“Ancient Doors”

The Pantheon doors, estimated to have been installed in 115 AD during Emperor Hadrian’s reconstruction, were solid bronze, measuring 7.53 meters high and 2.3 meters wide, with a weight of 8.5 tons. The hinge designed by Hadrian’s engineers consisted of what seemed to be a simple design. They were bronze pins set into the floor at the bottom and into an architrave at the top. The system was perfectly balanced such that anyone could easily open and close the doors when they were not barred.

In 1757 the bronze doors began to stick; the bronze pins had been riding perfectly on the stone shafts for over 1,600 years. The city authorities of Rome decided repairs were necessary to keep the Pantheon secure and accessible. The Romans had maintained these doors without difficulty for centuries, but somewhere along the lines the information failed to pass to the next generation. The 17th century craftsmen knew how the balancing of the massive doors worked, they knew how to adjust the architrave, lift the doors, and clean and replace the pins when necessary. However the 18th century craftsmen tasked with repairing the doors in 1757 did not know what they were doing; clearly something had been forgotten.

These craftsmen were tasked with replacing the pins and adjusting the plates; they took careful measurements and reproduced what they thought were exactly the same pins and plates. They even measured the consistence of the bronze to ensure the same material strength. However something went horribly wrong. On the day of the attempted repair they began removing the pins and the massive bronze door shifted. The weight distribution changed and the door fell, killing Corsini, the master mason. The other craftsmen on the job managed with great difficulty to reinstall the doors, but they were barely functional. The left door only partially opened and the right door was completely blocked. These ancient mechanisms which functioned flawlessly for sixteen centuries were now broken, and nobody knew how to fix them.

The city gave up and for 241 years, from 1757 to 1998, visitors to the Pantheon had to enter through one partially opened door. If you look up photos of the Pantheon from the 1970s, you will clearly see one door stuck and the other partially open—a living monument to lost knowledge. The 17th century Romans understood something that 18th century Rome forgot. The Pantheon doors were a masterpiece of ancient engineering. They contained no bearings, used no counterweights, and yet opened and closed with ease. The secret lies entirely in precise calculation and fine tuning of the moving parts. The bronze pins had to be perfectly placed, the weight had to be distributed evenly and the balance had to be flawless.

The ancient Romans were working without any modern technology, no computers, no software, no calculators, no modern metallurgy, no precision instruments, or at least precision instruments that we have found and understand. Yet they created something that functioned either without maintenance, or with very limited maintenance for nearly two thousand years. The doors were made of bronze, which is 90% copper and 10% tin, weighing 540 pounds per cubic foot. Casting even a single door of this magnitude is something that would be challenging, even today. Nobody knows exactly how the Romans made the doors. What we do know is that they created something that baffled later generations.

In 1998 modern engineers decided to make another attempt to repair the doors, this time with much more caution after the failure two hundred years earlier killed the lead engineer. They took very detailed measurements, studied the mechanism in great detail, created scale models, and tried several methods before arriving at a very simple solution.

After 241 years of being stuck and killing a master mason, and stumping two and half centuries of engineers, the Pantheon doors are fully functional once again. The repairs took several months, even with modern tools and knowledge. The ancient mechanism, once we fully understood it, required something that had the 18th Century Romans understood, they would never have disassembled the mechanism to replace parts in the first place. As it turns out, the ancient doors just required a special lubricant—an ancient mixture of soap.

These are now the oldest functioning doors in the world, including an over 2000-year-old functional lock. They survived the rise and fall of the Roman Empire, the medieval period when bronze was scarce and most bronze artifacts were melted down. They survived because the Pantheon was converted to a church in 609 AD, which gave it protection from looting. You can even find marks near the hinges where barbarians attempted to unhinge them and failed. The facts in the story come from The Medieval and Modern History Vault. Until next week, stay safe and learn something new.

Scott Hamilton is an Expert in Emerging Technologies at ATOS and can be reached with questions and comments via email to shamilton@techshepherd.org or through his website at https://www.techshepherd.org.