The temperature of the liquor may vary between 140° F. and 212° F.; but within these limits, the lowest temperature consistent with the sufficiently rapid conversion and solution of the litharge is preferable. The reason of this is simply that the less the heat employed, the less is the tension of the ammonia, and the chances of the loss of ammonia are thus minimised; in addition to this, less delay is involved and less refrigeration or cooling is needed before carbonating. The object of the violent agitation of the litharge amongst the heated acetate of ammonium in the digestor _a_, is that caking of the former may be prevented, and a most rapid conversion of the oxide of lead into basic acetate of lead, with minimum expenditure of heat, be secured. The effect of the heat in the closed space is greatly aided by that of the pressure due to the tension of the ammonia and aqueous vapour at the increasing temperatures. When the litharge is converted into tribasic acetate of lead, and brought into solution, the liquor and sludge of insoluble matters is preferably pumped through pipe _k_, by means of the filter-press pump _l_, and forced through the filter-press _m_, which removes and retains the insoluble matter, allowing the clear liquor to pass to the cooler _n_, and through this to the carbonator _o_, similar in construction to the digestor _a_. Here the cooled liquid is circulated by means of pump _p_ (in a similar manner to that adopted in the digestor process already described), carbonic acid being simultaneously pumped in through the pipe _q_, which is perforated as shown, or introduced in any other way. When carbonated to the desired extent, the white magma, consisting of basic carbonate of lead and mother liquor, is drawn through the pipe _r_, by the pump _s_, into the filter press _t_. The clear liquid flows through _u_, into the covered mother liquor tank _v_, whilst the press-cakes of white lead, after sufficient washing with water, are removed and dried in a suitable manner. The washings are run off to a weak liquor tank (not shown) for concentration for use over again. Any inert gases accompanying the carbonic acid, or the latter alone in excess, pass from the carbonator through pipe _w_, into the catch-box or other condensing and absorbing apparatus _x_, containing either dilute acetic acid or cold water, in order to retain any ammonia carried over, and furnished with perforated trays or baffle-plates.