Medieval formwork timber.
Situated at the far end of the prison (room 9), these wooden shingles and slats were retained in the 13th-century gothic vaults. They formed part of the old timberwork, laid in anticipation of the construction of the jack arches – between the ribbed vaults – and were designed to hold the bonding mortar between the blocks until it set.
These relics are a precious testament, perhaps the only one of its kind in Europe, to medieval construction methods; they were used to make the ribs, i.e. the wooden moulds that acted as the negative for the vault being made, and which kept the stones in place in the mortar until it was fully set.
In the early 1990s, some of the slats were dismantled. A dendrochronological analysis was then carried out – observing the rings in the cut wood –in a laboratory without core-sampling. This enabled the slats and shingles to be dated back to around 1250. This date, prior to that given in historical texts, was unsurprising given that the pieces had been reused from a castle roof they had formed part of for many years beforehand. In fact, the shingles were replaced by tiles in the 13th century.
These relics were probably forgotten about in the darkness of the prison, unless medieval masons simply did not see it fit to remove them, given the new penal role their surroundings would go on to play.
The entire north-west part of the castle is supported by these gothic vaults, and they themselves are supported by seven molasse columns.
Medieval formwork timber