The moka pot is three pieces screwed together: a lower boiler with a safety valve, a funnel basket of fine grounds sealed by a rubber gasket and filter disc, and an upper collector. Heat raises the pressure of the air and steam trapped above the water, following Charles’s law and the ideal gas law, and forces water up through the coffee bed, where Darcy’s law describes its resistance, and into the top chamber in a single pass. At one to two bar it is not espresso, which requires nine, and its coffee lands at about 3 to 4 percent dissolved solids, against 9 to 10 for espresso and about 2 for drip.

The principle traces to Romershausen’s 1818 patent and Louis Bernard Rabaut’s 1822 design, and in 1833 Middlesex coppersmith Samuel Parker patented the steam fountain, later known as the Vienna Incomparable, which set the vertical method. Alfonso Bialetti began selling the aluminum Moka Express in 1933 but sold relatively few before World War II; his son Renato’s 1952 campaign with the mustached little man made it an Italian fixture, its design essentially unchanged since. When the boiler nearly empties, steam and water sputter up together in what researchers Navarini, Nobile, and colleagues named the Strombolian phase, overextracting bitter compounds, so the pot should come off the heat as the gurgle starts.

• Measured caffeine in moka coffee ranges from 128 to just under 540 milligrams per 100 milliliters.
• The Brikka adds a weighted valve on the nozzle that builds higher pressure and produces a layer of crema.
• The Mukka froths milk and mixes it with the coffee inside the same pot.
• In a broken in pot, aluminum migration falls below 1 percent of the weekly intake, but after a dishwasher strips the oxide layer it rose to nearly 4 percent.
• Moka pots are found in the Museum of Modern Art, the London Science Museum, and the Smithsonian.