Of these there are two chief forms, the first being a tank or pan formed
of large pieces of slate, with the joints made with clay, and surrounded
with a mud wall. The whole is covered with an arch or vault and is
filled with the brine, which is then evaporated by surface heat, the
fire being placed at one end and the flue at the other.

The other form is very curious and interesting, and is almost identical
in its principle of construction with the pan I have referred to as
figured in Agricola, only in this case the materials are very different,
being, instead of wood and iron, nothing more than clay or mud.

It was described officially by the Japanese, in their publications at
the Philadelphia Exhibition in 1876. The Japanese description of this
apparatus is highly interesting. It is as follows:--

A low wall is built, enclosing a space of about 13 feet by 9 feet, the
bottom forming a kind of prismatical depression, 3 feet deep in the
centre line. An ashpit, 3 feet deep, is then excavated, starting from
the front wall, and extending about 4 feet into this depression at its
deepest place; it communicates with the outside by a channel sloping
gradually upwards, and passing underneath the front wall. The ashpit is
covered by a clay vault, with holes in its sides, so as to establish a
communication between the ashpit and the hollow space under the pan.
This vault is used as a fire grate, the fuel (brown coal and small wood)
being inserted by the fire-door in the front wall. The air-draught
necessary for burning the fuel enters partly by the fire-door, partly
through the ashpit and the openings left in the vaulted grate. Through
these same openings the ashes and cinders are from time to time pushed
down into the ashpit, for which purpose small openings are left in the
side-wall of the furnace, through which the rakes may be introduced. A