Rumors from ships flying through the Sky of Stones tell of a curious substance known as Ballast Mud.
Smelling like a mixture of clay and acetone, this sticky, heavy substance is reported to negate the buoyancy of bluewood when dry — a tragic state of affairs for captains whose ships have been plastered with the stuff by raiders in the Sky of Stones.
A report from a Crailese alchemist states that the base clay probably comes from the bedrock center of cloud-islands, and may be at least partially responsible for bluewood not being buoyant over the islands. Extremely deep holes must be dug (up or down) to get at the clay, which keeps it in short supply.
The fact that Ballast Mud seems to originate from the Sky of Stones may indicate that the residents there may have found a larger supply…