Webinar on Mortars for Conservation: Part 1 History and Materials

Mortars for Conservation Part 1 – History and Materials 

Speakers: Alison Henry and John Stewart 

Alice [00:00] Good. Well, over to you, Allison. 

Alison [00:03] OK, well thanks very much, Alice, and good afternoon everyone, and welcome to this Technical Tuesday webinar on mortars for conservation. My name's Alison Henry, and I'm head of building conservation in the technical conservation team at Historic England. 

John [00:25] And my name is John Stewart, and I am senior building conservation advisor in Alison's team. 

Alison [00:33] Thanks, John. So before we make a proper start, and for those of you who are new to Technical Tuesday, I'd just like to let you know a bit more about the technical conservation team. We're a group of technical specialists, including conservators, scientists and engineers, and we do research and develop advice and guidance on technical conservation issues. The first link that Alice has pasted into the chat for you takes you to the main technical advice page on our website, where you can find all our technical advice. And all our standalone research reports and advice notes are free to download as PDFs from our website, and the second link takes you to the catalogue shown on the right where they're all listed. 

We also thought it might be helpful if we let you know each time we produce a new technical note or new web guidance, so we've developed a monthly technical conservation update, and I think Alice is going to put a little poll on the screen. So if you'd like to be added to the mailing list, just click 'Yes', and I promise you won't be inundated with emails. It really will just be once a month, and it will be just the technical conservation update, not wider HE news. So, I will just give that a few minutes. If you don't remember whether you've already signed up, don't worry. If you sign up now, we won't send you multiple copies. So if in doubt, click 'Yes'. So, still people signing up. I think that's more or less stabilised. 

OK, well, let's crack on, and I'm going to hand over to John to explain what we're going to cover today. John. 

John [02:25] Thank you, Allison. Here is the structure of today's webinar. We'll outline the materials and broad development of mortars in England through the pre-industrial, industrial and modern periods, ending with a reflection on our current knowledge and practice. This is essentially an introduction to the following two webinars on conservation mortars, which will address mortar analysis and then specification. 

Alison [02:55] OK, thanks, John. So before we go on to describe the materials and history of mortars in England, it's worth considering how we know what those materials and history are, so what evidence do we have? And there are two main types. Firstly, documentary sources such as paintings, engravings and old photographs. These might show how materials were processed and they can give us a snapshot of how buildings used to look, which can tell us something about how mortar was used. And as well as images, there are also various contemporary written sources, things like [old estate?] records, building accounts and various instructional guides. 

But we've got to be a bit careful about relying on written sources, particularly some of the earlier ones. Firstly, in the past, of course, many people couldn't write. And if they could, it's fairly unlikely that they would record commonplace everyday activities such as mixing mortar, so much of what we would like to know now simply went unrecorded. Secondly, the people who could write were often the better educated and not the people who were actually getting their hands dirty doing the work. So it's quite possible that on some occasions this misunderstood what they were observing or missed some subtle point, and it's also more likely that those people were observing rather specialist or high-status projects rather than run-of-the-mill vernacular building.  

Another major problem is that terminology has changed over time. In many early documents, it's clear from the context that the word mortar referred to earthen mortar, not lime mortar. And in other documents, the word plaster refers just to plaster of Paris, which is made of gypsum and not lime plaster, and the word lime itself nearly always referred to quick lime and not lime putty. But by the time we get to the late-18th century, we start seeing treatises written by engineers, but it's really important to remember that for them the holy grail was strength, which of course was needed for civil engineering and military structures, but that's quite different to what was needed and indeed what was used for most other buildings. So we really can't assume that what engineers were recommending was used in other branches of building. And finally, we find that some writers simply copied the work of earlier authors, so mistakes were often repeated and indeed became sort of taken as gospel. So while these written sources are really useful, we've got to be aware of their limitations, and certainly we can't take them at face value using modern terminology.  

The second type of evidence is physical evidence. So this means looking at the actual mortars used on buildings, and we might be able to see different phases of construction and distinguish between mortars used for different purposes such as bedding mortar and pointing water, and identify surface treatments such as render and limewash. And once you get your eye in, you can tell a lot just by looking at or feeling old mortar, but for more detailed information we can analyse them. So this can give us detailed information about binders, aggregates and additives, but here too, a large degree of interpretation is needed because some materials are very hard to detect, the same materials might be present in a mortar of different forms and materials may have altered over time because of weathering. So mortar analysis is a highly skilled process, and in fact, we're going to cover that in more detail in our next webinar on mortars later this month. 

