The spectrophotometer was calibrated to the perfect white diffuser by a ceramic plate with tristimulus values X = 90.3, Y = 92.1, Z = 105.7 and chromaticity coordinates x = 0.3135, y = 0.3196.
The pH, water soluble substance, acid-soluble carbonate, and acidsoluble
iron of the samples were determined according to
the Turkish Standards adapted to European Norms TS EN
ISO 3262-10 and other Turkish Standards TS 2973 and TS
10521.