Study On The Color Change By Heating And The Origin Of Red Coral Color Based On CIE1976 L*a*b* Uniform Color Space

Red coral is a precious biological gem,which is quite popular with jewelry consumers all over the world for its vivid color and bright luster and has been a symbol of wealth and good fortune since ancient times.In this paper,the color appearance,chromogenic pigment and color change by heating of red coral were studied.In the experiment,X-rite SP62 integrating spherical spectrophotometer,LabRAM HR Evolution Raman spectrometer and 101-0AB electro-thermostatic blast drying oven were used,the heating temperature of the color change experiment was100?-230?,and the constant temperature lasted 0-50 minutes.In this experiment,coral samples with white and red colors were selected.Under the standard light source D₆₅,the hue angle of the samples was h₀??21.07,151.98?,the lightness value was L*??32.97,88.70?,which was above average,and thechroma value was c*??1.40,48.42?,whose variation range was large.According to Raman spectrum and the color appearance of red coral,the chromogenic pigment in red coral was a mixture of at least two or more kinds of polyene pigments,and their molecular structure had the same proportion of methyl?CH₃?,carbon-carbon double bond?C=C?and carbon-carbon single bond?C-C?,in which at least one pigment was red with a hue angle h₀ of about 20 degrees and at least one pigment wasyellow-orange with a hue angle h₀>50 degrees.Red coral also contains some white substances,which were a mixture of sugars,lipids and proteins and whose presence affected the lightness and transparency of the coral.The amount of these colorless and opaque substances was related to the species and genera of coral.In the future,if the specimens are further enriched and the database is established,it can be used todistinguish the species and genera of coral without loss.After the heating temperature exceeded 130?,with the increase of heating temperature and heating time,Raman signals of organic substances in red coral attenuated or even disappeared and the color faded.The attenuation degree of the organic signal in Raman spectrum was consistent with the fading degree,which verified once again that this organic substance was the chromogenic pigment of red coral.The structure of this pigment was destroyed above 130?,resulting in thefading phenomenon of red coral.This was because the C=C in the structure was unstable and easy to oxidize at high temperature.Corals discolored by heating had orange-yellow hue compared with natural white corals.When the heating temperature was lower than 130?,the color change of coral samples was not obvious.When the heating temperature was between 130?and220?,with the increase of heating temperature and heating time,the lightness of the sample color increased,and the saturation first stayed horizontal and then decreased.The hue shifted from red to orange and yellow,the color difference value increased,and the color perception showed a color change process of red,orange,yellow,light yellow and yellow white.The higher the temperature,the faster the color change.At the same temperature,the longer the time,the greater the degree of the color change.When the heating temperature is between 220?and 230?,the color of red coral no longer changed obviously,the lightness value remained basically unchanged,the color saturation slightly decreased,the hue angle rose and the color difference valueremained basically unchanged.