4 W/m*K, heat is conducted highly effectively through the brick; it is this characteristic, in combination with their specific heat capacity of 1.00 kJ/kg*K that makes chamotte bricks an excellent raw material for heat storage units. Depending on the initial materials used and the manufacturing processes employed, a range of different bulk densities can be achieved with chamotte bricks. ‘Kärlicher Blauton’, extracted from the Kärlich clay pit (Carl Heinrich pit; Middle Rhine clay region), is a material with unique properties. Its composition and characteristics enable the production of chamotte bricks with exceptionally high heat storage capacities. In addition, to mould the heat storage bricks, the material is compressed by a worm extruder, which subjects it to forming pressures of up to 18 bar. It is these factors that make the KTS KG a unique heat storage brick with outstanding characteristics. The bricks are produced for Fischer GmbH Heiz- und Umwelttechnik, who install them for heat storage use in electric storage heaters.”