They may not deliver the highest efficiency, but absorption refrigerators have earned a persistent place in camper vans and industrial plants thanks to a few distinct advantages: they can operate using waste heat alone as an energy source, and they can be quite reliable due to their lack of moving parts. It can be a bit hard to understand their working principles, though, which is why [Ben Krasnow] built a transparent Einstein-Szilard refrigerator.Most absorption refrigerators use ammonia and water as their working fluids; since the ammonia gas needs to condense at room temperature, the refrigerator requires pressures above what can be comfortably held in a glass or plastic enclosure. The Einstein-Szilard design, by contrast, uses a third liquid as its refrigerant. A strong solution of ammonia in water is separated by boiling, and the ammonia gas induces the refrigerant to evaporate by decreasing its partial pressure. The refrigerant and ammonia gasses then mix with water, in which the ammonia dissolves, leaving the refrigerant vapor to condense and start the cycle again. The newly reformed water-ammonia solution flows back into the boiler.The refrigerant is usually butane, but since [Ben] was powering this pressurized system with an open flame, he opted instead for nonflammable perfluoropentane. The refrigerator body itself was 3D printed in clear resin, with a copper block transferring heat from the flame to the boiler chamber; ammonia solutions do dissolve copper, so this isn’t a long-term solution, but it works for a demonstration. The single internal path of the refrigerator presented some difficulties: since almost no pressure builds up between different parts of the system, a bubble in the wrong place can easily airlock it. In the end, [Ben] managed to get it working well enough to reach five degrees Celsius below the ambient temperature. Conveniently, he notes that, for alcohol at a sufficiently high concentration, you could cool your drink by burning a portion of it.Ammonia-based refrigerators are a tough build for most hackers, but we have seen a salt-water-based absorption refrigerator before. For more less-known refrigerators, check out our article on strange cooling methods.Thanks to [spacedog] for the tip!