Dry sand moulds made in iron flasks are used for steel castings, although small steel castings are frequently made in green sand moulds contained in wood flasks. The boundry between the use of green and dry sand moulds in this work depends upon the care and skill of the moulder, but castings of over a few pounds are safest and soundest when made in dry sand. A dried mould must be dried thoroughly to prevent damage from steam, and it is the practice of some steel foundries to heat green sand moulds gently over night in a drying room.