This is in two parts: the firing of the paste and the firing of the decoration; sometimes even there is an intermediate firing for the glaze or dip, which is placed upon the biscuit and fixed before decoration.
Each manufacturer has his favourite kiln, so to speak, for firing the pastes; many direct-flame kilns are still to be found, although reverberatory kilns are increasing in popularity. The stacking is done in many ways, and most frequently without any precaution, so that the quarries receive the indirect action of the flame. Thus in the case of faiences with stanniferous enamels, they are arranged in parallel rows and the quarries are separated from one another by little pieces of refractory clay with angular and pointed surfaces called "spurs".
The consumption of fuel depends upon the nature of the objects fired; the aim of the manufacturer will be to make the best possible use of his kiln by means of a judicous stacking of the products. On account of the numerous hollows, the expense of fuel varies very little whatever the weight of the stacked products may be, and is about 80 to 100 kilog. per cubic metre.
The temperature of firing depends upon the composition of the pastes, and rises to about 1000° to 1200° C.
The firing of the decoration depends upon the process and the nature of the substances used. The stanniferous enamels used for the common quarries of Ponchon or Desvres are stacked in loads without special precautions.
Artistically decorated quarries naturally require more care; we must employ either saggers or stacking "par e"chappade," which consists in dividing the kiln into a series of divisions by means of refractory products in the shape of slabs resting on supports placed by the quarries and higher than them.
Fig. 822. Cross Section.
Fig. 823. Longitudinal Section.
The temperature necessary for vitrifying glazes is very variable. The enamels for Deck's silicious faience bear up to 1200 C.; stanniferous enamels also resist great heat if they do not contain too much lead, but over-glaze decoration requires a lower temperature: 700° to 800° C.
Many of these decorations cannot bear contact with the gases of combustion, and are fired in muffled kilns. Muffled kilns consist of a firing chamber round which the flames and gases of combustion circulate without ever penetrating into the interior.
The stacking is done through one side of the chamber, which is afterwards built up with a clay mortar. A wall is built at a certain distance from the kiln to enclose the space in which combustion takes place (Figs. 822, 823).
The expanded air of the firing chamber escapes through an opening which communicates with the upper air-channel and in front. Another opening allows of the degree of firing being observed. The temperature which can be attained in a muffled kiln does not exceed 1000° to 1100° C. at most, and is generally much less.
They are made in the same way as paving quarries, and with the same substances, machines, and kilns.
The glaze with which these quarries are covered is always colourless. It is applied in various ways, more commonly by salting, as in the case of pipes, by throwing sea-salt into the kiln when "en grand feu"; its nature is then silico-alkaline
Lead glazes are still used, and are applied by volatilisation in the following manner. The inside of the saggers is coated with a mixture of: sea-salt, 67 parts; carbonate of potash, 28 parts; minium, 5 parts. The heat volatilises these substances, and they condense on the quarries and attack their surface, forming a multiple silicate which forms the glaze. The silica is taken from the stoneware, which is always very rich in it.
Other glazes are also used which contain more lead and vitrify at a lower temperature, but which require a second firing. Here are two formulae -
Melted borax .
Carbonate of lime .
Frit for Glaze II.
Carbonate of lead .
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The glazes are ground and diluted with water; they are applied by sprinkling or immersion. When the paste of the stoneware is slightly yellow, the colour is disguised by lightly tinting the glaze blue with oxide of cobalt.
The East and especially the far East makes great use of porcelain quarries and plates for the ornamentation of walls; this substance does not, however, appear to be used in Europe for that purpose. Besides, felspar faience fulfils the same purpose in a more economical manner.