Abstract: Foundry moulds and cores of increased mechanical resistance are prepared from sand, a binding agent, a setting agent, and a surface active agent, the binding agent being a urea-formaldehyde resin or a urea-formaldehyde-furfuryl alcohol resin, the surface active agent being capable of producing a foam which begins to subside before the sand begins to set, the surface-active agent being selected from the group consisting of an alkylbenzene sulphonate, an alkylamine salt of formula RNH.sub.2,YH in which R is an alkyl group of more than 9 carbon atoms and Y is the anionic portion of an acid or a compound of formula: ##EQU1## IN WHICH R represents an alkyl group having more than 9 carbon atoms and Me is an alkali metal; in the proportion of 0.01 to 2% by weight of surface-active agent with respect to the total weight of the liquid sand. Particularly effective are n-dodecylbenzene sulphonate and tetra-isobutyl sulphonate.