The purpose of this study is to find out optimum conditions for zinc crystalline glaze under variables of firing: maximum firing temperature, crystal growth temperature, temperature increasing speed, annealing speed, holding time at maximum temperature and holding time at crystal growth temperature. Ferro Frit3110, ZnO and Quartz were used as starting materials and tested by three component system. The best result of test was selected and extended to its vicinity as five glaze formulas. And then the specimens were experimented by variable firing conditions and analyzed by crystal appearance observation, XRD, FT-IR and Raman spectroscopy. In result, main crystal was willemite in the zinc glazes. Some gahnite was detected in specimens which were fired at $1230^{circ}C$, $1250^{circ}C$ and $1270^{circ}C$, however gahnite was not identified at $1300^{circ}C$. Optimum zinc crystalline glaze was gained by following firing condition: temperature increasing speed $5^{circ}C$/min, holding 1 h at $1270^{circ}C$, annealing speed $3^{circ}C$/min till $1170^{circ}C$, holding 2 h at $1170^{circ}C$ then naturally annealed.