Reflectance (%)
25
20
15
10
5
0
400 450 500 550 600 650 700
Wavelength (λ) nm
Figure 6.6 Reflectance curves – synthetic and natural indigo, reduced in Bath A, steamed for 60 s
All pad-dyed samples exhibited a pale yellow-green colour when exiting the steaming chamber. This indicated that the indigo had been reduced to its leuco-form. The samples regained a dark blue shade on oxidation. The final shade became apparent after soaping off wherein the unfixed dye was removed. When soda ash replaced caustic soda as the alkali, dyeing did not seem to occur until the steaming duration had been increased to 2 minutes. It is probable that a comparatively higher amount of energy was required to reduce indigo in the presence of sodium carbonate.
The K/S values at maximum absorption wavelength (λmax) of 660 nm were compared to determine the effect of steaming time on the shade obtained by reducing indigo in either Bath A or Bath B. The value for 4 minutes steaming of the synthetic indigo–Bath B combination was assumed to be the benchmark (100% colour strength) in this thesis. Table 6.5 lists the numerical values for this comparison while Figure 6.7 and Figure 6.8 are graphic representations of the same.
Table 6.5 Comparative colour strength that demonstrates effect of steaming time
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