Proceedings e report - Firenze University Press

COLOUR MEASUREMENTS OF POLY-CHROMED WOODEN OBJECTS IRRADIATED FOR DISINFECTION REASONS
that even for the samples of bad uniformity (Solferino lake and cadmium red) for which the
disagreement (considering the colour differences on components) is not only quantitative, but also
qualitative, the total colour differences (dE* and dE CMC) are very close.
3.2. Colour stability over time
Colour stability over time for samples (B) was evaluated using HL. The interval of time between the
two series of measurements is one year. The results are presented in table 3; values obtained in the last
year are used as references.
Tabel 3: Colour shift over time for samples (B)
sample dL* da* db* dC* dh dE* dE CMC
raw Sienna -1.32 -0.77 -3.30 -3.32 -0.87 3.64 1.59
raw Sienna (d) 0.47 -0.34 0.18 0.03 0.47 0.61 0.41
raw Sienna (D) 0.71 -0.76 -0.88 -1.11 0.42 1.36 0.63
raw Sienna v 3.04 1.13 3.56 3.65 1.30 4.81 2.51
raw Sienna v (d) 6.28 1.83 7.24 7.25 2.78 9.76 5.12
raw Sienna v (D) 7.18 1.33 8.37 8.01 4.22 11.11 6.19
minium -0.29 -1.49 -1.28 -1.95 0.17 1.98 0.67
minium (d) -1.81 -3.27 -4.84 -5.78 -0.67 6.12 2.16
minium (D) -0.84 -3.00 -7.20 -7.31 -2.21 7.85 3.27
minium v -0.48 -1.96 -3.09 -3.61 -0.52 3.69 1.32
minium v (d) -1.91 -2.49 -4.74 -5.18 -1.08 5.68 2.19
minium v (D) -2.17 -3.45 -5.91 -6.69 -1.18 7.18 2.67
mars red 0.25 -0.81 -0.69 -1.06 -0.21 1.09 0.55
mars red (d) -0.28 -1.30 -0.82 -1.54 0.07 1.56 0.77
mars red (D) -1.06 -1.56 -0.59 -1.62 0.74 1.98 1.07
mars red v -0.44 -1.25 -0.60 -1.37 0.43 1.46 0.74
mars red v (d) -0.13 -1.40 -0.82 -1.61 0.23 1.62 0.80
mars red v (D) -0.57 -1.18 -0.57 -1.30 0.39 1.43 0.73
chrome yellow -1.14 0.41 -1.68 -1.67 -0.27 2.07 0.69
chrome yellow (d) -0.47 0.54 1.15 1.14 -0.37 1.35 0.49
chrome yellow (D) -2.30 -0.80 -4.25 -4.23 0.61 4.89 1.63
chrome yellow v -0.90 0.59 -0.21 -0.19 -0.39 1.09 0.45
chrome yellow v (d) -1.13 -0.14 -2.27 -2.27 0.12 2.54 0.83
chrome yellow v (D) -1.64 -1.95 -2.75 -2.70 1.42 3.74 1.49
Colour shift over time is showed as dE CMC total colour difference in Fig. 2 where a vertical line at
one dE CMC unit marks the limit up to which a normal standard observer could not detect any colour
change. In this respect colour shift over time is insignificant for samples prepared with mars red and
chrome yellow (except chrome yellow samples irradiated at a higher dose ratio). The colour of
varnished samples prepared with raw Sienna is the most unstable, even for the unirradiated ones.
Regarding the influence of irradiation on the stability of colour over time one cannot draw a general
rule. For example, in the case of minium irradiated samples are less stable than unirradiated ones but
for raw Sienna this effect can be seen only on varnished samples. The influence of irradiation is only
quantitative on lightness, chroma and hue. Where the colour shift is significant (i.e. more than one dE
CMC unit) it increases with the dose ratio. Varnish does not influence the colour stability over time
except samples prepared with raw Sienna where the colour shift is higher for varnished samples.
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