GAS RFTORTS.
Fire-clay gas retorts have almost entirely replaced the old form of iron retorts, as they possess when properly made the ex cellent advantage of neither expanding nor contracting upon being heated and cooled.
In Figs. 13o to 135 are illustrated the various forms of gas re torts in common use.
The manufacture of clay retorts for making gas is now car ried on largely.
The clay body is compounded to stand considerable heat without any tendency to soften, for so large a piece of hollow ware must be refractory to maintain even its own weight at the temperature used. The mixture requires more care in com pounding than a brick mixture, for more is involved in the failure of a retort to do good service than a few brick. Calcine is used in large amount, but crushed rather finer than common, and it needs a very good and plastic bond clay.
The retort is shaped from the tempered clay by filling the space between a large sheet-iron shell and a wooden core. The
shell is placed in position and the floor covered with clay four inches deep and tamped. The core is then introduced and adjusted so that four inches separate it from the walls on all sides. The clay is filled in small amounts at a time and tamped gently. When the retort is high enough, the core is withdrawn by a crane and the mouth of the retort built by hand. The shell is then unbolted and removed in two pieces, and the finished retort is standing on its end. It is left to dry for several weeks in this position, and is finally removed to the kiln to be burnt. It is put into an ordinary fire-brick kiln and brick are piled around it to keep it in position without sagging, and when in steady use, and never allowed to cool, they prove very durable.