When the CO
2 sand molds are put out in the air, sometimes the carbonate crystal grows on their surface. This tendency becomes promoted in the low temperature and high humidity air. The influence of water content in the sand, CO
2 gassing time and mole ratio of water glass added to the sand in crystallization are studied. Furthermore, the procedure of suppressing this surface crystallization is studied. The results obtained are summarized as follows : (1) The more the water content in the sand and/or the CO
2 gassing time are increased, the more promoting tendency of surface crystallization becomes. (2) Increasing the mole ratio of water glass, the crystallization becomes decreased. (3) As crystal grows more, the compression strength and the surface stability of the CO
2 mold go down. (4) The addition of alkylene oxide, such as ethylene oxide or propylene oxide up to 1% of the sand, suppresses the crystallization and the compression strength and the surface stability of the mold are improved.
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