46
which it has been added ; but the truth is that thiscolour is produced only in peculiar circumstances.The manufacturers of china, porcelain, and earthen-ware are well aware that oxide of iron is the colour-ing material of a fine purplish-red enamel; if thetemperature is raised too high, this enamel tendstowards orange.
If into a pot containing white melted flint-glass asmall piece of iron is introduced during,the blowing,it will from its gravity fall to the bottom, and thewhole of the glass in its neighbourhood will becoloured from orange to yellow. But when glassis melted at a high temperature, as, for example,in making window-glass, the addition of a smallquantity of ferric oxide produces a bluish colour.It is also known by the manufacturers of bottle-glass, that when cooled in the pot it becomes ojiaqueblue before being devitrified.
Thus glass may receive all the colours of thespectrum from oxide of iron ; and, moreover, it maybe observed that these colours are produced in theirnatural order as the temperature is increased.
Manganese gives to glass a purple or pink colour ;but if. the glass remains too long in the meltingfurnace, this colour becomes brownish-red, thenyellow, and afterwards green.
Copper, in its highest state of oxidation, gives toglass quite free from iron a sky-blue colour, and inits lowest state of oxidation a ruby colour.
At all times, as at the present day, red window-glass has always been coloured by protoxide ofcopper ;. but it is not very easy to obtain tills colour,because it is not at all fixed: it must be seized atthe proper time.
When the glass is in the proper state to be blown,if it is ladled into water, yellow-green cullet is pro-duced ; if this cullet is heated to the point of lique-faction and cooled slowly, the red colour willgradually show itself as the glass cools, becoming ofthe finest ruby, inclining more to orange than topurple. In some cases the cooling in the usual pro-cess of manufacture does not produce this colour,and it is necessary to expose the glass to the tem-perature of the annealing kiln, in which case the redcolour is seen to increase gradually till it arrives atits greatest intensity. If this ruby glass is over-fired it changes, first to crimson-red, then to purple,by a greater degree it takes a bluish tinge, andafterwards gets discoloured.
Flashing and Casing. —This process consists incoating a layer of colourless with one of colouredglass, which can then be reduced by grinding to theproper tint. For this purpose the glass blowercollects the proper quantity of colourless glass onthe end of his pipe, rolls it upon the marver, andthen dips it for a moment into a pot of meltedcoloured glass, and blows out the two together intoa cylinder or globe, which is flatted or flashed out inthe usual way. In this manner panes for the glass-painter are formed, which consist of two layers ofglass, one coloured, the other colourless ; and theformer can be ground down to any required tint ordegree of transparency.
Flint-crystal glass is usually coloured by a some-what similar process termed casing. Presuming thatany two or more glasses intended for casing havebeen mixed of the same specific gravity, to give themthe capability of harmonizing—that is, of contractingand expanding equally—the blower has to gather aball of solid glass, intended for the inner layer, inthe usual manner (Fig. 47). About the same time
Fig. 47.
his assistant prepares a casing of coloured glass, n,knocking off the knob to open and shape it some-what like the bowl of a wine-glass, or the broad endof a large egg-shell; this is set into a metal stand, E,on the floor, merely to steady the case or shell, D;while the blower takes the lump of flint or whiteglass, A, and gently blows it into the coloured caseor shell, d', to which it immediately adheres; andwhen submitted to the flame of a pot-hole, it is foundto weld perfectly, as F.
Glass-Painting. —Painting on glass is performedby means of a vitreous mixture termed the flux,combined with a pigment prepared from some me-tallic oxide. The flux and oxide are simply thatcombination of ingredients which is necessary toproduce a highly fusible coloured glass of therequired hue ; this mixture is reduced to a state offine powder, rubbed up in oil of turpentine, boiledoil, or sometimes simply with water, and laid uponthe glass to be painted by means of a brush. Theglass thus painted or stained with the intendeddesign is then exposed to a heat sufficient tovitrify the mixture without melting the glass; inother words, the painting is said to be burnt in; theingredients of which it is composed are convertedinto a coloured glass or transparent picture, whilethe pane or other article on which it is laid is onlysufficiently softened to cause the complete adherenceof the coloured glass to its surface.
It is evident, therefore, that the fusing point ofthe pigment must be much lower than that of theglass to be painted. Indeed, it is necessary thatthe former should vitrify at a temperature at whichthe latter does not sensibly soften; for any consider-able softening or yielding of the glass would distortthe design. Hence the manifest inapplicability ofcrystal or lead-glass for painted articles, on accountof its great fusibility. On the other hand, commonwindow-glass, or even plate-glass, may be success-fully used, and, best of all, the hard Bohemian glass,which, contains potash for a base.
When the colouring oxides employed are such thatthey are not injuriously affected by the oxides of lead