The use of attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectra combined with chemometrics were tested to assess the firing conditions of the shard samples excavated from the ancient city of Stratonikeia. Totally thirty-six principal component analysis (PCA) models were compared according to their ability to classify fifty-five shard samples. The PCA model using normal FTIR spectra ranged between 825 and 400 cm−1 with 25 smoothing points showed the best classification results by dividing the shard samples into five different groups (A, B, C, D, E) based on their different spectral information. To determine the firing conditions and the mineralogical compositions of the shard groups, the second-derivative ATR-FTIR spectra (two polynomial degrees and 11 smoothing points of Savitzky-Golay) between 1700 and 400 cm−1 was successfully used. It was determined that the minerals of pseudo-amorphous structure of meta-smectite, quartz, muscovite, gehlenite, and gypsum were the main phases in the shard samples fired in the oxidizing atmospheric conditions. The obtained findings indicate that the firing temperature is about 800–950 °C and 600–800 °C for the shard groups of A, B, C, and D and E, respectively.