Firing(Sintering)
After shaping but prior to firing, the ceramic piece is said to be green(same term as in powder metallurgy), meaning not fully processed or treated. The green piece lacks hardness and strength: it must be fired to fix the part shape and achieve hardness and strength in the finished ware. Firing is the heat treatment process that sinters the ceramic material it is performed in a furnace called a kiln. In sintering, bonds are developed between the ceramic grains, and this is accompanied by densification and reduction of porosity Therefore, shrinkage occurs in the polycrystalline material in addition to that which has already occurred in drying. Sintering in ceramics is basically the same mechanism as in powder metallurgy. In the firing of traditional ceramics, certain chemical reactions between the components in the mixture may also take place, and a glassy phase also forms among the crystals which acts as a binder. Both of these phenomena depend on the chemical composition of the ceramic material and the firing temperatures used. Unglazed ceramic ware is fired only once glazed products are fired twice. Glazing refers to the application of a ceramic surface coating to make the piece more impervious to water and enhance its appearance (Section 7.2.2). The usual processing sequence with glazed ware is: (1) fire the ware once before glazing to harden the body of the piece, (2) apply the glaze, and (3) fire the piece a second time to harden the glaze