Archaeologists unearth medieval convent’s heating system during renovations in Switzerland

Gnadental convent was built outside Basel's city wall for Franciscans around 1231, later becoming a temporary home for several orders.

A well-preserved medieval stone-chamber heating system was found during renovations of a property in Basel, Switzerland, the Canton of Basel-Stadt announced in late September.

The system had once belonged to the now-excavated Gnadental convent for women, established in around 1231 outside of Basel’s city walls, and was found beneath what had once served as the dining hall in the east wing of the convent.

It is almost completely preserved, and is one of the most significantly known examples of heating systems of its type in both Switzerland and Europe at large.

Based on the sites’s architectural history, archaeologists have estimated that the heating system was built between the 13th and 14th centuries.

The underground chamber where the heating system was found contained fieldstones which served to store heat, typical of stone-chamber heating systems. After removing the stones, archaeologists were able to study the firing channel beneath.

Made up of ashlar blocks, the channel also revealed two pointed arches standing in their original position.

Additionally, eight closable openings were still carefully preserved. Warm air would exit through these openings, rising from the heating chamber into the dining hall once the smoke from lighting the fire had cleared.

After uncovering the chamber, the Archaeological Soil Research Basel-Stadt created a 3D model of the find for additional documentation in research. The find will be added to the Petersgraben 52 renovation project and be subject to further study down the line.

What is the Gnadental convent?

Gnadental convent was built outside the city wall for Franciscans around 1231, later becoming a temporary home for several orders while their own convents were built inside the city or before they left.

Some 20 years after construction, the Franciscans moved from Gnadental convent to the newer Barfüsser Monastery, founded in what is now Barfüsserplatz. From then on, the older convent buildings housed several women's orders, including the Cistercian nuns and the Order of Poor Clares.

In 1279, lay sisters from Gnadenthal near Niederwil (AG) moved in, after whom the convent received its name. They were incorporated into the Poor Clares and lived there until the Reformation and the dissolution of the convent.

After the convent was abandoned, the buildings continued to be partially used. In 1573, building materials were taken from the convent to be reused into a granary.

Watercolor paintings done by Johann Jakob Schneider from the 19th century still depict the older convent buildings. However, towards the end of the 19th century, the granary was demolished to make way for a vocational school at the site.