But you might be interested in this publication from Historic Environment Scotland, and I think Alice is going to copy the link for that. They recently completed a research project, which involved digitising and reviewing all the records of mortar analyses carried out by the Scottish Lime Centre Trust over the last 25 years. And this allows us to see some of the spatial and temporal trends that we're going to cover later, and it provides some really useful information about the range of mortar types used in the past. 

So most mortars comprise a binder and some form of aggregate which provides bulk and strength, and from our assessment of the evidence, we can see that a wide range of mortar binders were used in the past, and this depended on the availability of materials, building status and to some extent function. And when we talk about historic mortars, we very often think just of lime and forget that earth and gypsum have had very long histories of use, and we also often overlook the fact that from the 18th century onwards, the nature of lime-based materials began to change enormously as natural and early artificial cements were increasingly used. So it's really important to be aware of this range of materials because you never know when you're going to encounter one of these materials. 

John [07:58] The performance of a mortar depends just as much on its aggregates as on its binder. Aggregates act as a filler, adding bulk and strength to the mortar, prevent shrinkage and also contribute to porosity, texture and colour. Aggregates used in the past include sub-soil and unwashed sand, washed sands [in grid?], [uncrushed?] chalk and limestone, seashells, ash and even lumps of old recycled mortar. And it's also worth reminding ourselves that mortars were used for a wide range of applications in the past. Obviously, things like bedding and pointing masonry and rendering and plastering are very wide spread, but mortars were also used for parging or lining chimney flues, for making solid and suspended floors such as lime ash floors and plaster floors, for floor pugging of timber floors – that's a sort of early form of stand-proofing for suspended floors – as well as for bedding stone and slate roofing, for torching the underside of roofing slates and tiles, for making artificial cast stone, such as polyimide on the far right, and for lime-based concretes. 

Alison [09:22] OK, so now we'll look at mortar materials in more detail, starting with earth, and it seems to me that we are really only just beginning to recognise the extent to which earth was used for mortars and plaster in the past. And by earth, I mean sub-soil in which there is a proportion of clay, and it's the clay that acts as the binder, with a course of sand and grit forming the aggregate. And I suppose it's not really surprising that in areas where earth was used as the main building material, such as for cob or wattle and daub and such like. Earth in daubs and plasters and even external renders were common, as shown on the two left-hand images. Also, in areas where there was no limestone to make lime, builders may have had no choice but to rely on earth for masonry construction, because the cost of importing lime over more than a few miles would've made it far too expensive for most people to use for general construction, so where lime was used in these areas, it was generally reserved for absolutely essential applications such as pointing earth-bedded masonry to give it a bit more weather resistance, or a thin plaster skin coat to give a fine finish over earth and plaster backing coats, and for protective lime wash. 

And this practice of very often applying a lime finishing coat over earthen materials perhaps explains why earth and mortars are so often overlooked, but I found really surprising when I started researching this is the extent to which earth was used in areas where lime was plentiful. The upper-middle picture shows the wall of a house in Dorset, which is built of limestone, and it's less than half a mile from a lime kiln, but the stone is bedded in earthen mortar with just a protective outer layer of lime pointing. And you find this time and time again that even in lime-rich areas, earth and bedding mortars were widely used. 

And another surprise is how much earthen plaster there is, but again it's usually hidden by a thin skim of lime or a few coats of lime, so very often we don't know it's there until there is some damage, as shown in the lower-middle image. And you might think that earthen mortars and plaster were just used for low-status buildings, but that wasn't the case. The top-right image shows York House in Malton in North Yorkshire, and this is a really high-status building, yet all the bedding mortars and all the plaster base coats on walls and ceilings are of earth. And we've even got earthen plaster base coats on some amazing high quality decorative plaster ceilings, such as this one in Dartmouth in Devon, shown on the bottom right. 

And why wouldn't we find it everywhere and on buildings of all status? It was readily available and cheaper than lime but also eminently fit for purpose, and as the cases above demonstrate it was the material of choice even where there were alternatives. And in fact, although its use started to decline through the 19th century, it was still being used into the early 20th century. But until recently, I think we've tempted to focus on lime, and we've had a bit of a blind spot when it comes to earthen mortars. We did a webinar on earthen mortars last year actually, so if you want to find out more, then you can find the recording of that on the Technical Tuesday website, but that's enough on earth for the time being, and I'm going to hand over to John now to talk about lime. 

