The residents of Dartford, England, erected one of the world’s most expensive walls of stone around 1579. They used more than 530 blocks of rock to reinforce the western perimeter of what at the time was known as King Henry VIII’s Manor House. The stone itself is not that exciting; it’s a 1.3- to 1.8-billion year-old, black and white gneiss. What makes the stone notable is how it ended up in Dartford, about 20 miles southeast of London.
In 1576, Sir Martin Frobisher had sailed with three small boats in search of a northwest passage to China. Two of the boats landed at a tiny dab of land, just south of Baffin Island. A group hiked to the island’s high point and mariner Robert Garrard picked up a rock the size of a “halfe pennye loafe…much like sea cole in colour.”
Back in England, a wife of one of the crew ended up with the black stone and threw it in the fire where it “glistered with a bright Marquesset of golde.” (We don’t know why she did this. Perhaps that’s what woman did with rock back then?) Assayers then determined that it was valuable ore. Others say that the stone ended up with Michael Lok, the principal financier of Frobisher’s expedition. Lok also took the stone to assayers. No one could find any gold until Lok gave a piece of the specimen to Venetian alchemist Giovanni Baptista Agnello in January 1577.
Within days Agnello had obtained gold. When asked by Frobisher how he could find gold when others could not, Agnello replied “Bisogna sapere adulare la natura,” or “It is necessary to know how to coax nature.”
No matter how it was determined that the “halfe pennye loafe” contained gold, it sparked so great of interest that Frobisher ended up sailing back to Canada, in 1577 and 1578. The first adventure netted more than 150 tons of black ore and the second more than a 1000 tons. Lok had the ore shipped to an empty chapel near Henry’s Manor House, where assayist Jonas Schutz had built a large processing plant. It consisted of two water mills, five melting furnaces, a coal house, and ten pairs of leather, four-meter-long bellows.
In the first stage of assaying, the ore was ground, placed in a mortar, and weighed. The assayer then heated a crucible, added lead and the ore, and melted them. More lead and glowing coal went into the crucible, which helped dissolve any gold and silver. After the mixture cooled, the alloy of lead, gold, and silver would rest on the bottom topped by the remaining slag. The alloy was then heated again with bone ash to created oxides that were poured off, leaving behind the gold and silver. A further addition of acid separated the two metals.
Unfortunately for Lok, no assayist could find any gold. By the end of 1578, everyone knew that that the three years of expeditions, the expenditure of £20,000 (~£4,000,000 in modern terms), and the loss of 24 lives had produced nothing of worth.
In addition to the stones in the Manor House wall, Dartford historians have excavated the stone from several localities. Some went into a sixteenth century cesspool, as well as a gunpowder mill site. Other blocks were found in a private garden, an orchard, and a farm. Frobisher’s three expeditions may not have produced any gold but at least they provided some very nice building stone for Dartford residents.
If you are interested in more details about Frobisher and his voyages, you can read a longer article of mine in the August 2011 Earth magazine.