An upper series of small tuyeres or jet pipes _l_ is provided for admitting air a little below the lowest diaphragm _g_, these jet pipes _l_ being supplied by a branch pipe _m_ from the main air blast pipe _n_, it being found that the introduction of the air and the production of heat can be best regulated by supplying a portion at the upper part of the furnace in this way, in addition to that supplied by the main tuyeres _d_; and a tap or valve _o_ is fitted on the branch pipe _m_ for adjusting the supplementary supply thus admitted. Above the highest perforated diaphragm _h_, the interior of the furnace communicates with a lateral flue _p_, through which the gases and sublimate pass; and at a part of this flue _p_, sufficiently near the surface for the gases to be still hot enough, an enlargement or chamber _r_ forming a descending part of the flue, is constructed. Into this chamber _r_ a solution of chloride of sodium is introduced as a spray from a number of jet pipes _s_, for the purpose hereinbefore explained. This chamber _r_ is provided with a door _t_ at its lower part, for periodically removing matters that become deposited in it. The continuation _u_ of the flue communicates with the lower part of the chamber _r_. Preferably the spray of chloride of sodium is fine enough to allow of all or nearly all of the water being instantly evaporated, so as to leave the salt in fine particles, and in a favourable condition for being acted upon by the sulphurous acid, steam, and oxygen present in the gaseous currents. The temperature of the chamber _r_ should not be allowed to fall below red heat. The proportion of chloride of sodium used will depend on the amount of chlorination of the lead that may be desired, in addition to what is necessary for converting the zinc and other metals into chlorides. In practice a proportion of salt 2½ to 5 per cent. of the weight of the sublimate formed answers the purpose and yields a good product, but a larger proportion may be used without injury.