John [12:43] Well, I'm sure that most of you are familiar with lime mortar chemistry and terminology, but I'll give a brief introduction for those of you who are not. In terms of lime mortars, the minerology of different limestone determines the properties of the eventual mortar, so simplistically this depends on the amount of clay and purities in the rock. Water made from pure limestone sets by reaction with air. Lime from rock with clay impurities sets in reaction with water as well as air, producing hydraulic lime. Hydraulic properties increase in proportion to the quantity of clay minerals present, as shown in the intermediate boxes. 

Most non-hydraulic lime was made by burning limestone or chalk containing a high proportion of calcium carbonate, at least, say, 94%, to form calcium oxide, otherwise called quick lime. The kiln temperatures need to exceed 800 degrees centigrade. It is then slaked by the addition of water to either a putty or a powder, which is calcium hydroxide, or slaked lime. When used as a binder in mortar or plaster or limewash, the calcium hydroxide absorbs carbon dioxide from the air and converts back to calcium carbonate. This is the lime cycle. Now, this and the following diagrams are from our book Mortars, Renders and Plasters in the practical building conservation series, for future reference, and of course there you'll find more legible copies. 

The cycle for hydraulic lime is more complex. When water is added to calcined hydraulic lime, it slakes in the same way as non-hydraulic lime. And if sufficient water is available, the calcium silicates and aluminates will also convert to form solid interlocking crystals of calcium silicate hydrate and calcium aluminate hydrate, which are responsible for the initial setting of hydraulic lime. 

The terms hydraulic and hydrated are often confused. As I said, hydraulic refers to the ability of certain types of lime to harden by reaction with water. Feebly, moderately and eminently hydraulic are the traditional designations of hydraulic limes. In contrast, hydrated refers to a fabrication process: the reaction in which water is combined to the crystalised structure of a material. It is synonymous with slaking. That is the reaction of quick lime with water to produce a powder or putty. It is also the process when calcium silicates and calcium aluminate in hydraulic lime react with water to form hydraulic compounds. 

Alison [15:56] OK, so let's look at methods of slaking in a bit more detail. With non-hydraulic lime, if you add more water than is needed to slake the quick lime, you'll end up with a wet material called lime putty. This process is illustrated in various historic images, such as this 18th-century French engraving – top left – and to the right you can see the same set-up in more modern times, with the lime putty collecting in a tank or pit. Lime putty can be stored indefinitely, provided it's protected from the atmosphere and not allowed to dry out. Then, when it's needed, it can be mixed with aggregates to form mortar, or fine finishing coats for plaster, and it was also sometimes used on its own for bedding very fine masonry or gauged brick work, or it could be diluted with water to form limewash. 

Now, hydraulic limes were generally slaked differently. Because they react with water to harden, they can't be slaked to a putty and stored for very long because the excess water will trigger the hydraulic reaction and the lime will set and become unusable. In the past, hydraulic quick lime was usually first mixed with the aggregate and then slaked by adding just enough water to slake the quick lime but not so much that it turns to putty. So you end up with a dry blend of slaked lime and aggregate, and this could be easily mixed together, and then more water added to make a usable mortar. And this dry slaking, or hot-mixing process, could be used for non-hydraulic lime, too. Making mortar this way, especially before mechanisation, was much easier than mixing stiff lime putty and aggregates together. And in fact, we now think that this was the way the majority of mortars and plaster-based coats were made, and again, this method of mixing as well, illustrated in historic images such as the 17th-century one on the right, they often show mortar being mixed under some sort of roofed cover, presumably to protect the quick lime from reacting with rain. And sometimes it's also possible to tell by analysing a historic mortar, whether it was made by this hot-mixing method. 

John [18:11] Turning now to gypsum mortars. When gypsum, or calcium sulphate dihydrate, is heated at comparatively low temperatures between 150 and 160 degrees centigrade, some of the water of crystallisation is driven off to form calcium sulphate hemihydrate. This is ground to a fine powder, or commonly known as plaster of Paris. When water is added to it, the plaster sets within a few minutes. The setting process recombines water into the crystal structure, forming hard crystalline gypsum. No aggregate is needed as it does not shrink on setting like lime without an aggregate. 

We'll now look at pre-industrial mortars, starting with Roman mortars. Well, lime burning technology dates back millennia, but little is known about the use of lime for building in pre-Roman Britain, as in this hypothetical hamlet in the left image. As far as I know, archaeological records haven't been systematically reviewed, but the Romans certainly imported their advanced lime-building technology into England for engineering works and buildings of high status. In contrast, their humbler buildings used earth and mortars. As for what went [indistinct] found in Roman London in waterlogged conditions around the Thames. 

