UPDATED - Barrier Materials + Spread Pattern
*Disclaimer: All information in this thread is based off a short series of tests, and it is possible that this information can change. Scientifically, I can only say what I observe happen, I can never truthfully say what can't happen, although sometimes I do anyway :tongue.gif:
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This is the pattern of what blocks lava can spread to. The orange block represents a lava block, blue blocks are Flammable Air Blocks(FAB), and red blocks are in danger of catching fire. The pattern is 3x3 on the level of the lava, 5x5 on the next level, 7x7(minus the corners) on the next level, and 5x5 on the top level. All of these blocks have been confirmed to be able to catch fire from the source block. The fire spreads by having an open FAB, and a flammable block touching it.
I haven't been able to get lava to spread fire below its own level, using contained, and uncontained lava. I have put in over 50 hours of combined tests with no instances of lava setting fire to any block on the level of its floor, or below.
Great news! Lava is a lot easier to use safely than fire. As long as it is completely encased(including corners!), lava will not spread fire. You can encase it with stone, glass, wood, wool... basically any block. Water doesn't work. All of these designs are safe(20 hours running with no fires). This is different than with fire. The enclosed lava block in the left example still has a 5x5 layer of Flammable Air Blocks(FAB) above the glass, but fire will not spread to any block in, or touching that layer.
Basically, any block, or any item, will work as a barrier material. The only caveat to this is if the barrier material is on the same level as the lava, also acting as a casing. In that case, you can see in this list what works and what doesn't. The items and blocks that don't work are insufficient because they are destroyed by the lava(in other words, can't encase it).
My favorite is definitely the pressure plates, as shown here.