'Worm Casts' in Historic Lime Render

My local church, St Mary’s, has stood at the heart of Billingshurst for over 900 years. With its soaring tower, graceful stone arches, flagstone floor, and stained glass windows, it’s a striking example of English ecclesiastical architecture — and a truly atmospheric place to visit.

But it’s not just the grandeur inside that tells a story. Walking around the church’s exterior, you’ll notice areas of beautiful pink lime mortar, studded with flint, shell, and local stone. Look closely at the render, and you’ll see something fascinating: the surface ripples in delicate patterns, almost like a fingerprint. These aren’t tool marks or intentional decoration — they’re the result of a natural process called hyaline weathering, more commonly known as 'worm casts.'

'Worm casts' in the lime mortar at St Mary's Church in Billingshurst

Despite the name, these markings aren’t biological. They form over time as the softer lime binder in traditional render gradually erodes under the influence of wind, rain, and freeze-thaw cycles. The coarse aggregates — like flint and shell — remain intact, while the surrounding lime weathers away, leaving a surface of meandering ridges and troughs that resemble the tracks of tiny creatures.

At St Mary’s, this effect is most noticeable on the southern elevation, where exposure to the sun and prevailing weather accelerates the process. Far from being a flaw, these textures are a testament to the building’s age, materials, and craftsmanship. They reflect the use of breathable lime mortars, applied by hand over irregular masonry — and they’ve evolved slowly, shaped by the passing centuries.

In conservation work, these features are often carefully preserved. They speak to the authenticity, resilience, and layered history of a building like St Mary’s — a structure that continues to weather time with grace.