Now, non-hydraulic lime can gain hydraulic properties with the addition of aggregates containing silica and alumina in a form which is reactive with calcium hydroxide, shown on the far left. These are called pozzolans or set additives. We know that the smaller particles act as the pozzolan and the larger ones as aggregates, albeit with some reaction around the edges. The Roman architect Marcus Vitruvius Pollio describes lime mortars in his book De architectura, also known as the Ten Books of Architecture from the first century AD. This includes the use of pozzolans of grand ceramic tile for hydraulic installations such as bath basins. These were the [indistinct] mortars that were ubiquitous around the Roman empire, including England. The engraving on the right is of an 18th-century mill to crush brick or tile from mortar, and the Romans probably had similar primitive crushers. 

Alison [20:55] OK, so moving on to the early medieval period following the collapse of the Roman empire, masonry building in England was on a much smaller scale, and mainly restricted to churches built of stone. There was a general demise of brick and ceramic tile production and revival only began in the 13th century. With the Norman invasion, there was a resurgence in stone construction, particularly for building castles, abbeys, churches and manor houses. To our knowledge, there's been no systematic study of medieval mortars, but we know from samples taken from buildings that earthen mortars were used, and local limestone would obviously have been exploited where it was available. The fact that so many medieval buildings survive is testament to the quality of their construction, and this must reflect to some extent the mortars that we use both for building and for surface protection. But what I find really amazing is the extent to which some of these survive as substantial ruins despite deliberate dismantling and robbing of the stone, deliberate attempts to blow them up, as happened in the 17th century at Bridge North Castle in Shropshire, shown on the bottom right, and often centuries of neglect. 

John [22:13] Calcined gypsum, or plaster of Paris, was imported into England from France from the 13th century, and it is cited in The History of the King's Works from that period for plaster work. Claire Gapper's research on renaissance plasters in England cites the use of gypsum in Henry VIII's palaces. 

As we've already stated, exploitation of local natural materials was standard in the pre-industrial age. So where natural gypsum predominated, as in Nottinghamshire, Staffordshire and Derbyshire, it was invariably used. The tradition of suspended floors of gypsum in these areas is well known, as shown on the left. Some samples of gypsum floors are illustrated in the middle, but the wider application of gypsum mortars in the pre-industrial period in England hasn't really been thoroughly studied. However, in France, Spain and Italy, there's been a recent, renewed interest in regional historic gypsum mortars. In these countries, they have been found on a regional basis as bedding mortars, plasterers, of course, and even external renders, as far north as Paris. 

Alison [23:32] OK, so turning back to lime, I mentioned earlier in the context of earthen mortars that in areas where there was no lime, it was imported for those absolutely vital protective or decorative uses. Over short inland distances, horse transport was used, but before the development of canals and railways, the easiest way to transport lime any distance was by sea. Where limestone outcrops close to the sea, there were usually lime kilns close by. The top-left image shows lime kilns at Lindisfarne in the 17th century, from which lime was shipped to Scotland as ballast in exchange for coal. And below are later 19th-century kilns on the beach at Lindisfarne. Sometimes kilns were very small, such as on the top right, on the Somerset coast, but in other places, such as Beadnell in Northumberland, shown bottom right, larger banks of kilns served a thriving coastal trade. 

Now, as we explained earlier, it was the geology that determined the nature of any limestone in a particular locality and therefore whether the lime produced from it was non-hydraulic or had hydraulic properties to some extent. And obviously, the properties of lime from different sources varied geographically, but it also sometimes varied within the beds of stone in a single quarry. For example, in some places the upper beds of the blue lias limestone yield a feebly hydraulic lime, whereas lower down the lime has stronger hydraulic properties. And use of these different types of lime was based on practical experience acquired over many centuries. 

In areas that had a range of limes available, there is evidence that they were used according to their properties, for example, their known resistance to weathering or their ability to set in damp conditions. And Richard [Niamh?] in his book first published in 1703 distinguished between lime made of soft stone or chalk, which was used for plastering ceilings and interior walls, and lime made from hardstone, which was suitable for bedding masonry and for plastering walls on the outside, and we now know that the hardstone lime he referred to was slightly hydraulic lime. So this sort of example, it illustrates how specific limestones were selected through local knowledge for particular purposes even though the mineralogical properties of the stone were not properly understood. 

