Medieval stone jigsaw keeps King's heads dry

The construction techniques that medieval masons used to create King's College Chapel in Cambridge, one of the most celebrated medieval buildings in Europe and an icon for higher education, have been revealed for the first time.

Scholars have long speculated on how the fan-vaulted ceiling was put together. But a five-year research programme, prompted by concern about possible instability of the ceiling, has now come up with answers.

The masons, it seems, used pegs and string to create an interlocking three-dimensional stone jigsaw.

Geometrical principles and gravity, rather than calculations and bolts, held it all together, echoing the myth of Cambridge's Mathematical Bridge, which is said (wrongly) to have been originally built by Sir Isaac Newton in such a way as to not need any screws or nails.

The research team, directed by Joe DiVanna, a business consultant with a strong interest in medieval history, now aims to build a full-scale replica of a single bay to test their ideas.

Work on King's College Chapel began in 1444 and was finished by master mason John Wastell, who constructed the ceiling between 1512 and 1515. It is divided into 12 bays that span 13.5m with massive stone bosses at their centres.

The research team mapped the ceiling using an astronomical telescope, a digital camera and a laser guidance system.

There were fears that the heavy stone bosses might be in danger of coming loose as it was thought they may have been pinned in place using iron rods.

But the team found that they were hollow, much lighter than previously believed, made in two tapering parts and were incorporated into the ceiling like stoppers in a bottle. "They are not in any danger of falling," Mr DiVanna said.

The joints between blocks showed that the vaulting had been worked out from its centre onto walls that were not quite square.

A network of fine marks and thousands of wooden pegs emerged on the upperside of the ceiling. These allowed the masons to plot out the design using string lines to ensure that the irregular space was compensated for and to align the complex mouldings on the underside of each block.

Mr DiVanna said there were still some outstanding puzzles. "We still don't have a definitive answer as to how exactly the masons built the ceiling, but we have gotten a lot closer," he said.