Over time a glass bell will start to grow an algae film, which rather defeats the purpose of having a clear bell. But it's fun for a science fair or classroom.
Small system I put together for a Feb 2012 science fair
That month I also put together my final rocket mass heater configuration, using cinder block and bricks with the 55-gallon steel drum.
My Rocket Mass Heater in Feb 2012
I like using the cinder blocks in the place of the insulated section of chimney because it is far less expensive. In addition, four stacked blocks are the perfect height inside a standard 55 gallon steel drum, leaving 1.5-2 inches of space for the gases to escape at the top.
Within minutes of me posting the cinder block rocket mass heater video, someone warned me that cinder blocks explode in fire. I have had no problem with this in the fires I've burnt in this system over the past six months. But I checked it out.
Allegedly moist cinder blocks can be a problem in fires - the water rapidly evaporates and the expanding gas can cause the blocks to shatter. My blocks aren't getting wet since they are covered by the steel drum. If they were to shatter, the flying bits would be contained inside the steel drum.
While I've read too many accounts to completely discount the danger of exploding cinder blocks, a search for exploding cinder blocks only yields videos of young men blowing up gunpowder inside a pile of cinderblocks (a really, really bad idea, by the way).
This fall I'm planning to reconfigure my greenhouse so the rocket mass heater is inside the enclosure itself. That way I'll capture the radiant heat off the heater itself, not just the heat traveling through the exhaust ducting running under my grow bed. I'll also be playing with solar heating. Fun times!