white lead will be made up of one part of hydrate and two parts of carbonate of lead.
The second compound roughly estimated
Pb H2O2,3Pb CO3
will be one part of hydrate, combined with three parts of carbonate or lead. The latter will be in the proportion of 75 per cent, of carbonate and 25 per cent, of hydrate of lead, and this represents the composition which has been assigned to good white lead by those most acquainted with the subject. The amount of hydrate contained in white lead should never exceed the proportion above named of 25 per cent., nor should its amount be much below the 25 per cent.
The hydrate contained in the substance serves to unite with the oil in the paint; it forms therewith a drying white and elastic varnish which embraces and holds the particles of white carbonate and prevents their subsidence and separation in the paint. There is a chemical action of a much more intimate character between the components of good white lead when mixed with oil which neither of the constituents of this compound can alone produce.
For instance, hydrate of lead and linseed oil produce a varnish-like substance, semi-transparent and of no covering capability.
Carbonate of lead and linseed oil produce a compound which is opaque, but has no body or covering power, and in which the white solid carbonate is held in feeble mechanical suspension.
Neither of them constitutes a paint, but when together as white lead they are mixed with oil, combination takes place, and serviceable paint of good body and covering power and enduring quality is produced. Good white lead is a dense, perfectly amorphous powder of perfect whiteness, possessed of great body and covering power when combined with oil. When mixed with linseed oil and used as paint it rapidly dries in the air and assumes a varnish-like, glossy, hard surface, and is capable when once dry of resisting the action of air and water for any length of time. It does not weep when laid on a surface with a brush, that is, the oil does not separate from the solid material of the paint.
Attempts have been made to produce white lead quickly and cheaply by precipitating processes, but in all such methods the resulting compound is deficient in certain special qualities absolutely necessary to white lead proper and to its uses. The precipitated white lead is always of a crystalline structure, and crystalline lead can never furnish a good body paint—no amount of pulverising and grinding of this crystalline material will correct this defect in its nature, and deprive it of its crystalline form.
“Once a crystal always a crystal” has an especial application to this point of our philosophy. Pulverising a crystal will not alter its structure, but simply reduces the size of the crystals. Crystals of white lead are unable to effect the necessary combination with the oil and form the true varnish which white amorphous lead so readily produces. Paint made with the precipitated white lead lacks body and covering power, and this because of the absence of this chemical union with the oil.