We have seen that the closing up of the kiln at a particular point is effected by means of a register. These registers may be of paper or sheet-iron: they take the exact shape of the gallery. They are placed against the stacked bricks, and the chinks between the sides of the kiln and the register are stopped up with diluted clay in order to close it hermetically.
When the next compartment is full a second register is placed in position. This compartment is thus enclosed between two registers. But the first register is then removed and the raught is advanced one compartment.
Sheet-iron registers are in two parts which fit together by Means of a groove plastered with clay. The lower piece is easily moved; the upper one, which is hemispherical in shape, is fitted with hooks; by means of chains passing through the heating holes it is hoisted above the first, and the whole is kept in position by a buttress which is taken away when stacking takes place. The removal of the registers is somewhat difficult on account of the lack of space. The upper part is held by chains, while the lower part is drawn out by means of a handle fixed to it; the upper part is then lowered and also taken out. For this removal, an opening of the thickness of the register is left in the thin wall against the bricks. The register must not be removed without first closing the door, for the draught would be stopped and the progress of the fire suspended.
These sheet-iron registers have several disadvantages: they are slow and difficult to move, they cause a loss of space, for the hooks project, and a space must be left for the descent of the upper part.
There are many advantages to be gained by substituting registers made of strong paper for the above. Yellow straw paper in rolls 1.4 metres high are used; this height, which is the width of the paper, should be a little more than half the height of the gallery. The kiln is closed by cutting out a first rectangle of paper and placing it against the bricks; care is taken to open the trap of the compartment in order to cause draught and draw the paper against the brick. The second part of the register is made by cutting out a piece of paper in the shape of the arch; this is also placed against the brick. The edges of the paper are pasted with a little clay diluted very thin, and they are stuck against the walls of the kiln. The top is pasted with clay and the whole is thus completely attached.
When the register is to be destroyed, a long iron rod made red-hot in the fire is passed in by a heating hole and sets fire to it. The expense in paper is trifling and is made up for by the time saved in placing and removal of the sheet-iron registers, and also by economy in space, for the bricks of the next compartment are placed against the paper and help to keep it in position.
Any burning fuel serves for heating continuous kilns: such are wood, peat, and especially coal, which is generally used in the form of powder, called commercially "fine." This is very suitable for firing; being finely divided it spreads all round, burns quickly, and is cheapest. When the Hoffmann kilns were first introduced the economy was all the greater because this fine coal, which was not otherwise used, encumbered the mine-yards and was sold at a price very much less than that of ordinary coal. This difference in price, however, has much diminished since the installation in various commercial enterprises of furnaces capable of burning this slack.
Rich and poor coals are also very suitable for continuous kilns. Some prefer rich coals, on account of their long flame; others prefer poor coal, which heats more uniformly and without sudden outbursts.