Both natural and cultured pearls are formed when an irritant lodges in a mollusk. The mollusk then deposits layers of nacre, covering the irritant and producing a pearl. In a natural pearl, the irritant may be organic or inorganic matter. In cultured pearls, the irritant is a mother of pearl bead or a piece of tissue placed in the mollusk by a technician. The mollusk covers the nucleus with alternating layers of conchiolin (a protein-like substance similar to human nails) and nacre. The latter consists of very fine crystals of calcium and aragonite, which are very soft minerals. It is delicate and can be scratched or chipped easily.
Among cultured and natural pearls, there are freshwater and saltwater varieties. These names simply mean that the pearl formed either in a freshwater or saltwater mollusk. The information outlined in this article pertains to working with pearls.
|Overaggressive drilling has damaged the nacre on this pearl.|
photo courtesy of Anton Nash.
Rock Solid Facts
Mohs Hardness: 2.5 to 4
Many pearls of light body color have been bleached to im-prove color matching. Bright pinks, purples, greens, yellows, and blues have been dyed. Irradiation can also be used to achieve exotic colors, such as bronze or black.
Dyes tend to be concentrated around drill holes or blemishes. Irradiation often turns the conchiolin layer of cultured pearls dark. To help identify these treatments, look down drill holes under magnification.
In the case of very high end pearls, coatings have been an issue. These are difficult to detect. The use of a qualified laboratory is recommended if there is any suspicion about coatings.
Cleaning pearls in an ultrasonic cleaner is not recommended. The vibrations of the machine can be harmful to nacre, and the chemicals used can attack the conchiolin layer, which can cause cracking over time. Steam cleaning is likewise not recommended. The temperature is damaging to the organic proteins in the pearls.
The best way to clean pearls is with a soft brush, such as one used for powder or blush, and a mild detergent, followed by a thorough rinse in clear water. If the pearls are strung at the time of cleaning, lay them flat to dry. This will prevent stretching the cord.
At the Bench
Pearls cannot tolerate heat or chemicals. In any repair situation requiring heat, it is best to remove the pearl. If the pearl cannot be removed for repair, use a heat-shielding product or immerse the pearl in water while doing torch work. Do not immerse the pearl in pickle solution, as it will etch the surface of the pearl.
Most pearls are drilled and glued onto a post with epoxy to set them in jewelry. Soaking the piece in methylene chloride solvent will usually allow the pearl to be removed. If methylene chloride solvent does not work, acetone can be used as a last resort. However, acetone may dull the surface of the pearl slightly, especially if the pearl is left in it for several hours.
An alternate method for removing a pearl is to heat the piece in water. To do this, heat a small pan of water with the piece in it. When the water is warm, try to pull the pearl from the post. Repeat this every minute until the water is too hot to touch. If epoxy was used to secure the pearl, this method of removal usually works. If super glue was used (which can stain the conchiolin layer and cause noticeable damage to the pearl), acetone often is required for removal. This is a very nasty situation in the case of an expensive pearl. The possible risks of acetone removal should be discussed with the customer prior to any repair that requires it.
Communicate with the Customer
Customers need to know that pearls are somewhat delicate. There are risks in the repair of any type of pearl jewelry, especially when removal of the pearl is difficult or not feasible. Customers need to be made fully aware of the risk to their pearl and any liability that you are willing or not willing to take.
When restringing pearls, count the pearls in the customer’s presence and inform her that the pearl strand will be slightly shorter after restringing. (This is often due to the fact that the silk was stretched out from wear.) A professional restringing also includes washing the pearls. Once cleaned, filthy pearls look quite different.