The products of combustion from the side fire-places, having heated the hearth and its contents, pass through an aperture in front of the charging door of the hearth, and thence go to furnish heat to an upper hearth where the conversion of the oxide into red lead takes place.
A period of about 12 hours is occupied in the oxidation of a charge, which is repeatedly “rabbled.” Even then a considerable amount of the metallic lead remains unoxidised and is returned to the calciner with the next charge. Half the oxide is utilised for making white lead, as described in a later chapter, and the other half is converted into red lead by the method detailed hereunder.
The crude oxide is pulverised in a small mill and separated from the unconverted metal. The mill takes the form of a flat circular cast-iron plate on which rotates a cast-iron muller. Water and an agitating arrangement are also provided.
As the muller revolves the material undergoes comminution, and the small particles of oxide as formed are disturbed by the agitator and kept in suspension in the water, by the overflow of which they are continuously carried away into settling pits. The residual metallic lead is not pulverised, and of course never becomes suspended in the water, consequently it accumulates at the bottom of the mill, whence it is occasionally withdrawn for re-calcination.
Sufficient oxide having collected in the settling pits, it is transferred to a shallow pan heated by the waste heat from the furnaces and is there rendered almost dry. In this state it is put into small square dishes made of sheet iron, and adapted to hold about 30 lb. each.
A charge consists of a hundred of these dishes, which are placed in the heated furnaces at the end of each day. The roasting is repeated several times, and the product is accordingly known as “two fires,” “three fires,” &c. The material at this stage is lumpy and coarse, and has to undergo dry pulverisation, the fine particles as they are produced being drawn off by means of a pneumatic fan, and collected.
(2) What may in contradistinction be called the English method of making minium does not differ materially from the preceding. The “drossing” furnace, where the metallic lead is first oxidised, receives a smaller charge as a rule, and perhaps greater care is given to the rabbling, and to the regulation of the temperature so that it is only just above the melting point of the metallic lead, and not sufficient to fuse the massicot.
Minium or red lead is one of the most important and useful red pigments, as it mixes well with oil, has good covering power, dries quickly, and is permanent except in presence of sulphur or sulphides.
Orange Mineral.—The pigment known as orange mineral or orange lead is simply minium which has been imperfectly calcined. Consequently it is almost identical with red lead in composition, qualities, and method of manufacture, the only exception being that, as the calcination is not carried quite so far, therefore the colour is not so fully developed, and is an orange rather than a red. As with minium, practically the only adulterant is iron oxide red, which may be detected by boiling the pigment to a colourless solution with nitric acid, when addition of prussiate of potash will give a blue precipitate.
Oxide Reds.—Under various names—such as Persian red, light red, Indian red, scarlet red, rouge, colcothar, red oxide, purple oxide, &c.—many pigments, of which the base is the ferric oxide Fe2O3, are now made. These vary in shade from a deep scarlet red to a dark violet. They are obtained both from natural and artificial sources. Oxide of iron occurs naturally as the mineral hematite, and some varieties of this are bright enough and soft enough to be used as pigment when ground up. These are usually nearly pure oxide of iron. Then the ochres, when calcined, yield red pigments known as light red, Indian red, &c., and a good many reds are obtained from this source. The composition of these is variable, being dependent upon that of the ochres from which they are made, and these, as has already been pointed out, vary very much. Then, in preparing fuming sulphuric acid from copperas, oxide of iron which is specially sold as rouge, is obtained. Colcothar is produced as a residue; this is nearly pure oxide of iron, and usually has a red colour. In the manufacture of sulphuric acid from pyrites, a dark violet oxide of iron is left as a residue, and much of this is used as a pigment under the name of purple oxide. Then a large quantity of oxide of iron reds are made artificially from waste liquors obtained in copper refining, galvanising iron, &c. The composition of the oxide of iron reds, therefore, is very variable.