Fig. 25.
Red lead is made in two ways—directly from metallic lead or by heating easily decomposed lead salts. When it is made from the metal, the following is the process: the lead, which must be very pure, is melted in a reverberatory or calcining furnace, oxidised to massicot by the air passing over it, and the massicot then, by careful heating, changed into red lead, a process in which particular care must be taken that the mass is not melted. By continued heating the massicot absorbs about 2 per cent. of oxygen, and changes in colour to a bright red.
The art in making red lead by this process lies in maintaining the proper temperature; the furnaces are constructed so that the temperature may be regulated during the heating. A reverberatory furnace is used, in which is a stirring apparatus, so that the heated mass may be continually turned over to accelerate the oxidation. Muffle furnaces are also used, in which the massicot is placed in crucibles on an iron plate, which can be pulled out of the furnace for observation of the change of colour. Whatever method is used, the temperature must be so regulated that over-heating of the material is avoided, otherwise litharge is formed, which is only very slowly converted into red lead.
Fig. 26.
Mercier states that the muffle furnaces are arranged as shown in [Fig. 25]. The muffle, a, is 2·5 metres long and 2 metres wide; its bottom rests on an iron plate. The passage, d, running under the muffle is 20 centimetres high; it is divided by a partition, and at each end are two hearths, c, 70 centimetres long and wide. The products of combustion pass from the long passages into side channels, f, provided with dampers, go round the muffle and unite in the space, g. The flue, k, at the back of the muffle is provided with dampers, m, which exactly regulate the current of air through the muffle; n is the chamber in which are collected the particles of oxide carried over by the draught. The furnaces used for manual labour consist, according to Percy (Figs. [26] and [27]) of a rectangular or circular hearth, a, of about 3 metres diameter, which is deeper in the middle and has two fireplaces, b. The low arch surmounting the hearth is covered with sand in order to prevent cooling.
Fig. 27.
When finished, the red lead is drawn out of the furnace and finely ground under edge-runners, or occasionally levigated. The temperature necessary in making red lead is that at which the angles of the muffles, when these are used, begin to show a dark red glow.
Orange lead, which is a brighter variety of red lead, is prepared from lead salts; white lead or lead nitrite is used for this purpose. The latter salt is made by the process of Pischon, by heating 1 equivalent of lead nitrate with 4 equivalents of granulated lead and water at a temperature between 50° and 60° C. After about 2 hours, the lead nitrite separates in the form of a granular yellow mass. According to Burton’s process, lead carbonate is oxidised by heating with 20 per cent. of sodium nitrate and extracting the mass with water. There are also other methods by which red lead is obtained from litharge by the use of potassium chlorate or saltpetre; but these methods, without producing a finer product than those previously given, are more expensive, and consequently have found no application on the large scale.