The vital role of mortar analysis in heritage restoration
Just a couple of decades ago it was common for heritage structures to be restored with cement. The mindset tended to be along the lines of “chuck some cement in, make it strong and that’s that.” But experts in the heritage field are now seeing the negative impacts of taking this approach to the restoration of sensitive masonry features.
The use of cement has meant these structures can’t breathe, moisture is trapped, and salt ends up deteriorating the stone. The heritage structures end up being more expensive and time-consuming to fix in the long run, and their integrity can be jeopardized.
Mark Milevski, Materials Testing Specialist at Stone Initiatives, explains that sound heritage restoration is carried out with methods and materials that are a close match to those of the original structure – not just for aesthetic reasons, but for the health and longevity of the building.
“Modern materials can negatively impact the life of the building. Rather than just making it really strong and resistant to everything, we need to take a more sympathetic approach in order for any new work to be compatible.”
Mark is a geomaterials scientist who specialises in heritage and conservation, particularly in relation to the use of natural stone. One of the most fascinating elements of his role is analysing 100+-year-old mortar mixes and coming up with mix ratio guides so that heritage architects and stonemasons can recreate them, for use in restoration works.
Moonta Mines National Heritage Project
At South Australia’s Moonta Mines site, at the northern end of the Yorke Peninsula on traditional Nharangga land, Mark recently spent two days working alongside stonemason Stone Ideas on the National Trust SA’s Moonta Mines National Heritage Project. The federal government-funded project seeks to “restore, conserve and build new experiences” for the national heritage-listed precinct.
The site began operating as a copper mine in 1861, and today contains an important collection of nineteenth-century mining structures, including the Hughes and Richmans Enginehouses. Restoration works are being carried out in line with the Moonta Mines Conservation Management Plan prepared by heritage architect Swanbury Penglase for Copper Coast Council and National Trust SA.
Several mortar samples were taken by Swanbury Penglase from different locations at the site – both bedding and pointing mortar. These were analysed in the laboratories of Stone Initiatives, who then provided the mortar analysis results to the project team, to aid them in creating new mortar mixes that closely match the mortar used on the original structures.
Mark and the team at Stone Initiatives made a fascinating discovery in the process of analysing the mortar samples in the laboratory – one that turned out to be quite useful to the stonemasons in their restoration works.
Images: While Mark was on site, works included general stabilisation of the mining structures, as well as repointing and rendering of internal walls in the 1865 Hughes Enginehouse. Top–bottom, L–R: Moonta Mines Hughes Enginehouse (right) and chimney stack (left); Preparation of NHL lime render; Appearance of build-up render coat (building render up to existing original render thickness); Mark on site assisting with render application; Appearance of finish render coat (float finish – sponge).
“Through our analysis of the mortar samples we discovered that some combustion by-products from a nearby smelting facility – another mine on the Yorke Peninsula – had been used in the sand for the original mortar mix.”
“The stonemason was able to find something similar at a nearby facility,” Mark says. “They incorporated that into the new mortar mix to create a close match to the original.”
In addition to information such as aggregate sizes, sand types and details of any additives, Mark says the visual descriptions and images supplied in the mortar analysis reports were key to finding a match for the sands.
“In most cases that’s the crucial part. Lime hasn’t changed a lot over time, but it can be difficult to find the right sand. Back when most heritage projects were constructed, the stonemasons just used what was nearby and available to them – there was no standard sand type or size in a sand yard like today.”
The process of mortar analysis is not always straightforward, and often requires a degree of investigative work to reveal the information required to create the ‘recipe.’ The mix ratio guide formed by Stone Initiatives is then used by stonemasons and heritage architects, who might make a number of different ratios – more lime, less lime, more sand, less sand, for example – before coming to an ideal match.
Usually, a mortar board is created by the stonemason, to work out a colour palette using the aggregates, binders and any other additives identified in the report.
Sam Pentelow, founder of Stone Ideas, explains in a video filmed at the project site for National Trust SA that the stonemasons are “really looking to match the mortar exactly, because mortar reflects the time and the types of stone and brick that were used in the structure. In a conservation project it’s really important to get that right.”
