There's two envelopes of bricks, that contain the fire box and the circuit for the hot exhaust gases. The inside envelope is not visible, it is air-gapped 1/2 inch from the outside one, providing for hot gas circulation and heat exchange. It is made up of firebricks to withstand the 1000 C* temperature that the firebox produces (due to a venturi effect into the secondary chamber that induces complete combustion). There's no visible smoke whatsoever above the outside chimney 10 minutes after the fire is started; the exhaust pipe is barely 100 C* where it exits upward. Inside, the exhaust gases are directed towards the floor, in brick chambers for heat exchange traveling under the firebox before reaching the chimney at floor level. Unlike most mass-stoves, heat is released almost immediately upon the heavy steel cover reaching 150 C* to 200 C*. The firebox is filled twice a day, maybe three times on a cold day, leaving it to cool off during the night after hours (wood consumption is minimal). 1 1/2 hr after starting the fire and afterwards when the combustion is over, the surface temperatures of the outside clay/sand bricks is too hot to keep your hands on it. The design that was developed in France and the company sells on-a-pallet kits they claim can be assembled quickly. The key features are the two kinds of interlocking bricks, low emission levels, low kit price and ease of start-up (a few kindlings, fill the box with wood (+/- 10 kg.), and close the door).
uzume.fr in France manufactures thermal mass ovens and technology
Their Batch Block (aka Rocket) stoves are substantially different from the classic mass stoves of Scandinavian and Russian origins. They operate at much higher temperatures, hence higher wood consumption efficiency and low rate of un-burnt gas emissions (as per tests in the Netherlands and France). Their website has animations and images that clearly show how their ovens are built and function (e.g. the inner box - outer box design).
Clay for Ovens and Heaters