John, I think you're on mute. Ah, right. 

John [26:13] Yes, I've cited the use of ceramic pozzolan in the Roman period. It's not clear if crushed tile or brick was subsequently used in lime mortars. Brick and tile largely ceased in late antiquity, as Allison has said, and only really revived in the late medieval period, so the practice may have been lost. But mortars with guaranteed hydraulic properties were still required for important engineering works as bridge foundations.  

From the late 17th century, there are citations of the use of German trass to provide hydraulic set-in-lime mortars. Trass is a greyish volcanic tooth from the Eiffel region in western Germany. It consists of silica and feldspar and reacts with calcium hydroxide to form tricalcium disilicate hydrate. It compounded of hydraulic lime. On the left is a trass quarry in the mid-18th century. Ground trass became the pre-eminent pozzolan in the 17th and 18th centuries, at least in parts of England. The quarry material was shipped to Holland for grinding and was then exported in casks. According to modern historians, this practice was commercialised around 1600. 

So the import of trass into Britain probably occurred shortly after. In the year from March 1691, 175 barrels of trass were imported into the port of London through Custom House, shown here on the upper right. Trass was expensive, so it was limited to specific applications, for example, facings of embankments, drains, sewers, mill heads, the types of works illustrated in the contemporary French engraving in the centre. And probably the largest use of trass was in the new Westminster bridge, designed by the Swiss engineer Charles Labelye. It was begun in 1738 and completed in 1750 and was one of the largest engineering projects in the country at the time. 

Alison [28:30] OK, so turning now to mortars in the industrial age. At the beginning of the industrial revolution, the first holy grail of mortar production was a reliable hydraulic mortar for engineering and military construction. In the 1750s the engineer John Smeaton was commissioned to design a new Eddystone Lighthouse off the coast of Plymouth, which was in a notoriously hostile environment. This prompted him to undertake detailed research into hydraulic mortars, including trips to the continent to investigate trass production, but he also surveyed existing structures in the South West. 

And in 1757, he was struck by the mortar in the foundations of a little bridge at Dunster in Somerset, which was built with a local blue lias lime, and that proved remarkably durable. So he did some experiments with different limes, including blue lias lime, and as well as using various pozzolans to determine their ability to settle underwater, and this confirmed that blue lias lime indeed had excellent hydraulic properties. But more importantly, Smeaton accurately related hydraulicity to the presence of reactive clay minerals in the limestone, and this was hugely significant because it meant that for the first time, it was possible to predict which limestones could produce a useful hydraulic lime. 

John [30:05] As Allison has said, Smeaton's experiments included lime with pozzolans. These were of German trass and Italian pozzolano. The pozzolan is a pulverised volcanic product, primarily composed of impure illuminous silicate and varies from dark red to purple, in colour. It reacts with calcium hydroxide in the presence of water, similarly to trass, to form hydraulic compounds. The material derives its name from Pozzuoli in the bay of Naples north of Vesuvius, approximately where this 18th-century painting was executed. There are other Italian sources and notably north of Rome, and it was famously exploited by the Romans for their lime concrete. On the top right is a Pyreneesian engraving of 1756, which dramatically shows the collapsed monolithic mass of lime concrete in this building on the Appian Way. For the Eddystone Lighthouse, Smeaton ultimately chose a mortar of [Aberfar?] blue lias lime with pozzolana, as this combination produced the fastest setting mortar. This was the first major use of pozzolana in Britain. By the third quarter of the 18th century, Smeaton was advocating and specifying the use of Italian pozzolano over trass in his other works as the many harbour enclosures he built around England. One of these is the breakwater at Ramsgate Harbour of 1787 on the bottom right. 

As Smeaton's book was only published 32 years after the completion of the Eddystone Lighthouse, the impact of his research only really began in the early 19th century. This had a profound influence on engineering works and research for the next 40 years of so. Hydraulic limes could be variable in nature as [indistinct] has said, depending on their quarry source and mineralogy, so the addition of pozzolano showed a good hydraulic set. So Italian pozzolano supplanted German trass for engineering applications from the end of the 18th century. Many major engineering works of the early 19th century emulated Smeaton's Eddystone precedent, using hydraulic lime with pozzolano. For example, water for Robert Stevenson's Bell Rock Lighthouse on the fourth, shown on the left, utilised blue lias transported from Somerset and pozzolano from Italy, as Smeaton's lighthouse, and there are many other examples of lime pozzolano mortars of the period, including Waterloo Bridge in London, designed by John Rennie, on the right. 

