| People usually treat pearl as queen of gem and deeply love it from time immemorial andalso regard it as a sign of beauty and wealth. As the natural pearls are extremely scarce,farmed pearls are prevalent. However, environmental pollution results in the degenerationand even inferiority of the cultured pearls. Some of the cultured pearls possess irregularsurface shape and tinge. Especially there are variegated substances in those inferior pearls.The ultimate goal is to find a processing method which can remove the variegatedsubstances without distroying pearl stucture. The seawater cultured southern pearls havebeen used for researches. We have systematicly analysed variegated substances whichcause the low quality of pearl with advanced instruments. The researches include analysingcomponent and structure and the effect on pearl structure to find out the effect on pearlquality and the reason of corrosion during processing and optimize pearl bleaching forimproving pearl processing technologies.Fristly, in order to oberseving the inside structure and growth pattern of variegatedsubstances and exploring the formation cause of black variegated substances, these blackvariegated substances have been investigated by using scanning electron microscope.Secondly, we have analysed its thermal degradation and composition by using thermalgravimertic analysis, infrared spectrum, X-ray photoelectron spectroscopy and inductivelycoupled plasma atomic emission spectrum. And then, in order to understand factors whicheffect on qulity during bleaching process. We have oberserved and recorded corrosion ofthree different pearls (“White mist” pearl, Stain pearl, Corrosion pearl). Finally, we haveoptimized the existing bleaching process by using TAED as activating agent for improvingpearl processing technologies. The experiment process and the conclusions are as follows:1. Structure analysis of variegated seawater cultured southern pearl. These blackvariegated substances have been investigated by using scanning electron microscope formainly oberseving the the inside structure and growth pattern of variegated substances andexploring the formation cause of black variegated substances.This article shows different structure in inferiored pearl and founds that there are somedifferent crystallization structures which are black.Five sequential lamellar structures areclearly observed in research, which indicates the practical growth direction of pearls. From bottom to top, the lamellar layers are nucleus of pearl, viriegated layer, intermediate layer,prismatic layer and nacre, respectively. The thickness of the prismatic layer and theintermediate layer are around20and10um, respectively. But the thickness of viriegatedlayer is nonuniform, it can even reach50um. Between nucleus of pearl and intermediatelayer where viriegated substances disappear, it appear normal structure. Once the pearlnucleus is covered by variegated substances, the variegated substances will lead to theirregular pearl growth. This irregular growth will eventually result in the irregular pearlsurface. It`s the existence of variegated substance that directly leads to the formation of theirregular pearl appearances.The schematic microstructure of variegated substances indicatethat variegated substances are the characteristic of stacks lots of spheroids with many nanoholes and loose stacked together, and also there have some loose network fibers linkedsome spheroids.2. Composition analysis of variegated substances. We have analysed its thermaldegradation and composition by using thermal gravimertic analysis, infrared spectrum,X-ray photoelectron spectroscopy and inductively coupled plasma atomic emissionspectrum.The research shows that the thermal degradation could be divided into4stages. The friststage (30-150℃) is related to evaporation of adsorbed water. The second stage (150-450℃)is related to decomposition of organic substances. The third stage (450-610℃) is related tobound water loss in the black variegated substances. The last stage (610-750℃) is causedby CaCO3decomposed into CaO and CO2. Therefore, it can be calculated that the weightproportion of CaCO3in black variegated substances is about53.9%and38.1%for organics.This elucidates that the black variegated substances contain more organic matter.The result of infrared spectrum shows that the viriegated substance is mainly consists ofaragonite and proteins. With SEM images analysis, it can be inferred that black viriegatedsubstances have unique structures which are mainly consists of proteins and aragonites.The result of XPS analysis indicates that the main elements of blak variegatedsubstances and inside surface of normal pearl samples are Ca, C, O, N and the fewerelements are Zn, S, P, Na. We find sodium in variegated substances and phosphorus innormal pearl. Compairing the content of C, O, and N in both samples, we find that theorganic matters in variegated substances are different from which in high quality pearl.According to analysis of carbon spectrum, the main differents between these two samplesare proteins. It’s obvious that black variegated substances contain more organic matters andless calcium carbonate than inside surface of high quality pearl.There are lots of mineral elements in both samples according to ICP analysis. The main elements are Al, Fe, Mg, Na, Zn. Comparing those elements of two samples, Al and Na aremore contained in normal pearl, but Mg and Zn are more contained in inferiored pearl.3. Structure characteristics of pearl during different bleaching stage. We have oberservedand recorded corrosion of three different pearls (“White mist” pearl, Stain pearl, Corrosion pearl)during bleaching process.Among the results of pearl bleaching,“White mist” pearl belongs to mildly corrosion.Scattered aragonite crystal cause the “White mist” surface. When these scattered crystalsare wiped out, the pearl also has value. Stain pearl belongs to further corrosion. It looks asif there are lots of pits on the surface, those are corrosion pits which caused by bleaching.When corrosion pits appear, the pearl is already broken and lose value.4. Research on the effect of TAED in pearl bleaching. We have optimized the existingbleaching process by using TAED as activating agent for improving pearl processingtechnologies based on existing technology.The research shows that TAED has a great effect on catalyzing pearl bleaching compareto which not using TAED as activating agent in hydrogen peroxide bleaching. When theconcentration of TAED exceeds to1%, it will leads to intense bleaching reaction andbrings lots of damage on pearl surface. For the better bleaching condition, theconcentration of TAED should be0.5%. Hydrogen peroxide/TAED bleaching system hasbetter effect than hydrogen peroxide bleaching. The advantages of this system are faster,less corrosion and lower energy consumption. |
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