So I thought I'd try out DF again and play around on a map with a combination of volcano and shallow aquifers to try my hand at obsidian casting.
My dwarves built some temporary housing from wood and whatever lesser stones they could find and set upon building pump-stacks to bring water from the aquifer up to the rim level of my local volcano. Forms were made from the same sorts of wood and stone, and the plan set into motion: to cast a large solid structure, from which a worthy natural-obsidian dwelling could be carved, with engravings depicting its creation upon the walls and floors.
Magma descended upon the form, filling the bottom layer just before the water came pouring down. Thrice more the layers were poured, and no sooner was the fourth layer cool to touch than the miners set to work in shaping their new home. Only... there was one misgiving. Instead of the solid obsidian megalith that they had toiled to create, only strata of their dreams were found, set between layers of ugly sand. The walls could be smoothed, engraved, made to depict their adventures and toils, but the floors not so. The ground beneath their feet was only something to kick around or cover. Never would they find themselves trudging with their chins down to find the stories of their brethren lifting their spirits.
Seeing their work inferior to their aspirations, the dwarves tore down the mass of stone, reassuring themselves that the blocks and stones would make fine material, as they began to pour anew. This time, the mud and grime would be stripped away with each added layer. Each level would be smoothed anew before laying down the next strip of molten rock. Alas, they found themselves once again with sand below their feet and nothing but more callus to show for their efforts. Before a third vain effort, the dwarves implore the Mountainhomes to lend their wisdom of the magma-crafts. How can one cast solid obsidian, with floors created of equal quality to the walls?
tl;dr What are the mechanics on casting obsidian floors (smoothable-engravable floors)? I can't seem to get non-sand floors after pouring large 3+ tile thick slabs, despite trying both magma-water and magma-water-smooth layering procedures. I'm sure my question ties into the natural-soil layer mechanics, but unusre if there is some simple procedure I am overlooking.