Alison [33:02] So, pozzolano was imported into Britain as ballast, but during the Napoleonic wars in the early 19th century, the French blockaded imports from the Mediterranean to Britain, and this was at a time of huge industrial development, when there were burgeoning demands for strong, quick-setting cements from new transport infrastructures and construction of factories as well as the military requirements of the empire. So clearly relying on imported materials for these projects was hugely problematic, so the new holy grail of mortars in the early 19th century was to reduce reliance on such imports and instead develop indigenous sources of reliable hydraulic materials, and ironically it was the French engineer Louis Vicat who had a major influence. He continued research in a similar vein to Smeaton, exploring the properties of lime with more scientific rigour and for the first time defining classes of limes – air lime, feebly, moderately and eminently hydraulic limes. His most influential publication, Mortars and Cements of 1828, was translated into English by Captain Smith of the Madras Engineers and published in 1837. 

And in parallel with Vicat's work, there was further research in Britain, notably by the Royal Engineers, and one of the most important works was Observations on Limes, Calcareous Cements, Mortars, Stuccos, and Concrete by Captain Charles Pasley of the Royal Engineers, and that was published in 1838. But all the researchers of this time were still fascinated by the technology of ancient Roman construction, with its tenacious mortars and enduring concretes, and the picture on the right is a French engraving of 1812 of a Roman ruin in Italy, and it shows how the strong mortar is holding the structure together despite the collapse of the masonry below. 

And this growing understanding of the relationship between clay and hydraulicity was further exploited in the use of very clay-rich limestones for the so-called natural cements. Now, these are like strong hydraulic limes but with a higher proportion of silica, alumina and iron oxide to calcium carbonate. They were made by burning septaria, which are these sort of huge nodules of clay-rich limestone criss-crossed with calcite veins, shown on the top left. And these occur in the cliffs and on beaches at various locations around the coast, including Dorset, Hampshire, Suffolk and Yorkshire, and James Parker's Roman cement, patented in 1796, was the most prominent natural cement. But other manufacturers soon followed suit, and by the early 19th century, there were dozens of natural cement plants around the country. Natural cement was very hard and strong and set very quickly, within about 30 minutes of mixing with water, which had obvious advantages for engineering, as it meant that less centring, and works could progress much faster than when using slower-setting mortars, and the engineer Brunel used bricks set in Roman cement to line the walls of the Thames tunnel, which was completed in 1843, shown in the painting at the bottom left. 

Natural cements were also widely used for external rendering, often to imitate more expensive ashless stonework. Sometimes they were painting with oil paints, as shown in the centre, but often they were self-coloured and could be remarkably convincing. The natural cement render in the bottom right-hand image, which is on a country house in Dorset, was described both by Pevsner and the Historic England listing inspector as natural stone. And unfortunately, natural cement was also sometimes used for repointing, which often caused accelerated erosion of the stone around it. 

Both natural hydraulic lime and natural cements depended on the mineral composition of the parent rock and the firing temperature. Now, natural variations in rock within a quarry and inconsistent firing temperatures in early kilns would have resulted in variable products. But for exacting engineering applications, you need really reliable performance. So the obvious answer was to artificially replicate the best natural combinations of lime and clay, and by burning crushed limestone with clay or shale, and artificial cement could be produced. Joseph Aspdin's patent of 1824 is generally accepted as the first variety of Portland cement, so named because it was supposed to look similar to Portland stone. However, it was burned at too low a temperature to realise the full reactive potential of the clay minerals, so it was probably more like an eminently hydraulic lime than a modern Portland cement, and it was Aspdins assistant Isaac Johnson who realised that burning at a higher temperature produced a better material. So kilns were modified to increase the draft and raise the temperature, which is Aspdin's works at Gateshead, which was later taken over by Johnson. 

From the 1840s, there were many patents and hundreds of small firms around the country were producing artificial cement but often alongside hydraulic lime and natural cement, such as those at Rugby and the Isle of Sheppey, shown on the right. And it wasn't until the 1890s that large scale commercial production of Portland cement started. 

John [38:49] Of course, all of these developments we've described in the past few slides were about serving engineering transport and military works and, in fact, improved transport networks also facilitating movement of hydraulic mortar materials around the country. But these hydraulic limes and cement mortars emphasised in contemporary research and publications overshadow the fact that many traditional practices continued throughout the 19th century. Even by the end of the century, vernacular and low-status buildings continued to be built with local materials and lime mortars, particularly weak lime mortars where available. 

