The concavity defects frequently appear at the surface of porcelain tile produced with the blank via SNCR (Selective Non-Catalytic Reduction) denitrification using urea reductant. The general chemical analysis of the blank cannot judge the causes of the defects. A multispectroscopy approach combining XRD, FTIR, TGA-FTIR adopted in this study reveals the forming mechanisms of the concavity. The results demonstrate that the blank denitrification by urea and that by ammonia mainly release CO₂ and H₂O during heating. However, the former release not only much gases than the latter but it can releases HNCO and NH₃. The residual urea in the blank could augment the gas flow further. The blank denitrification by urea contains cyanic acid, isocyanic acid and their salts, which could decompose beyond 500 ℃ and release additional CO₂. In the high temperature stage of sintering, the massive liquid is formed and seals the surface pore of the body. The cyanate and isocyanate in the blank decompose continuously and produce excessive gases, which cannot escape from the body and congregates to be bubbles in the liquid. During the cooling stage after sintering, the bubbles break or collapse, resulting in a concavity at the surface of porcelain tile. Using the multispectroscopy approach and adopting characteristic parameters such as the peak of sodium cyanate at 17.80° in XRD pattern, absorbance peak of cyanate radical at 2 226 cm^-1 in FTIR spectrum and absorbance peak of gaseous cyanic acid at 2 241 cm^-1 in TGA-FTIR spectrum as the criterion, the quality of the blank could be valued previously and thus the porcelain tile yield could be promoted.