He successfully used shellfish pigment extract for colour, and the rotting flesh of tinned cockles to start the fermentation. Edmonds knew that shellfish dye was an indigoid, requiring a reduction process. The exact method for creating a true shellfish purple reduction vat was lost to us until the painstaking work in the 1990s of the late John Edmonds. On removal, colour will return to dyed items as they re-oxygenate in the air. The vat turns a yellowy colour and fibre, threads or fabric are introduced. This reduction process is achieved by removing oxygen from the pigment molecules, or by adding hydrogen to them. In reduction, the pigment’s molecules are converted to a slightly altered, but more soluble, molecular structure. Readers who are indigo dyers will be familiar with its process because an additional requirement for dissolution of the pigment is that it must undergo ‘reduction’. Shellfish pigment must first be made soluble, (as with any dye), and this only occurs in alkaline conditions of around pH 8 and higher.
When the dyers ‘disappeared’ it is believed the method went with them. The biochemical method of shellfish dyeing was complex, relying on processes which were not understood chemically, and were probably only passed down within families. In the eastern Mediterranean, purple dyeing ceased almost a millennium earlier as a result of the Arab conquest at the beginning of the seventh century. It’s well known – in the dye world, at least – that shellfish dyeing largely ceased around the time of the Fall of Constantinople in 1453, after which the last dyers seemed to have disappeared. A small envelope of purple threads fell from the hinges of an upturned rusty trunk: I stared with some incredulity at handwriting in red ink claiming the envelope’s contents to be ‘Tyrian Purple, dyed with the bodies of shellfish found on the African coast'. In late 2008, I was sorting and listing the contents of a nineteenth and twentieth century industrial archive relating to dye manufacture in Leeds.
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