Flagstaff, AZ – Earth Notes: How To Be A Trash Detective
In the arid Southwest, discarded cans and bottles can last a long time and they provide archaeologists with important clues about the history of the old buildings and abandoned camps where they're often found.
The first tin can was patented in 1810. The oldest examples can be identified by lids with a central cap through which food was inserted before sealing. By World War Two this large hole had morphed into a tiny vent in the cap's center.
The seams of early cans were sealed with globby lead solder, a feature famously responsible for poisoning the crew of the 1845 Franklin expedition to find the Northwest Passage. Around the beginning of the 20th century machine-crimped cans began to take over.
Bottles often have identifying code letters on their bottoms, but seam type, glass color, and labels can also give approximate dates. Free-blown irregular bottles are usually pre-Civil War, while embossed lettering became common after 1869. Around World War One modern machine-made bottles became common; they have seams running down the bottle's entire length and over the finish.
The mixture of old trash can also tell a story. Large piles of coffee, cooking-oil, and tobacco cans often represent the kitchen area for a work camp, while small amounts of varying food cans might indicate a household dump.
Other clues can be gleaned from items such as condensed milk tins, first made in 1856, and aerosol cans, introduced in 1945. But pull-tab beverage cans are a thoroughly modern invention they didn't appear until 1962.