Many different forms of kiln are used for burning Portland cement.
At the bottom of the kiln is a grate of iron bars, and on this wood and coke are piled to start the fire.
This is employed in the shape of lime-kiln gases, obtained in a comparatively pure and strong form (up to 33% CO 2), in very large kilns, charged with limestone and coke.
The manufactured goods are either removed gradually from a constant source of heat by means of a train of small iron trucks drawn along a tramway by an endless chain, or are placed in a heated kiln in which the fire is allowed gradually to die out.
Before the glass is introduced, the annealing kiln is heated to dull red by means of coal fires in grates which are provided at the ends or sides of the kiln for that purpose.