The history of architecture is in some respects the history of not only the materials that make up a particular building but also the binders and mortars in between that keep them together.
This is why there were notable shifts in the scale, scope and types of buildings constructed when hydraulic lime mortar was first used by the Roman Empire in Great Britain, and again when it was rediscovered for the construction of Smeaton’s Tower in 1759.
However, prior to 43 AD when the Romans conquered Britain and took their well-documented construction techniques with them, it can sometimes be more difficult to determine whether an ancient building used mortar or freestanding dry stones, a difference vital to authentic building restoration.
Probably the most mysterious long lasting yet largely lost construction technique is the use of earth mortars, the rediscovery of which is not only of importance to conservationists of heritage buildings but also could be a potential option for green construction in the future.
The Forgotten Core
Part of the reason why earthen mortars primarily composed of subsoils with a lot of clay material have gone somewhat underappreciated was that they were primarily used as the central core mortar that would be pointed or repointed with another mixture such as lime.
This meant that a lot of older buildings were misidentified as being made completely with lime mortar binding or without any binding at all in old enough cases where the original earth and clay have been removed completely.
According to a guide by Historic Environment Scotland, a lot of clay mortar buildings were discovered by accident during other building works, typically when a facing stone has been removed to reveal the material backing it.
Part of the reason for this is that earthen mortars such as clay are exceptionally vulnerable to water, so a preventative measure is typically to use a more weather-resistant material for pointing, typically in the form of a weak lime mortar.
Because it is often hidden, there is a misconception that clay mortars are weaker than their lime alternatives for construction, which is typically only the case when the mortar is already exposed.
In well-maintained, regularly repaired historic buildings such as churches, clay mortar can reside for centuries and retain the same level of compressive strength.
If it is exposed and suffers a high level of water ingress, the clay can be broken down by the water and effectively flow out of it, leaving a dry stone structure and often leading to misconceptions that a majority of mediaeval structures were constructed in this way.
How Do Moisture Levels Affect Earth Mortars?
If moisture levels in walls and other such structures are raised, this can cause significant issues for earthen mortars. If enough of this moisture builds up, structural strength can be compromised and it can return to a plastic state.
Of course, it’s important to note that this is a rare occurrence and is generally only found in old ruins. However, you can also ascertain damp levels in structures by assessing moisture in earthen mortar, thus potentially being able to resolve issues in a timely fashion to ensure the health of buildings and other structures.
Interestingly, the grading of earth mortar has a part to play in how resilient it is and how well it will hold up against progressive decay. Research from Historic Environment Scotland, for example, shows that particle size distribution is the main factor in how well mortar performs.
If it is well graded with a good range of particle sizes, it will then have good working qualities and stand up well over the years.
Why Were Earthen Mortars Used?
The primary reason for the use of earthen mortars is that they could be made with plentiful local materials, particularly in places such as Scotland, where the subsoil has an abundance of clay.
Of course, earthen mortars have been used in some of the oldest cities in the world such as the ancient cities of Jericho and Ganj Dareh.
The key difference, and why far more evidence of early clay mortars exist in Palestine, Iran and Pakistan is that they are all hot countries with a relative lack of humidity in dry seasons, meaning that there lack the prevailing damp conditions that can completely wipe out earth mortars.
However, in the famously rainy United Kingdom, a lot of early evidence of clay mortars is sadly lost, although why the practice rapidly died out is far more well-known.
Why Did It Stop Getting Used?
Tracing the history of earthen mortars in the UK is exceptionally difficult, as a lot of early evidence of ancient British buildings has been lost to time, either through erosion, destruction or reconstruction using more advanced building techniques.
After the Romans abandoned the United Kingdom in an era that has since become known as Sub-Roman Britain, a lot of Roman technologies left with it, including hydraulic lime.
This meant that from the fifth century until the mid-18th century, a lot of buildings relied on the use of cured lime mortar, often backed by clay and earth mortars in the core, to say nothing about the thousands of years of construction before the Roman Conquest which used the same materials.
To this day, debates continue to rage about exactly why so many technologies were lost and rediscovered thousands of years later.
Regardless, for as long as there has been masonry, earth mortars have been used, and whilst the rediscovery of hydraulic lime and the development of portland cement and concrete would invariably lead to changes in construction methods, it would take much longer than one would expect for this to take effect.
According to the Earth Mortars research project, which attempts to estimate the reach of earth and clay mortars in Scotland specifically, approximately three-quarters of masonry buildings were constructed from earth mortars prior to 1800.
This was helped by many areas of Scotland having subsoils rich in clay, meaning that rural areas had a strong building material all around them.
However, by the start of the 19th century, the Industrial Revolution was beginning to take hold, and with vast changes to agriculture and the development of industry, so many historic buildings created using earthen materials were demolished and replaced.
As industrial transport links became wider and more efficient, older building techniques started to dry out. In just 50 years, the number of buildings using earthen mortars reduced from 75 per cent to just 25 per cent. Fifty years later, the practice was almost entirely eradicated.
That failure to understand the potential of earthen and clay mortars, as well as a lack of understanding of the importance of architectural conservation, nearly wiped out a construction tradition that had existed for over 5500 years.
This has been harmful in a number of ways, as it has not only hurt the understanding of how we historically constructed homes and shelters and affected potential surveying and restoration efforts, but it has also stymied the potential for its use in a future construction industry that cannot depend on concrete.
With the carbon dioxide emissions of concrete a substantial sustainability challenge, earth mortars could potentially be a locally sourced, easily reused, low-carbon alternative, strengthened using modern technologies and building techniques mindful of clay’s unique properties.
Repair Techniques
When carrying out repairs on earth mortar structures, similar processes and techniques will be used for other masonry work, such as lime mortar repairs.
However, it’s generally a lot safer to conduct earthen mortar repairs because you don’t have to worry about the likes of caustic, heat and chemical issues that you get when you work with lime.
Many churches and masonry structures in the UK and Ireland that were built before 1800 contain earth mortars. It’s important to note that using incompatible cementitious or hard-setting natural hydraulic lime mortars for repointing can cause serious issues for earth-built structures.
To ensure the best results, hot-mixed air lime mortars and renders should be used for surface treatments, as they offer high breathability for earth mortars.
By repointing earth-mortared masonry structures with hot-mixed air lime mortars, they become more thermally efficient, less vulnerable to salt or frost attack, and generally healthier. When repairing earth mortar, it’s important to follow similar principles and techniques as lime mortar repairs, but with lower health and safety risks.
It is recommended that consolidation and initial deep pointing be done in a minimum moisture earth mortar to minimize shrinkage and reduce the time before pointing can be applied. Some residual moisture can be beneficial for reducing the risk of shallow lime pointing drying out too quickly.
Repair Mortars
Repair mortars should ideally match the original construction material in character, appearance, and performance. In some cases, altering the mortar for better performance may be necessary for places such as church ruins that are exposed to more moisture than the original construction.
The first step in repairing mortar is understanding the original material, which can be done through visual inspection, field tests, and laboratory analysis. The local subsoil is the most appropriate material for repairs, but it can be difficult to source, and time should be allotted for this task.
Pre-mixed earth mortars are available but expensive, and tailored mixing using bulk clay and sand materials is often more cost-effective and time-efficient.
Non-local earth mortar is preferable to lime mortars, and hydraulic limes and cement should never be used for repairs. Quicklime can be added to new earth mortar mixed to a sloppy consistency to engage the clays most effectively, and sand can be added to poorly graded local earth if necessary.
Other additives can be included to manipulate the mortar’s performance in special situations, but good grading is key to ensuring good performance.
