Chinese Pearl Enhancement Techniques

(Gemmology Queensland, Volume 3, No.6, June 2002)

According to Guo & Shi on pages 32-36 of Volume XXII of The Journal of the Gemmological Association of Hong Kong, Chinese pearl processors apply a range of enhancement technology to both their freshwater and saltwater cultured pearls. These routine treatments include:
- Pre treatment
- Bleaching
- Increasing whiteness
- Adding luster
- Dyeing
- Polishing

Pre Treatment
Pre-treatment process includes sorting, drilling, expansion and dehydration steps. Pearls are sorted on the basis of thickness of nacre, color, luster, shape and size. Drilling allows stringing and setting removes some external defects, and is an essential prerequisite for subsequent treatments for reducing oil content, bleaching, adding whiteness and dyeing pearls. Expansion looses the pearl’s structure and so facilitates future treatment. Two techniques of expansion are used at a temperature of 70-80°C and for times that depend on the depth of color of the pearl (darker color, more time). These techniques of expansion are:
- Heating pearls that have been wrapped in gauze in deionized water.
- Immersion of pearls in an unspecified liquid.
- Dehydration follows expansion. The removal of water from cracks in the pearl is accomplished by immersion in anhydrous ethanol and glycerol.

Bleaching
Bleaching is used to create white pearls and to remove disfiguring colors and stains from the pearls. The common bleaching solution consists of a mixture of hydrogen peroxide, a solvent of surface active reagent (detergent), and a pH stability regulator. Factors affecting the efficiency of bleaching include concentration of hydrogen peroxide, temperature pH of bleaching solution, composition of surface active agent, pH stabilizer and solvent duration of light exposure, and the frequency of stirring and renewal of the bleaching solution.

Two formulations are commonly used for bleaching pearls:
- Hydrogen peroxide carbinol ammonia, 12-alkyl sodium sulphate, Britton-Robinson buffer solvent.
- Hydrogen peroxide chloroform, ethanol amine, 12-alkyl sodium sulphate, Britton-Robinson buffer solvent.
- Common bleaching conditions are a temperature of 30-35°C in association with light exposure. Hydrogen peroxide of 2-3% is commonly used, while bleaching times range from 2-3 days for light colored pearls to more than 10 days for darker colored pearls. The bleaching solution is changed every four days.

Increasing Whiteness
Following bleaching some pearls still retail a yellowish tone due to the presence of pigments resistant to the bleaching effect of hydrogen peroxide. These pearls are further treated with a fluorescent whitener. This is a dye molecule that is activated by the UV in visible light to emit a bluish fluorescence which neutralizes the residual yellow color of pearl and produces a whiter pearl. A Swiss manufactured whitener FP, dissolved in acetone and the surface active agents acrylic acid for 12-alkyl sodium sulphate is the fluorescence whitener commonly used on Chinese pearls.

Adding Luster
This is claimed to be one of the most important processes in pearl enhancement. It involves immersing the pearls in a weakly alkaline TGP (magnesium compounds) solution and keeping them constantly stirred at even temperature for several days.

Dyeing
As bleached pearls usually have some residual uneven color, they are dyed to yield fashionable and marketable colors. Specific dyes are used to produce predictable colors, e.g. potassium permanganate solution with golden NH-R yields golden pearls, while Luo Dan Ming B in alcohol produces pink pearls. These dyes can be either water, oil or alcohol based.

Polishing
Polishing, the last step enhances both the smoothness and luster of the pearls. This step involves surface polishing, usually by tumbling in a mild abrasive, followed by coating of the pearls with a thin layer of wax.

The Problem
- Which of these treatments should be detected?
- Which of these treatments should be disclosed?