A rectangular prism (any solid XxYxZ shape, even 1x1x1) of pure Wood Piles, must be surrounded on every face (above, below, and all four sides, but not the edges or corners) by a compatible block. JEI lists all these blocks, of which Gravel and Dirt are worst, Ash Bricks or End Stone much better. One Wood Pile in the set must be lit on Fire (such as with Flint and Steel), and then before the fire can destroy the block the final piece of casing must be put in place.
After smoldering for c. ten minutes, during which the pile emits smoke particles visible through the top casing layer and makes burning sounds, all the Wood Piles will be replaced with 'Ash Blocks'. These can be broken very easily, and drop 1 Ash and ten to fifteen Charcoal each, depending on which casing material was used.