L*a*b* colour parameters and spectral curve of coloured
glazes were measured with an spectrophotometer (MINOLTA
3600 d). These parameters were measured for an illuminant D65,
following the CIE-L*a*b* colourimetric method recommended
by the CIE (Commission Internationale del’Eclairage). In this
system, L* is the degree of lightness and darkness of the colour
in relation to the scale extending from white (L* = 100) to black
(L* = 0). a* is the scale extending from green (−a*) to red (+a*)
axis and b* is the scale extending from blue (−b*) to yellow
(+b*) axis.
3. Results and discussion
3.1. Decomposition of zircon with NaOH
Zircon sand powders were mixed with NaOH and heated at
temperature between 600 and 950 ◦C for 5 h and their XRD patterns
were taken to find out present phases at different temperatures
(Fig. 1). These diffraction patterns showed that the reaction
was not complete at temperatures below 850 ◦C and the reaction
products contained Na2ZrO3, Na2SiO3 and unreacted zircon. At
around 850 ◦C, zircon phase disappeared, Na2ZrSiO5 formed
and Na2ZrO3 phase content increased. Although temperature
was increased to 900 and 950 ◦C, zirconwas observed with other
phases. It is apparent from Fig. 1 that zircon decomposed completely
to intermediate products with NaOH at around 850 ◦C