![]() ![]() He "found this species on the shores of the Bristol Channel, which on cracking and picking off the shell, exhibited a white vein lying transversely in a little furrow or cleft next the head of the fish which must be digged out with the stiff point of a horse hair pencil being made short and tapering which must be so formed by reason of the viscous claminess of that white liquor in the vein so that by its stiffness it may drive in the matter into the fine linnen or white silk. William Cole clearly described this process in which the contents of the hypobranchial gland of Nucella lapillus (at Minehead in the UK) are spread on to linen. However, based on the successful woad vat parameters (pH 7.8 50 ✬), and the long (10 day) fermentation period, this process has been successful in producing a dye bath from Murex trunculus and reducing synthetic dibromoindigo using cockles. The alternative suggestion is of a biochemical reduction, analogous to that for indigo in the woad vat, which has recently been shown to utilise Isatis clostridium, but was reportedly unsuccessful for dibromoindigo. Experiments with iron reducing systems, mentioned by the ancients, did not succeed, but may have referred to other dyes which were cheap alternatives to the purple. Honey has been suggested, although the reducing properties of glucose are not very powerful. Experiments with dodecanethiol in 1M NaOH at 78-88 ✬ were successful. Other suggestions for reducing agents are mercaptans methane thiol is a potential byproduct but the amount present is small and would be insufficient to convert all the dye into the leuco form. Experiments with lead and tin usually in strongly alkaline solution have met with variable success but tin is the stronger reductant. In Pliny’s description, the word plumbum has no adjective and depending on whether nigrum (“black”) or album (“white”) is added, could be translated as either lead or tin. Much effort has been devoted to the discovery of potential reagents which could have been used at that time to reduce the dibromoindigo to the leuco form. It is generally believed that the dyeing process involved generation of the purple dye from the precursor(s), followed by reduction to a leuco compound and subsequent oxidation on the cloth to give the colour in the same way as modern vat dyes. Meyer Reinhold has reviewed in detail the importance of the purple in ancient Greek and Roman times and a large body of literature has accumulated which has been summarised by Dedekind, Becker and in less detail by Born. In more recent times, a distinctly different process for obtaining the purple has been described, first by Cole in 1685, in which the contents of the hypobranchial gland are spread on to cloth and the colour develops in response to air and light. ![]() The wool drinks in the dye for five hours and after carding is dipped again and again until all the colour is absorbed. A frankly red colour is inferior to one with a tinge of black. The liquid is then heated till the colour answers to expectations. Meanwhile, the flesh which necessarily adheres to the veins is skimmed off and a test is made about the tenth day by steeping a well-washed fleece in the liquefied contents of one of the vessels. Next, five hundred pounds of dye-stuff, diluted with an amphora of water, are subjected to an even and moderate heat by placing the vessels in a flue communicating with a distant furnace. It should be soaked for three days, for the fresher the extract, the more powerful the dye, then boiled in a leaden vessel. The vein already mentioned is then extracted and about a sextarius of salt added to each hundred pounds of material. The process is described by Pliny, writing in the 1st century AD: But it is clear that the dye does not exist in the mollusc and is generated from precursors, sometimes termed chromogens, contained in the hypobranchial gland. Surviving details of the ancient process are insufficient to explain the chemistry involved and this is the subject of continuing speculation. The ancient industry was distributed world-wide. ![]() This molluscan dye has been known since pre-Roman times and in the Mediterranean region there is evidence for the industry around the 13th century B.C. This review attempts to set the record straight. The long history, stretching back well into the pre-chemical era, and embracing chemistry, biology and sociology, contains not a few misconceptions and erroneous conclusions. The colour is derived exclusively from marine shellfish of the Muricidae and Thaisidae families. Arguably, it is the oldest known pigment, the longest lasting, the subject of the first chemical industry, the most expensive and the best known. 6,6’-Dibromoindigo is a major component of the historic pigment Tyrian purple, also known as Royal purple, shellfish purple and Purple of the Ancients. ![]()
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