Cores are more or less surrounded by molten metal and are therefore subjected to more concentrated heat than other parts of the mould. To serve their purposes they must be made especially (1) to resist erosion from flowing metal, (2) to resist fusing, and (3) to allow the escape of air, steam and carbon gases contained in the core.

Clean silica sand of more or less coarse grain is the principal material of cores. About 15% of flour is mixed with this and the mass is wet with thick clay wash until the sand grains stick together.

All cores must be carefully vented to assist the natural porosity of the sand. Vent holes are usually formed by straight wires. Crooked vent holes are formed by greased strings. Vent wires and strings are pulled out before the core is taken from the core box. Large cylindrical cores may be made in halves for better venting, and all large cores contain cinders at the center to increase porosity. All vents must lead to the core bearings from which gases escape along the mould parting or through especially provided pipes or channels.

Small cores are strengthened by wires. Specially made core irons are enclosed in large cores to make them rigid enough for handling, and to support them in the mould.

The baking of cores makes the sand much firmer, and decreases the gases and moisture in them. Baking is usually necessary, but there are many shapes of cores which can be made of green sand as a part of the mould. These are well vented and are well enough supported to avoid the necessity of baking.

Large cores for loam moulds are built of bricks and surfaced with loam and slurry.

For smoothness of surface, cores are blacked and slicked. This is done after drying.