A treatise on chemistry / by H.E. Roscoe and C. Schorlemmer.

  • Henry Enfield Roscoe
Date:
1883
Catalogue details

Licence: Public Domain Mark

Credit: A treatise on chemistry / by H.E. Roscoe and C. Schorlemmer. Source: Wellcome Collection.

    502/520 (page 490)
    metal. The glass thus prepared is much less brittle, and better able to withstand sudden changes of temperature than ordinary glass. Table-glass thus toughened is not readily broken when thrown on to the ground. If, however, it be thrown from a great height or with great force, it breaks into fine splinters. Although a crack may occur in a vessel of tough- ened glass without the whole of the mass breaking up, yet it cannot be cut with a diamond; from this it would appear that the molecules of toughened glass are in a molecular condi- tion somewhat similar to those of the Rupert’s drops. 299 Etching of Glass. The etching of glass with hydrofluoric acid was known in the 17th century, but it is only recently that this process has been employed for the decoration of glass, and now etched glass is much used in place of the more ex- pensive cut-glass. Gaseous hydrofluoric acid as well as many soluble fluorides produce an opaque surface 011 glass, but the surface of glass etched with liquid hydrofluoric acid presents, according to the concentration of the acid, a more or less trans- parent character. According to Tessie de Mothay and Marechal, the best mode of obtaining opaque- or ground-glass by means of hydrofluoric acid is to employ a bath made of 1000 parts of wrater, 250 parts of crystallized double fluoride of hydrogen and potassium, 250 parts of commercial hydrochloric acid, and 140 parts of potassium sulphate. This last salt renders the fluorides of calcium and lead which may be formed less soluble, whereby they are separated out in the crystalline form on the portions of the glass which have been etched and thus tend to render the surface more opaque. POTTERY, PORCELAIN, AND EARTHENWARE. 300 The potter’s art is one of the oldest in existence. The Egyptians were well acquainted with the means of glazing ware made of burnt clay, as well aa with the art of enamelling in colours. Amongst European nations the Etruscans were espe- cially celebrated in early times for their proficiency in the manufacture of painted pottery-ware, and in Pliny's time seve- ral towns in Greece and Italy were well known for the beauty and artistic taste of their vases, urns, and other pottery wares.