Fig. 99 is the section, and Fig. 100 an elevation, of a form of kiln commonly used for burning Portland cement in the Medway district.

Fig. 99. Section.

Fig. 99. Section.

Fig. 100. Elevation.

Fig. 100. Elevation.

It is worked upon the intermittent system; the coke and slurry are, however, packed in alternate layers 6 and 4 inches deep.

Such kilns hold about thirty tons. Their contents are burnt in forty-eight hours, and are drawn about once every four days.

The kiln is lined with firebrick, sometimes only up to the line a, but better throughout, and loaded at the holes HHH.

The firebrick lining should be detached from the mass of the brickwork, so as to be free to expand and contract under the great changes of temperature to which it is subjected.

The inside should be painted over with wet stuff from the bach each time before the kiln is charged. This will greatly increase its durability.

In some forms of this kiln the top has a wider opening, and the short vertical neck or chimney is frequently omitted.

The description of kiln used varies in different places. A modification of Hoffmann's kiln similar to that used for bricks may be economically adopted where a large continuous supply of cement is required year after year.

The time required to burn a kiln varies according to the proportion of the materials, the position of the kiln, etc.

The contents are burnt at a high temperature, but the amount of firing depends upon the proportions of the mixture. If the lime be in excess it can hardly be overburnt, but if there be too much clay it will fall into dust.

The Michele-Johnson Kiln is a modification of the kiln mentioned at p. 161.

The arched chamber there referred to as branching out from the kiln has a very thin arch over it. Above this the cool slurry is spread for a preliminary drying before it is forced through the openings of the arch and spread over the floor of the chamber; there it is further dried by the hot air and gases from the kiln, which pass through the chamber on their way to the chimney.

Roman Cement Kilns

Fig. 101 is a plan and Fig. 102 a cross section of the kiln used for burning Roman cement. It is worked on the constant system. The stone is packed in strata separated at intervals of from 6 to 9 inches by thin layers of fuel. The cone in the centre guides the burnt cement to the drawholes D D where it is taken out.

Fig. 101. Plan.

Fig. 101. Plan.

Fig. 102. Section.

Fig. 102. Section.