clinkers

noun

1. 1.A very hard brick used for paving customarily made in the Netherlands.
2. 2.A mass of bricks fused together by intense heat.
3. 3.Slag or ash produced by intense heat in a furnace, kiln or boiler that forms a hard residue upon cooling.
4. 4.An intermediate product in the manufacture of Portland cement, obtained by sintering limestone and alumino-silicate materials such as clay into nodules in a cement kiln.
5. 5.Hardened volcanic lava.