The next real milestone in mortar development was the creation of formal national standards. One of the earliest British standards was for Portland cement, and that was BS 12 of 1904. This served the need for reliable materials for exacting engineering applications, but a British standard for building limes did not appear until 1940 as BS 890, reflecting their diminishing importance to industry. BS 890 included classifications for high calcium lime, that is non-hydraulic lime and semi-hydraulic lime or feebly hydraulic lime. A further standard just for hydraulic limes was planned, but this never appeared, perhaps because of the difficulty of commercial classification of such a variable geological material. 

Against this contemporary emphasis on cement, Alfred Denise Cowper produced a report in 1927 for the building research station, entitled Lime and Lime Mortars, which influenced only 20th-century building conservation practice for [indistinct] for example of the Ministry of Works in their extensive programme of restoration of ancient monuments as Bodiam Castle. Cowper adapted Vicat's lime classifications, shown here in the centre table from our book Mortars, Renders and Plasters. In practice, these classifications were only used informally by producers and practitioners to describe commercial products from specific geological sources. 

Alison [41:24] Even by the end of the 19th century, problems of cracking shrinkage and excessive strength were being observed in some cement mortars, and there were those who questioned the sense of a bonding agent that was stronger than the materials that had to be joined together. So one solution to this problem was to mix lime with the cement to reduce its strength, and various cement-lime-sand mixes were developed between the Wars, such as 114, 116, 129 and 1312, and most of those remain familiar even today. And early in the 20th century, these led to the development of patent cements, modified with hydrated lime or powdered limestone fillers, and these were known as masonry cements, which became mainstream products in construction and repair. 

As these new cement-based materials were increasingly adopted, the hydraulic lime industry faltered in the third quarter of the 20th century. Similarly, the production of lime putty declined as plaster board and gypsum became the standard materials for internal plastering. However, the interest in hydraulic lime was not entirely lost, and in the early 1970s, the Cathedral Works Organisation at Chichester Cathedral began importing a hydraulic lime from France. But the revival of lime mortars received its greatest boost with the programme of conservation of the 12th-century limestone sculpture and architectural detail on the west front at Wells Cathedral. This was done between 1974 and 1986, and the stonework was conserved using new techniques developed by Robert Baker, involving the use of non-hydraulic lime putty mortars, which came to be known as the lime method. 

Refractory brick dust was added as a pozzolan, but this was used very differently to the way in which pozzolans had been used in earlier centuries. Then they had been added in large quantities as reactive aggregates, but at Wells the pozzolan was very finely ground, which made it more reactive and added in only very, very small quantities. And this has now become standard practice when using pozzolans in conservation mortars. 

Sorry, I've just overshot, I think. So after Wells, to a large extent, modern lime mortar practice derived from a combination of trial and error: the reading of selective historical texts, observations of existing mortars and accounts of the last few surviving tradesmen experienced in traditional crafts. But there was, in fact, some misunderstanding of the historical literature, particularly as I mentioned earlier, failing to recognise that the term lime nearly always referred to quick lime and not slate lime. And also, many sources were ignored, and the materials available were different to those used historically, so all this meant that modern practice focused on the very pure non-hydraulic lime binders that were commercially available. And though the aim was to replicate historic mortars, the mortars that were made this way were really significantly different in many ways. 

John [44:46] So as Allison said in this period, non-hydraulic lime was widely used but sometimes indiscriminately in inappropriate locations such as highly exposed wall heads of ancient monuments. This resulted in several large mortar failures, costing tens of thousands of pounds. One of these was at the gatehouse of Corfe Castle, where a new wall head capping mortar of non-hydraulic lime was applied in 1988/89, and this soon failed. It resulted in stone falling from height and calcium hydroxide leaching through the core and carbonating on the gateway ashlar below. 

In the private domain, various lime training centres were established in this period around the country, specifically to promote good practice, but in the public domain, several research initiatives sought a more scientific understanding of lime mortar from the mid-1980s. The Smeaton project of English heritage was begun in 1985 at Hadrian's Wall, initially with exposure trials of commercially available materials. This defined the need for more controlled laboratory investigations that followed with the building research establishment. Other research programmes were undertaken by the National Trust and a consortium of interests led by Bristol University. 

