Blister
A cavity or hollow usually found in the upper surfaces of castings. They are imprisoned gases, which, having no means of escaping before the metal congeals, arrange themselves in various sizes and shapes under a thin film of metal. They are found sometimes on the top side of pipes and columns, and in this case may be caused by the steam from a damp core, which, not having a ready means of escape through the vents, finds its way into the mould. Another fruitful cause of blisters is rusty chaplets and studs, which give off considerable gaseous compounds as the rust decomposes. Blisters are almost certain to ensue when a green sand surface, core, or cope is too damp or wet in spots. Should there be no vents at that particular part to lead away the steam as fast as it generates, it must inevitably find its way into the mould, the result being blisters.
Sometimes blisters are caused by the sulphurous gas contained in the iron itself, which, if it once enter the mould, acts exactly like the gases we have noticed above. Such gas as may be mingled with the iron will naturally ascend to the top if the mould's formation is favorable to its rapid transit in that direction; but, should it be otherwise, the probabilities are that it will be found imprisoned at whatever part of the mould it happened to be when the latter had received its fill of metal. Remote risers do not in the least affect this phenomenon, as the currents of metal leading thereto may, and usually are, far removed from the already formed blisters in congealed parts of the casting. Increased pressure will assist to force mould-gases out at the legitimate vents, but will render small help to expel such as may be contained in the metal itself, when the ordinary processes of moulding is employed. See VENTING; PRESSING FLUID STEEL; RUST; PASTE.