Baryta-Water

Baryta-water has its uses, but cannot replace lime-water in fresco-painting. It is a solution of hydrate of baryta, barium hydrate, barium hydroxide, for these names all belong to the compound, in distilled water. The distilled water used should have been recently boiled and then cooled out of contact with the carbonic acid of the air. The barium hydrate used may be purchased in the form of colourless crystals having the formula Ba(OH)₂8H₂O. These, if not sufficiently pure, may be washed with cold distilled water, or recrystallized from boiling water, in which they dissolve very abundantly. A saturated cold solution is made by placing rather more than 1 ounce of these crystals in a bottle containing a pint of distilled water: the bottle should be almost full, the stopper should be smeared with a little vaseline. If the crystals dissolve completely, after repeated agitation, a few more should be added so as to leave a small excess at the bottom of the bottle. If the solution be clear it may be used directly from the bottle, as required; if filtration be needed, a glass plate should be placed on the funnel during the operation to prevent free access of air, and the clear filtrate should be received at once in the bottle in which it is to be preserved.

A solution of barium hydrate saturated at 15° C, contains nearly 2.9 grams of BaO in 100 cubic centimetres, or 2,023 grains per gallon. It is thus about seventeen times stronger than a solution of calcium hydrate saturated at the same temperature. Baryta-water, as it is called, is a powerfully alkaline liquid, becoming covered with a film of white barium carbonate on exposure to the air. By blowing air from the lungs through a glass tube into baryta-water, a dense white precipitate is formed.

Unfortunately, the binding power of barium carbonate is almost nil, so that baryta-water in mural painting is of service, not directly as a medium, but for destroying traces of calcium sulphate (gypsum) in the plaster-ground, and thus liberating a corresponding amount of lime-water. It may also be used for testing the effect of an alkaline earth on the powdered pigments which it is proposed to use in the work, in order to see if they can withstand its action; those unaffected by baryta will prove to be unchanged by lime.