of a heat to cure the binder. Resin binders are quite popular because different properties can be achieved by mixing with various additives. Other advantages include good collapsibility, low gassing, and they leave a good surface finish on the casting. MDI (methylene diphenyl diisocyanate) is also a commonly used binder resin in the foundry core process. Sodium silicate Water glass ( sodium silicate [Na2SiO3 or (Na2O)(SiO2)] ) is a high strength binder used with silica molding sand both for cores and molds. To cure a mixture of finely ground sand (e.g. by using a sand muller) and 3 to 4% of sodium silicate the binder, carbon dioxide (CO2) gas is used. The mixture is exposed to the gas at ambient temperature reacting as following: {Na2O(SiO2)} + CO2 <=> {Na2CO3} + {2SiO2} + Heat The advantage to this binder is that it can be used at room temperature and is fast. The disadvantage is that its high strength leads to shakeout difficulties and possibly hot tears in the casting. The mixed sodium silicate and sand may also be heated by a heat gun to achieve better rigideness. Additives Additives are added to the molding components to improve: surface finish, dry strength, refractoriness, and "cushioning properties". Up to 5% of reducing agents, such as coal powder, pitch, creosote, and fuel oil, may be added to the molding material to prevent wetting (prevention of liquid metal sticking to sand particles, thus leaving them on the casting surface), improve surface finish, decrease metal penetration, and burn-on defects. These additives achieve this by creating gases at the surface of the mold cavity, which prevent the liquid metal from adhering to the sand. Reducing agents are not used with steel casting, because they can carburize the metal during casting. Up to 3% of "cushioning material", such as wood flour, sawdust, powdered husks, peat, and straw, can be added to reduce scabbing, hot tear, and hot crack casting defects when casting high temperature metals. These materials are beneficial because burn-off when the metal is poured creates tiny voids in the mold, allowing the sand particles to expand. They also increase collapsibility and reduce shakeout time. Up to 2% of cereal binders, such as dextrin, starch, sulphite lye, and molasses, can be used to increase dry strength (the strength of the mold after curing) and improve surface finish. Cereal binders also improve collapsibility and reduce shakeout time because