Pearl blister

Pearls are growths that form inside the shells of mollusks, usually in response to the presence of a parasite or other irritant. They are composed of layers of nacre that the animal secretes over the foreign body. Pearls tend to be the same color as the nacre inside the shell. Sometimes pearls are loose, forming in the mantle cavity, and sometimes they become attached to the inside of the shell. These latter pearls are called blisters and may contain rotten organic matter or a smaller, loose pearl. 

Mabe pearls are based on a hemispherical nucleus that is inserted through a hole drilled in the outside of the oyster s shell and glued into place. The animal inside will secrete nacre over this round knob which, when harvested, will yield a large pearl that is flat on one side. Mabes and other blister pearls can only be harvested by killing the oyster, since they must be cut out of the shell. 

Pearls with a nacreous sut ce can be divided into three groups natural pearls, nucleated cultured pearls, and non-nudeated cultured pearls. (Pearls without nacre will be dealt with separately at the end of the Spedes secdon.) A further group - blister pearls - are hemispherical pearls that grow attached to the oyster s shell. These can also be cultured. 

Natural blister pearls can occur when a parasite bores through the mollusc s shell. The mollusc seeks to protect itself by increasing the rate of nacre secretion at the point of invasion. This leads to a bump on the inside wall of the shell, which may finally become a hemispherical blister pearl, or which may simply outline the shape of the parasite (Fig. 9.2). 

Composite cultured blister pearls are composed of a hemisphere (or other shape) of nacre that has been cut out firom the shell, where it had formed to cover a nucleus that had been inserted artificially and glued to the shell. It is filled with resin or wax, and backed with a slice of mother-of-pearl. The hemispherical blister pearls are often called mabes (Figs 9.2 and 9.3). 

Figure 9.2 Broken blister pearl in shell, showing nucleus, and parasite causii natural blister in shell.

Shell )- The pearls are iridescent, with the same blues, greens, purples and pinks that are displayed by the shells. The pearls are natural, and are usually hollow and of unusual shape - often cusp shaped - and very seldom round (F%. 9.8). Nowadays cultured composite blister pearls are being produced in abalones. 

Figure 9<11 Composite nucleated cultured blister pearl ( mab ), showing the motfaer of-pearl back (magnified).

As already stated, a mollusc can also cover an object that is attached to the inside of its shell. This can be a natural occurrence, but is also copied to produce cultured blister pearls. A hemisphere of mother-of-peari or plastic is attached to the shell underneath the mantle, and the mollusc is left to coat the object with nacre. Usually the blister pearl will be cut away from the shell and the shell back and nucleus removed, leavir a hollow hemisphere of nacre. This is cleaned, and filled with resin or wax. The hemisphere is then backed with a new piece of policed mother-of-pearl. The result is a composite cultured blister peari. The term mab is now generic and used to describe these pearis, although, strictly speaking, it should only refer to blister pearls fix)m a single species of oyster - the black willed oyster (Figs 9.3 and 9.11). 

White composite cultured blister pearls can also be dyed (Fig. 9.3). Pearls are drilled li t throng the centre for stringing as beads, or part way throu to be mounted and glued on a pin, in, for example, a ring mount. 

A dyed composite blister pearl may show accumulations of dye around the join of the top section and the mother-of-pearl back. Those that have been colour enhanced by painting the inside of the nacreous top are almost impossible to detect. 

Pearl cultivation is a much older science than is generally realised. The Swedish botanist Carl Linne had attempted, with some success, to culture blister pearls in the eighteenth century. And long before that - some say as early as ad 500 - the Chinese had been culturing blister pearls, using half spheres or images of the Buddha as the nucleus. 

Mab pearls are cultured blister pearls, produced against the shell of a mollusc. They are dealt with in detail in Chapter 9 (TeariO as they are used and sold as pearls (Figs 9.2 and 9.3). 

Erythema is evident within 15 to 30 minutes after exposure to liquid Lewisite, and blisters start within several hours these times are somewhat longer after vapor exposure. Lewisite is absorbed by the skin within 3 to 5 minutes (compared with 20-30 min for an equal amount of mustard) and spreads over a wider area than the same amount of mustard. The Lewisite blister begins as a small blister in the center of the erythematous area and expands to include the entire inflamed area, whereas vesication from mustard begins as a string of pearls at the periphery of the lesion, small blisters that eventually merge.58 Other differences between the lesions produced by these two chemical agents are... 

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