Color Treatments
Most white pearls have been bleached. It stands to reason that a laboratory should be able to determine whether a pearl contains residues of chemical treatments, but many gem labs do not offer this service.
Natural pearls had been bleached for centuries before cultivation began, suggesting that aesthetes felt the need even then to improve on nature. The most benign method of bleaching is exposure to sunlight, which is effective in lightening some typical colors of freshwater mussel pearls. This was rediscovered in 1960′s Japan after a parcel of freshwater pearls sent to India by Uda Pearl Co was returned. While being reforwarded around India for 2 years in translucent airmail packaging, the peachy-creamy pearls had become brilliantly white. Freshwater pearl processors began modifying ice boxes, which were being discarded in large numbers following the introduction of affordable refrigerators, with light bulbs to simulate the process. This kind of bleaching appears to have little or no detrimental effect on pearl lustre.
However ocean pearl color is mostly unaffected by visible or uv light irradiation. Sunlight bleaching of China fw pearls seems to have been largely abandoned during the 1980′s in favor of chemical treatments, which are faster and more powerful.
Practically all akoya pearls are treated with hydrogen peroxide bleach. This serves two purposes. One is “spot removal”, done on all pearls except for some to be dyed dark colors. The vast majority of akoya pearls are harvested with small grey spots that respond well to mild bleaching, which may not significantly alter their color. Until a recent craze for “golden” akoya has led to quite a lot of pearls being dyed yellow, it has been most common to lighten the color in the same operation, by bleaching more intensely, for longer time and/or at higher temperature. The main cost of more bleaching is in the form of more pearls that lose lustre and thus value. Most of this loss occurs during treatment, and some durimg subsequent months and years. In the latter case, dealers are forced to cull pearls from strands, and commonly consumers are told that their use of cosmetics (or constitutional chemsitry) is at fault.
Bleaching is usually done after pre-sorting and drilling akoya pearls, and before final sorting into loose pearl lots and strands. Undrilled pearls are slower to bleach because the peroxide must penetrate the nacre, but the drill holes allow it to be absorbed below the nacre. Though preferable to all alternatives, hydrogen peroxide has a solubility problem. Despite the use of modern equipment such as high-speed centrifuges, it is difficult to eliminate the oxidant completely from treated pearls. This helps to explain why highly-bleached akoya pearls tend to change more rapidly than typical creamy colors.
The state of the art of pearl processing is a trade secret of each processor, and far from seeking to provide details of color removal treatments, the convention remains nondisclosure. Only treatments obviously adding color, by means of dye or nuclear irradiation, are usually disclosed when pearls are sold. Cobalt 60-gamma irradiation turns freshwater pearl nacre grey – and affects akoya pearls because their nuclei are freshwater. Ironically, added color is much less likely to impact pearls’ durability than bleaching. The addition of blue color was a prerequisite for exporting naturally blue-grey baroque akoya pearls from Japan while the government inspection was required. This is because the natural color is unstable, ie fades and may become spotty, thus dye treatment was considered a necessary improvement.
It took many years for golden color, long appreciated in southsea pearls, to be a desired color of akoya pearls. Since for the preceding half century, akoya shells have been bred largely for their whiteness, not enough pearls with strong goldy yellow color occur naturally to meet the new demand. Yet most lots of extremely yellow pearls are marketed as natural color, leading us to believe that nondisclosure is as much the norm with yellow dyeing as it is with bleaching.
Color treatments affect the nacre of a pearl, either bleaching or staining it. That nacre coating remains the source of the pearl’s lustre after treatment. Paints like nail polish, which cover a pearl’s surface, providing lustre from the paint layer, are not pearl treatments. When they are applied to pearls, they transform them into imitation pearls. Similarly, faceting pearls by cutting away their surface transforms them into the equivalent of shell beads.
There has always been a need to improve the lustre, especially of pearls that lost it to bleaching. Tumbling with bits of bamboo impregnated with silicone and/or all manner of shiny resins is very common for all types of Chinese pearls. In Japan there was a treatment for akoya pearls with eye-poppingly unnatural results that required the use of ether. It was called bakudan-zome (bomb dye) because of what happens when one is not careful with ether. More recently, there have been occasional announcements of treatments likely to affect pearl lustre, such as polymer coating, never to be heard of again. Successful lustre enhancement treatments can add more value if undisclosed, might actually improve durability, and may remain undetectable for a long time. Uncertainty about this has deterred us from investing heavily in high-cost akoya pearls with extremely unnatural color and/or uncanny lustre.
The monochrome image above was sold (including publication rights) by Japan Pearl Export Processing Assn (JPEPA) to its members, including Pacific Pearls, circa 1960. Soon after that it disappeared, and by the time color photos became available, only cultivation and sorting scenes were offered. We surmised that some other member(s) had pointed out that, since by convention pearl bleaching is not disclosed, it would be better not to publish photographs of the equipment used to do it.