This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
White-lead is a form of lead carbonate. The best kind is produced by the Dutch process, which consists in placing gratings of pure lead in tan, and exposing them to the fumes of acetic acid; by these they are corroded, and covered with a crust of carbonate, which is removed and ground to a fine powder. There are other processes for manufacturing white-lead, in which it is precipitated by passing carbonic acid through solutions of different salts of lead. "Clichy white" is produced in this way by the action of carbonic acid gas upon lead acetate. The white-lead produced by precipitation is generally considered inferior to that prepared by corrosion. It is wanting in density or body, and absorbs more oil, but does not require grinding. Pure white-lead is a heavy powder, white when first made; if exposed to the air it soon becomes grey by the action of sulphuretted hydrogen. It is insoluble in water, effervesces with dilute hydrochloric acid, dissolving when heated, and is easily soluble in dilute nitric acid. Heated on a slip of glass it becomes yellow. It may be used as the basis of paints of all colours.
It is often sold mixed with various substances - such as baryta sulphate, lead sulphate, lime sulphate, whiting, chalk, zinc-white. These do not combine with oil so well as white-lead, nor do they so well protect any surface to which they are applied. Baryta sulphate, the most common adulterant, is a dense, heavy, white substance, very like white-lead in appearance. It absorbs very little oil, and may frequently be detected by the gritty feeling it produces when the paint is rubbed between the finger and thumb. White-lead is sold either dry in powder or lump, or ground in oil in a paste containing 7-9 per cent. of linseed oil, and more or less adulterated. When baryta sulphate has been added, its presence is in most cases avowed; the mixture is called by a particular name, which indicates to the initiated the proportion of sulphate of baryta that it contains. "Newcastle White," "Nottingham White," "Kremnitz" or "Krems White" (known also as "Vienna White," imported from Austria in small tubes), "French" or "Silver White" (in drops, from Paris), and "Flake White" (made in England in small scales), should all be pure white-lead, but they differ considerably in density. "Venice White" contains 1 part white-lead to 1 part baryta sulphate; "Hamburg White" contains 1 part to 2; "Dutch" or "Holland White" contains 1 part to 3. When the baryta sulphate is very white, like that of the Tyrol, these mixtures are considered preferable for certain kinds of painting, as the barytes communicates opacity to the colour, and protects the lead from being speedily darkened by sulphurous smoke or vapours.
White-lead improves by keeping. It should not be exposed to the air, or it will turn grey. Old white-lead of good quality goes farther and lasts better than if it is used when fresh; moreover, the paint made with fresh lead has a tendency to become yellow, and the fresh white-lead itself often has a yellowish tinge, from the presence of iron. Of all the bases for paints, white-lead is the most commonly used; for wood surfaces it affords in most cases the best protection, being dense, of good body, and permanent. It has the disadvantage, however, of blackening when exposed to sulphur acids, and of being injurious to those who handle it. Testing its quality is a very simple operation. In the case of dry white-lead, digest it with nitric acid, in which it dissolves readily on boiling. When ground with oil, the oil should be burnt off, and the residue treated with nitric acid; or the whole may be boiled for some little time with strong nitric acid, which destroys the oil and dissolves the lead on the addition of water.
Baryta sulphate being insoluble in the acid remains behind, and can be collected on a filter, washed with hot distilled water, and weighed.
Red-lead or minium is produced by raising "massicot" (lead oxide) to a high temperature, short of fusion, during which it absorbs oxygen from the air, and is further oxidized. It is usually in the form of a bright red powder. The colour is lasting, and unaffected by light when the article is pure and used alone; but any preparation containing lead, or acids mixed with it, deprive it of colour, and impure air makes it black.
Bed-lead is used as a drier; also for painting iron; and in the priming coat for painting wood. It is sometimes adulterated with brick-dust, which may be detected by heating in a crucible, and treating with dilute nitric acid; the lead will be dissolved, but the brick-dust will remain. It may also be adulterated with "colcothar." As a substitute for it, antimony sulphide ("antimony vermilion") has been proposed. It is sold in a very fine powder, without taste or smell. It is insoluble in water, alcohol, or essential oils, is but little acted upon by acids, and is stated to be unaffected by air or light. It is adapted for mixing with white-lead, and affords an intensely bright colour when ground in oil.
Zinc oxide, the basis of ordinary zinc paint, is prepared by distilling metallic zinc in retorts, under a current of air; the metal is volatilized, and white oxide is condensed. This is filled into canvas bags, and pressed to increase its density. It is durable in water and oil, dissolves in hydrochloric acid, does not blacken in the presence of sulphuretted hydrogen, and it is not injurious to the men who make it, or to the painters who use it. On the other hand, it does not combine so well with oil, is wanting in body and covering power, and is difficult to work. The want of density is a great drawback to its use, and the purest quality is not always the best for paint on account of its low specific gravity; in this respect the American zinc whites, which are frequently very pure, do not generally compete with the zinc-white supplied by the Vieille Montagne Company, in Belgium. Zinc oxy-sulphide is used as the basis of Griffith's patent white paint, stated by Dr. Phipson to be prepared by precipitating zinc chloride or sulphide by means of a soluble sulphide - of sodium, barium, or calcium.
The precipitate is dried, and levigated, while hot, in cold water.
Iron oxide is produced from a brown haematite ore found at Torbay in Devonshire, and forms the basis of a large class of paints of some importance. The ore is roasted, separated from impurities, and ground. Tints, varying from yellowish brown to black, may be obtained by altering the temperature and other conditions under which it is roasted.