Nacre Thickness
Akoya pearls with adequate to thick nacre coating are a magnificent product which usually retains its beauty through remarkably long use. Such pearls usually have some degree of deviation from the bead nucleus’ spherical shape. Slightly ovoid shapes are called semi-round, more irregular ones semi-baroque, and a majority of thick-coated akoya will be quite baroque. The most valuable akoya pearls are those which have been left in long enough to acquire adequate nacre thickness, but have exceptionally few irregularities and surface flaws in spite of it. However, it is far more cost-effective to produce pearls with near-perfect surface by keeping nacre thickness minimal. Of course, such pearls are not as durable, but hardly anyone asks about that.
With the introduction of akoya pearls to the mass market, the perfectionist approach affected the durability of pearls produced to disadvantage. Round pearls with good nacre coating could not be produced to match retail price points. No pearl producer wishing to take advantage of the expanded market had any choice but to make pearls with very thin nacre coating, as that saved costs and largely eliminated the occurrence of baroques, for which demand was low.
Extremely thin-coated pearls are called “sudare” in the trade in Japan, a reference to chick blinds. When a thin coated pearl with a bead nucleus is rotated, a deep reflection from the nucleus’ near-flat nacre layers (moonstone effect) can be seen. 90 degrees from that, the edges of the nucleus layers are sometimes visible as stripes. The simile is between that striping and light filtering through a straw or bamboo chick blind.
Many such pearls, after being subjected to strong bleaching, were so weakened that they lost all of their nacre coating, and reverted to bare mother-of-pearl beads. Sometimes nacre can be removed by prying at drill holes with fingernail or penknife – if a hard, brittle layer usually much thinner than an eggshell can be removed, you have transformed what got by as a cultured pearl back into a mother-of-pearl bead. Such beads cannot be re-used for cultivation because they may have absorbed enough peroxide to kill the mollusc if implanted.
K. Mikimoto generated a lot of publicity by acquiring a quantity of extremely thin-coated and misprocessed pearls and publicly incinerating them, causing of course a great stench. Rudolf Voll contributed his concerns about the durability of the akoya pearls being exported on a large scale during the 1960′s to the English-language press in Japan. For nearly four decades, pearls exported from Japan were required to pass an inspection by fisheries ministry officials, chiefly aimed at ascertaining that the nacre coating was adequate. This determination was made by visual inspection, without reliance on radiography or any device to measure nacre thickness in microns or as a percentage of mass. However, orders continued to insist on round pearls with smooth surface at low prices… and to enable the industry to meet those demands, the unwritten standard of adequacy had to be kept quite low.
A decade after the end of the government export inspection, pearl grading companies in Japan offer various kinds of certification for akoya pearls, especially strands. An advertisement from a vendor of strands so certified defines “hanadama” (flower pearls, traditionally the top circa 5% of round akoya pearl strands produced in a year) as a grade by stipulating nacre thickness as 5% of the mass, perhaps calculated by comparing the pearls’ weight to that of bare nuclei. An 8.5mm diameter pearl with 5% nacre could be expected to have approx. 0.42mm of nacre on both sides combined, a single nacre thickness of less than 0.22mm… and 95% of akoya with similar nacre thickness are too imperfect for this grade. Most akoya pearls with as much as half a millimeter of nacre thickness are excluded from top grades, because they deviate from the smooth and spherical.
It is necessary to make pearl purchasers aware of nacre thickness and its effects on shape, surface, and durability. The preoccupation with round shape and smooth surface is usually learned rather than innate. It has been our experience that, given this knowledge and the opportunity to choose from a variety of akoya pearl qualities (not all round and smooth), people quickly discover an individual set of priorities and preferences. They are much less likely to demand an unreasonable degree of surface smoothness relative to cost, which can only be satisfied with thin-coated pearls.