Characteristics of good brick earth

Good brick earth should have the following characteristics.

1. Plasticity: when mixed with water it should be workable and hold its shape on moulding. Alumina provides this.
2. Proper composition: silica 50-60%, alumina 20-30%, lime 5-10%, iron oxide 5-6% and magnesia below 1%.
3. Free from harmful substances: no lumps of lime, pebbles, iron pyrites, gravel, organic matter or excess alkalies.
4. Low shrinkage: should not warp, crack or shrink excessively on drying and burning.
5. Uniform fineness: fine-grained, with a balanced sand to clay ratio.
6. Binding power: strength after drying so that the green brick can be handled.
7. Good fusibility: it should fuse slightly (but not melt) at burning temperature to give a hard and strong brick.