Glorious mud

On a sunny Wednesday May afternoon, I popped along to site to chat to Ben Morton, our Project Manager at Doughton Manor. Ben has a background in ecclesiastical stone masonry and has worked on some incredible structures, including Worcester Cathedral. His enthusiasm for the subject was surprisingly contagious and his exceptional knowledge on stone and mortars became clear immediately (specialist subject for Mastermind perhaps?). However, so rarely is it used, that the actual application of earth mortar isn’t common for him either!

Grade I listed buildings represent the pinnacle of national heritage significance in the United Kingdom. As structures of exceptional historical and architectural value, they demand a meticulous approach to conservation. Among the most under-appreciated yet vitally important aspects of preserving these buildings is the correct specification of mortar. Mortar is not merely a binding agent; it is a component that directly affects a building’s breathability, stability, appearance, and longevity.

Traditionally, historic buildings – particularly those built before the 20th century – were constructed using lime-based mortars. These mortars, made from lime putty or quicklime and aggregates (and occasionally earth), were softer and more porous than modern cement-based mixes. They allowed buildings to “breathe”, accommodating moisture movement and thermal expansion, while also enabling easier maintenance and repair. At Doughton Manor, there are a multitude of different applications of earth and lime mortars across different areas of the building and it has been critical to identify them all.

One of the most common and damaging mistakes in historic building conservation is the use of hard, impermeable cement-based mortars. Such mortars can lead to:

• Moisture retention and freeze-thaw damage: Impermeable mortar traps moisture within the masonry units, which can freeze and expand, causing spalling and structural degradation.
• Mechanical incompatibility: Cement mortars are significantly harder than historic masonry, which can result in stress concentrations and cracking of the original fabric.
• Aesthetic dissonance: Modern mortars often differ in colour, texture, and profile, detracting from the visual authenticity of the building.
• Loss of reversibility: Cement mortars are difficult to remove without damaging surrounding historic materials, compromising the principle of reversible intervention.

Before any intervention, a comprehensive mortar analysis is essential. At Doughton Manor www.roseofjericho.co.uk have provided this service. Techniques may include:

• Microscopic examination to determine the binder-to-aggregate ratio.
• Chemical analysis to assess the composition and identify additives.
• Petrographic analysis to study the mineral content and bonding characteristics.

So, Rose of Jericho were asked to analyse the bedding mortar found at Doughton Manor. The samples extracted when Tom Balch (Director) visited, were very dark brown in colour and extremely soft and crumbly. He said, “It wasn’t clear whether they were simple earth mortars or whether a binder had been added. Historically many early mortars were comprised of little more than sub soil dug locally to the site. Being rich in clay they had good binding properties and were often used as bedding mortars. However, such mortars are not very durable when wet and can be washed away by rain being directed into joints. Often such walls would then have been finished with a lime rich pointing mortar, often buttered over the arises of the stones.”

Tom expanded on his theme and added, “Sometimes such earth mortars would be stabilised by the addition of lime to the mix. We chemically and microscopically analysed the bedding mortar sample and found it to be a lime stabilised earth mortar.” He went on to explain that the earth, or sub soil, was likely originally dug within the grounds of Doughton Manor and represented the colour of the local geology.

To replicate the mortar, rather than dig sub soil on site, it was decided to use a commercially available processed, screened loam from Ongar in Essex and combine this at a ratio of 1 part lime to 3 parts Ongar Loam. The loam is a mix of sand, silt and soil, similar, but not an exact match to the earth found at Doughton Manor. The mortar produced was a little lighter and greyer than the original but it will function and perform in a similar way, being flexible and highly permeable.

As per the original mortar the joints will be finished with a separate lime rich rough cast render to protect this soft earth mortar.

When it comes to application, the services of skilled artisans is also vital. At Castellum we have our own stone masons who are experienced and adept, particularly when it comes to Cotswold stone of course. At Doughton Manor we are also using the services of a specialist stone masonry company www.brunelstonemasonry.co.uk

• Preparation of joints: Mortar joints should be raked out carefully to avoid damaging adjacent masonry.
• Pre-wetting: Damping the masonry before repointing prevents rapid drying and improves bond strength.
• Layering and compaction: Mortar should be applied in layers and well-compacted to avoid voids.
• Curing: Slow, controlled curing (ideally protected from direct sun and wind) is vital to prevent premature drying and shrinkage. At Doughton Manor, the new mortar is daily misted and protected by hessian sacking to slow the drying process further.

Getting the mortar right in a Grade I listed building is not a minor detail – it is a fundamental component of responsible conservation. Earth based and lime-based mortars, when specified and applied with sensitivity and expertise, not only protect historic masonry but also ensure the building’s legacy for future generations. As stewards of cultural heritage, conservation professionals have a duty to respect the material language of the past, and earth and mortar, though humble, speak volumes.

Click here to see some work in action!

This month we are pleased to announce that Oliver Stockbridge (Construction Manager in London) has been confirmed as a CIOB member. This is a massive achievement – brilliant news Ollie.

In February’s journal, we shared the happy news that Flo (Assistant Site Manager in Surrey) had said YES. Well, her and Tom tied the knot at the end of last month and are now husband and wife. Huge congratulations to them!