Thursday, 11 February 2010

Alum and the Secret of the Floating Egg

Dress fabric dyed without alum mordant is like a world without sunshine. There is no brightness, only subdued hues doomed to fade even more in natural light and with washing. Alum imports came originally from the East - from Phocaea, north of Izmir, on the Anatolian coast (Turkey). Carried overland and by ship, the cost on the European market was such that only the rich could afford the resulting bright coloured textiles. In 1453 Constantinople fell to the Ottomans and though exports of alum continued, the search for European sources of alum began. Deposits were found on Papal territory, at Tolfa near Rome. Prices did not fall as a consequence, since the Pope's administration guarded the monopoly jealously. Various attempts had been made to discover Alum sources in Britain. Henry VIII had encouraged prospecting in Ireland. Then some alum was found on what would be called Alum Bay on the South Coast of England, opposite the Isle of Wight, but these sources were never viable. In 1600, Thomas Chaloner began mining near Guisborough, on the North Yorkshire Moors, again with no economic success. The breakthrough came with finds on the North Yorkshire Coast. Called the Jurassic Coast, the coast could equally be called the Great Alum Coast, since alum quarrying changed the whole face of this 30 mile stretch of coast - carving away some 25 million tons of rock! Huge boiling pans were constructed on the beaches - see the picture below.
The rocky shore is striated still with special ruts chiselled out to enable wagons to carry the alum to waiting ships on the tide. It took over 100 tons of rock to produce between just 1-3 tons of alum. The manufacture of alum depended also on urine which was carried from the surrounding country in barrels, the emptied barrels being replenished with fish and sent back inland. The process for making alum takes months, so it was a high risk and complex affair. In the age when chemical knowledge was uncertain, it was difficult to gauge when the boiling pans of alum solution were ready, and that once cooled, alum would crystalize out of solution. Someone hazarded dropping a hen's egg into the solution - and found that when it floated the solution was ready to be harvested. Was the floating egg the world's first hydrometer?


  1. i am always amazed at what i learn on your site. thank you once again for all your research.

  2. What an interesting story! Though according to Wikipedia, hardly the earliest hydrometer, as there is description of one in a letter from Synesius of Cyrene to Hypatia of Alexandria.

  3. Thank you very much for finding this out for us - I am always astonished by history.