Heat resistance
A pioneer in soapstone research
NunnaUuni's unique new gasification burning method for wood, Golden Fire, burns wood at an exceptionally high and even temperature. Therefore, the fire chamber material also needs to withstand extreme heat. At the same time, the stone should store thermal energy in the structures of the fireplace and release it gently and evenly.
NunnaUuni is a pioneer in soapstone research. In 1993, we began research at the geology department of the University of Oulu. The heat resistance capacities of various soapstone types and the effect of high temperatures on the mineralogical changes of soapstone types had not been studied before.
The research showed that the stone in NunnaUuni's uniform soapstone deposit consists of strongly foliated and crinkled talc and fine-grained magnesite. The deposit was named the MammuttiStone deposit, and its soapstone MammuttiStone.
A MammuttiStone type from the deposit was chosen, and its heat resistance was verified with thermal shock tests (DIN 51 068). The scale used in the test is 1–30. MammuttiStone received an incredible result, handling a transfer from glowing heat to running water without breaking 28 times. The resulting thermal shock rating of 28 that was measured for the heat resistance of the best MammuttiStone indicates why this type of soapstone is particularly suitable as a fireplace material.
Thanks to the foliated talc…
The talc keeps the thermal conductivity high and steady up to 1,400oC. The research showed that the talc that is part of the crystal structure of the MammuttiStone binds the small magnesite grains, enabling the stone to resist heat. The foliation of the talc helps in passing the heat quickly into the stone and promotes the efficient storage of heat in the structures of the fireplace.
An occlusion of pure talc, found in the MammuttiStone deposit. One part has been heated in an annealing furnace at 750°C for 10 hours. No changes took place in the structure of the talc.The dehydroxylation of talc starts at 820°C. At this stage, water molecules in the talc structure are removed. Foliated talc conducts heat well even at high temperatures, up to the melting point of talc (about 1,400ºC).MammuttiStone used for years
When the surface temperature of the fire chamber stones rises above 520ºC, a layer of 5 to 10 mm of the surface magnesite (MgCO3) at the hottest parts of the combustion chamber turns into harder magnesium oxide (i.e., periclase). With the Golden Fire, the temperature in the fire chamber can be as high as 1,200ºC. The talc of the MammuttiStone remains unchanged and efficiently conducts the heat deep into the stone. At the same time, it functions as a binding network structure for the periclase.*
MammuttiStone reveals its secrets
We performed tests during which we heated different soapstone types. These tests were conducted by heating the stone samples in an annealing furnace at a temperature of 700ºC for over 10 hours.
MammuttiStone - Before and after 
Heat resistance