Industrial Processes of Harvested Pearls
In the current pearl industry, most cultured pearls will, to some extent, experience post-harvesting processes before being sold. These processes include washing, buffing, polishing, sorting, drilling, bleaching and stringing. Honest and reputable pearl retailers should be fully upfront in disclosing these processes to their customers.
10) Washing and buffing
All cultured pearls must be washed and dried soon after harvest to remove unwanted mucus or tissue clinging to pearl surfaces. The washing is often conducted in a rotated tumble where pearls are dipped in a concentrated salt solution.
Buffing is performed by mixing pearls with abrasive grains in a rotating canister or vibration polisher. The pearls are then cleaned and polished by mild abrasion or sometimes with the use of a waxy substance. The buffing process will remove some surface imperfections and also add a polished look to pearls.
Pearls arrive in a processing factory in bulk and not yet sorted in regards to size and quality. Therefore, the first steps are to sort them by size, quality, colour and their intended usage in different types of jewellery. The sorting work of thousands of pearls a day requires a staggering amount of labor. Processors first remove low quality pearls—those which are heavily spotted, have very thin nacre or are of undesired shapes. Sorting may take several steps, depending on the potential usage of the specific jewelry type. Uniformity is the goal when sorting pearl strands.
To combine sorted pearls with specific jewellery settings, they must usually be drilled. Full-drilled pearls, such as those strung in necklaces or bracelets, are drilled all the way through. Half-drilled pearls, such as those to be mounted on a post, are drilled halfway through. The technician must decide on the best drilling location and direction, the chief aim of which is to drill into the blemishes in order to maximize the amount of clean surface that will be visible.
Almost all Akoya and Freshwater pearls are bleached. Bleaching is done after drilling to allow for a deeper penetration of the bleach solution. Bleaching will usually whiten the conchiolin between nacre layers and improve the uniform appearance of cultured pearls. Akoya pearls are typically bleached using hydrogen peroxide combined with bright fluorescent light to improve lustre. South Sea and Tahitian pearls are not bleached.
14) Matching and stringing
Matching is the final step before the cultured pearls are ready to be marketed, as pearls with different levels of quality will to be priced differently. Full-drilled pearls are matched uniformly in size, luster, shape, surface quality and colour. Half-drilled pearls are matched according to jewelry type, such as in pairs for earrings.
Sieves with series of hole sizes are traditionally used to filter out different ranges of pearl diameter size. The pearls are first separated into broader ranges, such as 7-8mm, then more precisely sorted into half millimeter ranges, 7-7.5mm and 7.5-8mm. Pearls with a similar size range are then re-matched by luster, surface, color and nacre quality. Sorted pearls with acceptable uniformity are then strung to make strands, and usually five strands are bundled together to be sold as hanks.