Lead white brightens...

Lead white - a white mineral paint based on lead. Previously, for writing pictures and icons, leaded white was used. Looking at the picture darkened by time, painted in oil paints, the owner sighs sorrowfully. Meanwhile, if you know the secrets of chemistry, this grief is easy to help. Moisten cotton wool in a 10-15 per cent solution of hydrogen peroxide and wipe the picture. That's a miracle - the picture becomes lighter, the colors look quite fresh. What happened?

Lead white - a white mineral paint based on lead. Previously, for writing pictures and icons, leaded white was used

The composition of lead white is the main carbonate lead 2PbCO3*Pb(OH)2. In our homes, in exhibition halls, as a result of the activity of living microorganisms, there is always a small amount of hydrogen sulphide in the air. Hydrogen sulphide (H2S) interacts with the basic carbonate lead, and lead sulfide (PbS) - a substance of black color is obtained. That's why the picture, painted with lead white, darkens over time. When sulfur dioxide is treated with hydrogen peroxide, it turns into lead sulphate (PbSO4) - white matter: the picture is refreshed - it becomes lighter. Lead compounds are poisonous, so now lead white is replaced with other colors.

The sold lead white had not all the same white color and the same covering ability. The latter, apparently, is dependent on the quantitative content of lead oxide hydrate, and with increasing its content (up to a certain limit), the covering capacity also increases. Lead white was very often falsified (for example, chalk, gypsum) and mixed with other whites, for example, barite whites, lead sulfate, barium carbonate and others. Mixtures of lead white with heavy spar (barium sulfate) at 50% of the latter are called in the trade by Venetian whites, with a content of 66% - Hamburg, and at 75% - Dutch whites.

People received lead white in ancient times. The Dutch way of cooking is white (the oldest) is as follows. Thin lead sheets, coiled into a spiral, are placed in clay, pots covered with glaze, on the bottom of which a little acetic acid is poured. Pots are closed with lead plates, placed several floors one on the other and buried in horse dung. The carbon dioxide released from decay of manure together with acetic acid causes the transition of metallic lead to the basic carbonate salt, as a result of which a more or less thick white plaque forms on the surface of the lead. This scum is scraped, dried, crushed and wrung out. The German way of preparing whitewash has much in common with the Dutch, with the difference that instead of pots, chambers are used where lead sheets are hung, and where vapors of acetic acid are first introduced, followed by carbon dioxide.

In the English method, lead is converted first into a glue that is mixed in a homogeneous dough with a 1% solution of lead acetate and through this way the prepared mass, which is pulled through closed chutes, is passed through a jet of carbon dioxide. The French way of obtaining lead white is that the jet of carbon dioxide is sent to the solution of the basic lead acetate obtained by dissolving the glue in dilute acetic acid; the precipitation is white, the latter are separated from the solution and washed thoroughly. Since this method completely eliminates the harmful operations in the hygienic operation of grinding and soaking, it presents undoubted advantages.