For Brianehon's lustre, whieh is sirnilar to that used at Belleek and Worcester, a mixture is used of bismuth nitrate, resin, and essence of lavender. If ferric or uranic nitrate be added to this mixture, the glaze of the ware wilt be tinted by the ft rric or uranic oxide, and the °filet of the lustre will be heightened. Instead of applying the reducing agent together with the metallic salt, a reducing vapour may be directed upon a pigment rieh in copper, silver, or other rra tals, with similar results. The rod lustre of Gubbio ware is due to the action of smoke upon cupreous oxide; it is usually epplied to a coloured body. Gilding and silveriog are perforrned by fixing metallic gold and platinum upon glazed ware by partially fusing the glaze in smell muffles or kilns adapted to the purpose. The metals tnay be used in leaf, as amalgams, in powder, or as pre cipitates from solutions. When used a.3 an amalgam or in powder, a small quantity of flux is added.
Gold may be precipitated from solution by ferrous sulphate ; the precipitate, after washing and drying, is mixed with bismuth oxide, and rnixed as a pigment with thickened oil of turpentine. Platinum sponge may be sitnilarly treated. A bright silver may be obtained by using as a pigment a mixture of platinie chloride and essence of lavender. The gold powder and amalgams require prolonged and careful grinding before they are fitted for use as pigments.
The metal, after firing, is generally dull, and if a bright gold is needed, it rnuat be burnished by rubbing over with an agate or bloodstone. " Chasing" is marking with a burnisher a bright pattern upon a dull unburnished ground. The gold pigment may be applied over raised bosses of paste, or over depressed patterns eaten into the glaze by hydrofluoric acid. Patterns rnay be painted with the brush, and lines may be accurately described upon the edge or sides of ware by applying the brush, and causing the vessel to turn upon the operator's hand, or upon a hori zontally revolving wheel. Patterns may also be stencilled on the ware with charcoal, and gilded over.
By mixing gold with silver in various proportions, and using the mixtures as pigments, a large number of tinted golds and bronzes may be obtained. Coloured decoration npon the surface of ware is produced by metallic oxides dissolved in or covered by the glaze, or by the application of opaque coloured glasses. Metallic oxides applied on the body of ware and under the glaze, as well as those mixed with the glaze, obtain the glass necessary for their development from the glaze. Metallic oxides applied on the glaze are used in the form of coloured glasses or enamels. The latter may always be detected by the touch, as they are raised above the level of the glaze. Colours produced by certain oxides are developed by special media, and certain colours are able to with stand a much higher temperature than others. To the latter class, belong the oxides of cobalt,
chromitun, iron, titanium, and uranium ; these are adapted to English underglaze decoration ; but the blue produced by cobaltic oxide is the only colour able to resist the intense heat required to fuse the glaze of Oriental and Continental hard porcelains.
Zincic oxide tends to brighten a large number of colours ; others cannot be developed without the addition of stannic or sodic oxides; aod others, especially the pinks produced by gold, deteri, orate in the presence of plumbic oxide. The subjoined oxides when mixed with a glass give the following results :—Ferric oxide, yellow ; ferrous oxide, green ; manganic oxide with sodic oxide, violet, passing to grey and black ; chromic oxide, yellow ; a trace of chromic oxide with stannic oxide, pink ; pink, crimson, brown, black, and green are obtained for underglaze printing colours from various combinations of chromic oxides; all kinds of overglaze greens, and some yellows, are due to the same oxide ; cobaltie oxide, blue, deepening to black, and brightened by ziocie oxide, but injumd by the presence of manganese or nickel as impurities ; cupric oxide with sodic oxide, turquoise-blue, passing to green ; cupreous oxide, red ; auric oxide with stannic oxide, pink to purple; uranic oxide, yellow to orange ; titanic oxide, yellow ; antimonic oxide, yellow ; plumbic oxide, pale-yellow ; iridic oxide, black or opacity. All intermediate tints may be obtained by mixing the oxides.
Certain colours cannot withstand a high temperature, and other colours vary in tint with variations of temperature. Upon this fact, pyrometrical tests for burning-in coloured glasses and enamels are based. The heat of a muffle is ascertained by withdrawing from time to time a piece of china or porcelain marked with a glaze containing auric and stannic oxides. As the temperature rises, the tint changes from brown to brick-red, from brick-red to rose, from rase to purple, from purple to violet, from violet to pale-rose, and from pale-rose to a colourless stain. Wedgwood's pyrometer, which has been sometimes used for the same purpose, is based upon the regular con traction under the influence of heat of an uuhumt clay mixture of known composition. Small cylinders of this clay mixture are expressed through a gauge, and after being dried, but before being placed in the kiln or muffle, are tested in a gradually tapering groove cut in copper or gun metal. They are then placed in the muffle, and withdrawn when the temperature is to be deter mined. The temperature can be roughly estimated according to the position in the groove to which the contracted cylinder can be advanced, by reference to a scale determined by previous experiment.