13th November 2018 – Tufa stone in medieval Fingal and beyond




by  Niall Roycroft
(Skerries Historical Society Meeting – November 2018)

Tufa stone in medieval Fingal and beyond
How many people know what tufa is? If you’re one of the ones who does, it’s odds on you’re a geologist or you were at the Skerries Historical Society’s November talk where Niall Roycroft presented ‘Tufa Stone in medieval Fingal and beyond’. Niall’s talk was really two talks in one as it explored both geology and history/archeology.
Tufa is an unusual type of stone in that it is neither igneous nor sedimentary rock. It’s not even metamorphic. It’s organic! How’s that? Isn’t stone, a mineral, the very antithesis of organic? Niall explained that when spring water, supersaturated with lime, trickles over vegetation, the lime is precipitated out of the water to form a matrix around the plants. The plants are slowly petrified – they’re turned to stone: tufa stone.
The East coast is peppered with tufa – Skerries in particular boasts a multitude of petrifying springs at Barnageeragh. And we have a responsibility to look after them as the plants associated with these petrifying springs are often rare mosses and liverworts that only grow where tufa forms.
But that’s not the only reason that tufa is important. As far back as the 11th and 12th centuries, stonemasons had discovered that tufa was an ideal building material. It’s immensely strong but very
light – perfect for creating a vaulted ceiling on which to pitch a roof. Tufa, by its nature, is porous so any tufa vault would have to be covered in slate or wooden shingles to keep the rain out.
Tufa’s other virtue is that it is very easy to cut and shape. It was frequently used for window surrounds. The fact that it’s not waterproof wasn’t a problem when it was used on the inside of buildings. The medieval builders simply covered the tufa with a good plaster render and it looked fantastic – as though it were a much more expensive type of worked stone.
Both these uses of tufa can be seen in the ruins of the 12th century monastery on St Patrick’s Island. Did the tufa come from Barnageeragh? We can’t be sure. And sadly the deterioration of the monastery is accelerating. Niall showed us paintings and photographs dating from 150 years ago through to the present day. In the 1860s the vault over the chancel, where the altar would have stood, was still intact. Fifty years later the vault had fallen and now only a couple of bits of wall remain. It seems that the cost of restoring it would be prohibitive. A piece of history important not only to Skerries, but to Ireland as a whole, will surely be lost within the next century.

Report by Oona Roycroft