A system for manufacturing three dimensional objects can include logic to apply a gas with at least one flow rate to a vessel of a three dimensional printer. The logic can also detect a permeability of a build material residing within the vessel, wherein the permeability is calculated based on a plurality of pressure values detected by a plurality of pressure sensors residing in the vessel, and apply the gas with a predetermined humidity level at a second flow rate to modify the permeability of the build material residing in the vessel. Furthermore, the logic can initialize manufacturing a three dimensional object with the build material in response to detecting the permeability of the build material is below a predetermined threshold.