Madrid, 1787
Proust was inaccurately thought to be the discoverer of silver in seawater. Actually he had observed tarnished silver in San Pedro de Alcantara's shipwreck off the coast of Portugal, which was later misinterpreted in 1859.
San Pedro de Alcantara was a 64-gun Spanish man-of-war, built in Cuba in 1770, which sank near Peniche, Portugal, loaded with so much treasure that her shipwreck shocked the financial markets of Spain in 1786. The cargo was about twice as much as usual for such a ship. She sailed from Peru in 1784 with a load of 600 tons of copper, 153 tons of silver and four tons of gold from Peruvian mines. In a dark February night of 1786, she hit a rock near Peniche, 90 km north of Lisbon.
Shipwreck of the ship of War San Pedro de Alcantara near Peniche, Image: Vicente Mariani
The silver was mined in the tiny Bolivian village of Potosi that was once the largest industrial mining complex in the world
Proust suggested the acidic seawater tarnished the silver. This was alarming because the silver was minted to coins in Lima, mostly 8 reales pieces, "pieces of eight". Although the mint said the silver could be fully recovered, there was a fear of loss of considerable amount of silver due to corrosion. King Charles III of Spain then ordered these silver should never leave the country in fear of fall of economy. Proust also mentioned gold was unaffected in seawater.
The pieces of eight were minted at Casa de Moneda de Lima
Silver and Gold Eight Royals Coin of Charles III of Spain
The copper, silver, and gold lost in San Pedro’s cargo corresponded to about 1/12 of the total currency circulating in Spain. The Spanish king was alarmed and immediately sent his representative to the site. An enormous diving enterprise was initiated and divers were hired from all around. During the following three years more than 40 divers worked full-time, and almost all of the cargo was salvaged. This was considered a great success, since the cargo was worth much more than the ship. The diving operation was by then the largest ever in European history.
Joseph Louis Proust
Joseph Louis Proust (26 September 1754 – 5 July 1826) was a French chemist. He was best known for his discovery of the law of constant composition in 1794, stating that chemical compounds always combine in constant proportions. In 1787 he was in service of the Spanish king and was teaching chemistry in Madrid.