This is the most important of the moulding materials. In different forms, it has different uses and is given different names, such as green sand, loam, facing sand, core sand, brass sand. All moulding sands are essentially mixtures of silica to give porosity and clay to give tenacity. Silica grains are refractory and have no cohesion. Clay is a fine powder, is refractory and when wet is adhesive and plastic. These two ingredients are mixed in different proportions, the silica grains varying in size, to make mixtures suitable for different uses.

Other materials are mixed with moulding sands for various purposes, as will be mentioned.

All moulding sand mixtures are more or less wet when formed into moulds. The requirements of a good moulding sand are:

(a) Sufficient porosity to allow the escape of gas and steam generated in the body of the mould by the heat of the molten metal. The greater the bulk of metal the coarser the sand used, and the greater the need for careful venting.

(b) Sufficient plasticity and tenacity to hold its form in the mould and to resist the erosive action of hot metal. These qualities are due to clay, which is more or less detrimental, because it fills the spaces, or pores, between the silica grains.

(c) A high enough fusing point not to melt and stick to the face of the casting.

There are many trade names for foundry sands, but all must fulfill the requirements mentioned. Vegetable or other combustible matter, sea salt, lime, and substances easily decomposed by heat should not exist naturally in moulding sand, as they may cause failure in casting.

Brass sand is green sand of fine grain used in moulds for brass castings.

Facing sand is also a fine-grained sand placed next to the pattern in small moulds to make a smooth casting.

Core sand is used for cores.

Loam is a very course moulding sand made up for loam moulds.