chemical inertness, permeability, surface finish, cohesiveness, flowability, collapsibility, and availability/cost. Refractoriness — This refers to the sand's ability to withstand the temperature of the liquid metal being cast without breaking down. For example, some sands only need to withstand if casting aluminum alloys, whereas steel needs a sand that will withstand . Sand with too low refractoriness will melt and fuse to the casting. Chemical inertness — The sand must not react with the metal being cast. This is especially important with highly reactive metals, such as magnesium and titanium. Permeability — This refers to the sand's ability to exhaust gases. This is important because during the pouring process many gases are produced, such as hydrogen, nitrogen, carbon dioxide, and steam, which must leave the mold otherwise casting defects, such as blow holes and gas holes, occur in the casting. Note that for each cubic centimeter (cc) of water added to the mold 1600 cc of steam is produced. Surface finish — The size and shape of the sand particles defines the best surface finish achievable, with finer particles producing a better finish. However, as the particles become finer (and surface finish improves) the permeability becomes worse. Cohesiveness (or bond) — This is the ability of the sand to retain a given shape after the pattern is removed. Flowability – The ability for the sand to flow into intricate details and tight corners without special processes or equipment. Collapsibility — This is the ability of the sand to be easily stripped off the casting after it has solidified. Sands with poor collapsibility will adhere strongly to the casting. When casting metals that contract a lot during cooling or with long freezing temperature ranges a sand with poor collapsibility will cause cracking and hot tears in the casting. Special additives can be used to improve collapsibility. Availability/cost — The availability and cost of the sand is very important because for every ton of metal poured, three to six tons of sand is required. Although sand can be screened and reused, the particles eventually become too fine and require periodic replacement with fresh sand. In large castings it is economical to use two different sands, because the majority of the sand will not be in contact with the casting, so it does not need any special properties. The sand that is in contact with the casting is called facing sand, and is