The crystal structure rainbow – Imperial violet

What does it look like?

Image generated by the Mercury crystal structure visualisation software http://www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/Mercury.aspx

What is it?

What are our statuses of wealth? A sporty car? A swimming pool? Or maybe a beach house? In the time of Pliny the Elder a sure sign of a wealthy status was to wear purple. To colour material purple 2000 years ago required a dye known as Tyrian purple, which at the time cost as much as silver did. Made from thousands of crushed shells, much of the early production of this material was in the ancient city of Tyre (hence the name).

Pliny the Elder wrote about the production of Tyrian purple dye in his 'Natural History'

The most favourable season for taking these fish [i.e. shellfish] is after the rising of the Dog-star, or else before spring; for when they have once discharged their waxy secretion, their juices have no consistency: this, however, is a fact unknown in the dyers' workshops, although it is a point of primary importance. After it is taken, the vein is extracted, which we have previously spoken of, to which it is requisite to add salt, a sextarius [just over 560 grams] about to every hundred pounds of juice. It is sufficient to leave them to steep for a period of three days, and no more, for the fresher they are, the greater virtue there is in the liquor. It is then set to boil in vessels of tin [or lead], and every hundred amphoræ ought to be boiled down to five hundred pounds of dye, by the application of a moderate heat; for which purpose the vessel is placed at the end of a long funnel, which communicates with the furnace; while thus boiling, the liquor is skimmed from time to time, and with it the flesh, which necessarily adheres to the veins. About the tenth day, generally, the whole contents of the cauldron are in a liquified state, upon which a fleece, from which the grease has been cleansed, is plunged into it by way of making trial; but until such time as the colour is found to satisfy the wishes of those preparing it, the liquor is still kept on the boil. The tint that inclines to red is looked upon as inferior to that which is of a blackish hue. The wool is left to lie in soak for five hours, and then, after carding it, it is thrown in again, until it has fully imbibed the colour.

Where did the structure come from?

Though it was synthesised in 1903, the crystal structure of Tyrian purple was not determined until 1980 with X-ray crystallography by Larsen and Watjen in 1980. The crystallographic information file for this structure can be found in the Cambridge Structure Database, refcode DBRING01.

Tags: colours   dye   imperial   Tyre   violet   molecular