Potsdam boiler, manufactured c. 1800. Rote Fingur / wikimedia / 2009 / CC BY-SA 3.0
A Potsdam boiler coffee maker (aka Russian Egg) is a flip coffee maker similar in principle to a Neapolitan flip coffee maker.
Made of metal, it consists of two pots, one with a spout on it. Both are mounted on a stand, with the top one resting upside down on the bottom one. The top — or rather, the one at the top at the start before you flip it — has the spout.
You fill the bottom, plain pot with water, then fill a filter insert in it with ground coffee. You then put the spouted pot on it upside down to act as a lid, and mount the two pots on their stand.
The stand suspends the pots above a spirit burner. You light the burner and bring the plain pot with the water in it to a boil. When the water is boiled, you use a handle to rotate the pots head over heels, so that the pot with the spout on it gets flipped to the bottom. At this point the pot with the hot water is now on top.
The water drizzles down through the filter holding the ground coffee, and into the spouted pot at the bottom.
History Notes
Potsdam boiler coffee makers were made in France sometime around the 1840s. They were often ornate enough for use at the table.