Mortar analysis and recipe process explained
- A mortar sample of about 100 grams is requested (sometimes that much isn’t possible, in which case the lab works with what’s available)
- A portion of the sample is pulverised and undergoes X-ray diffraction testing to determine the mineralogy of the whole mortar, and to identify any unique features of the lime or cement.
- Another portion is dissolved in a dilute acid to remove the binder component (lime or cement). The sand component is usually a quartz, which is non-soluble in the acid solution, so this is left behind. Extra steps are needed if it is determined that the sand component includes soluble sand minerals (such as shell grit).
- The sand component is sieved through standard sieve sizes, to determine the size distribution (how course or fine).
- Under the microscope, the sand’s textural characteristics are determined (sharp, soft, staining, pigment etc).
- Using information gained through the XRD testing and the acid dissolution process, the volume of binder to the volume of sand is determined, to get the lime:sand ratio. Also identified are any additives or other unique features or components, such as brick dust or smelter by-products.
- All the information gained from the above processes is pulled together to create a mix ratio guide for the mortar, which includes comments regarding the likely source of the original sand, and descriptions of the sand’s physical appearance.
Images: Aggregate analysis at Stone Initiatives; mortar dissolution; Gold chalcopyrite on aggregate – chalcopyrite mined for copper; Combustion by-product – black vesicular material.
“When it all lines up and makes sense, it’s definitely satisfying,” Mark says. However, a lot of that does depend on sampling and the condition of the mortar. With heritage mortar, often the first thing to deteriorate is the lime; this means that a mortar sample can sometimes appear more sand rich than it really is, or was.
“If the sample is quite soft and sandy and falling apart, I’m already thinking ‘okay, there might be some portion of lime and/or sand missing due to environmental factors and weathering,’ and we take that into account in our analysis.”
Why shouldn’t cement be used in heritage restoration?
As we’ve already heard, the lime element in mortar is often the first part to deteriorate. This links directly to why cement should not be used in place of a lime:sand mortar on heritage restoration projects.
Essentially, it is best that the mortar around stone or brick is softer than the stone or brick itself – that way, if any moisture or salt gets absorbed though the masonry, the water can be evaporated through the softer and more porous component, the mortar. As it evaporates it leaves behind the salt, which deteriorates the mortar.
“If the mortar starts to deteriorate you can just scratch it out, repoint it, and the stone or brick is fine,” Mark explains. “But what was happening [in a lot of earlier heritage projects] is that when the lime mortar was deteriorating and falling apart it would be scratched out and replaced with cement, because, you know, it’s hard and strong. But then instead of the moisture and salt evaporating through the mortar, it’s evaporating through the stone, because that was now the more porous component.”
“If the stone itself starts to decay, you’re left with this picture-frame maze of hard cement pointing and the stones or bricks have all decayed. Mortar is a lot cheaper and easier to replace than stone.”
Mark says it is a similar situation with render. If a lime render is covering the stone, it’s still vapour permeable (breathable) and can let salts and moisture out of the structure. It will decay over time, but it can be scratched off and new lime render can be applied. Cement render, on the other hand, can trap salts and moisture behind it – eventually, everything behind the cement can deteriorate and the integrity of the heritage structure is jeopardized.
This is one of the reasons the work that Stone Initiatives does in mortar analysis is so vitally important to the future of our heritage structures, such as those at the Moonta Mines site.
“Knowing the history of the buildings and what they mean to the local community – there’s a rich history there and it’s rewarding being a part of restoring these significant structures,” Mark says.
“What we do is support the project team to make informed decisions about how best to care for the heritage structures, in a way that is respectful of the built heritage and the stories it tells so that they can have life into the future.”
Project: Moonta Mines National Heritage Project
Stone consultant: Stone Initiatives
Scope: Laboratory analysis prior to restoration works – mortar compositional analysis
Client: National Trust
Heritage architect: Swanbury Penglase
Stonemason: Stone Ideas
Photography: Mark Milevski
Find out more about mortar analysis and heritage services here.