i tried to build an obsidian room. i do have obsidian on my map, but it's quite an advantage to have a permanent obsidian source (and it is fun to design it)
i've dig out a 5x5 room, made channels to the river and to the magma vent, built lever connected floodgates (steel of course) and then wanted to start the action. i will tell what happened: i connected a channel to the magma vent and tried to close the floodgate to control the stream. however the magma stream took the floodgate away and the magma went uncontrollably into the designated room. then i opened the water floodgates - but i connected the water channel directly to the magma room, so the water met the magma turned into obsidian, blocking the water supply.
i propably have a solution to the second problem. next time i will build a 5x5 room, directly above my magma room, completely channeled, with floor grates, and a connection to the river. this way i should be able to turn all the magma into obsidian.
but i really have no idea how to handle the magma floodgate problem. i will build several floodgates, but losing one floodgate every time i open the magma channel is definitely not satisfying.
if you have any experiences or suggestion concerning magma/obsidian room please post them.