So we stated the lime revival was based entirely on non-hydraulic limes slaked to putty. In reality, UK production of hydraulic limes had ceased, as Allison has said. During the 1980s only one was being imported from France. John Asher's hugely influential practical building conservation series did present hydraulic limes and the historical practice of hot mixed slaking, but these were ascribed for academic interest rather than practical advice or application. More thorough historic research only began to appear in the early 1990s, based on historical literature such as builders' manuals. Gerard Lynch's book, with a history of brick mortars, was published in 1994, the same year that my initial research results were published in the Ashby transactions. Over the 1990s, I undertook a literature review of a very large number of historical publications in various languages from the Middle Ages to the early 20th century. This helped to clarify real historical practice, including the prevalence of hot mixing, historic mixes and the strategic selection of hydraulic limes where available. 

Alison [48:02] Increasing awareness of the potential of natural hydraulic limes for conservation, and even for new-build construction, led to more hydraulic limes being imported from continental Europe, and in 1995 production of blue lias lime resumed at Torr Quarry in Somerset, but very sadly this production was short lived, mainly because of local opposition. In 1995, the first modern lime standard, which included natural hydraulic limes, or NHLs, was adopted, and this was EN 459. This classified natural hydraulic lime, mainly according to its minimum compressive strength. It also introduced standards for other limes with hydraulic properties, termed either, simply, hydraulic lime, or HL, or formulated limes, or FL, but it's the NHRs, the natural hydraulic limes that are most widely used in conservation. 

But the standard is problematic for specifiers and users. Firstly, testing is done using standard mortars made with a special laboratory sand and a ratio of water to binder that results in a mortar with unworkable consistency for building, but it just happens to be ideal for ramming into moulds to make little prisms for mechanical testing. Secondly, all testing is carried out at 28 days. Now, this is the same age at which Portland cement is tested, but Portland cement gains 80% of its strength in the first four weeks, so testing then makes sense, whereas lime mortars continue getting stronger over a much longer period. So testing strength after 28 days isn't very informative. 

So classification of an NHL according to EN 459 doesn't really give us the full picture about the long-term performance of these binders when made into real mortars for building or conservation work. And there's another problem. The parameters for each class of NHL are very widely set, so for example, a standard sample of an NHL 2 mortar must reach a minimum of 2 megapascals at 28 days, but it could reach up to 7, as shown on the first line of that table, and still be classified as NHL 2. So this means that potentially, limes with significantly different properties can be classified as the same type. But also, there is overlap between the classes, and this means that limes with similar strength between 5 and 7 megapascals at 28 days – that's the darker green section on the table – could be classified as any one of the three different types. And the objective of this methodology is simply to ensure that test results from any laboratory in Europe are comparable. The N 459 is a production standard for manufacturers, and it was never intended to produce relevant data for the end-user. 

John [51:18] So by the early-21st century, mortars for conservation could be characterised by better understanding of historical materials in practice, material properties and availability of a broader palette of materials. However, there was still a great disparity between historical and mortar and lime binders. This diagram of the lime spectrum shows relative compressor strengths of historical and modern binders along with their mineral content. The historical designations of Vicat are in the red box in the centre, and the modern British-standard ones are in the blue box below. There is certainly no correspondence between the two, and the modern ones are much stronger. 

Also, a major omission from the current British standard is feebly hydraulic lime, shown in the red oval within the red box, which was so much used in traditional construction. And some modern lime producers falsely equate the historical and modern designations, for example, describing their NHL 2 product as being a feebly hydraulic lime. This is incorrect. It is much, much stronger than the historical material. 

Alison [52:42] OK, so we now know from documentary research and analysis of historic mortar samples that the vast majority of lime mortars used in the past for most ordinary construction were either non-hydraulic or feebly hydraulic, contained a much higher proportion of lime than typical lime putty or NHL mortars and were generally made by hot-mixing quick lime and aggregates rather than first slaking the quick lime to putty. And recognition of this has been growing in the last decade or so and has prompted renewed interest in mixing. Typical mixes consist of one part of quick lime and three parts of aggregate. As the quick lime slakes, it expands in volume, making the mortar more lime rich than, say, A 1–3 lime putty mortar, and pozzolans can be added if additional strength is needed, and although some lime mortar suppliers are making ready mixed non-hydraulic mortars by hot-mixing, it is essentially still a site-based craft practice. 

And finally, in the spectrum of materials, we should also mention that large numbers of pre-mixed proprietary mortars are available nowadays. Now, some of these may be suitable for conservation, but as most of them contain a lot of undeclared ingredients, it's hard to be sure. But one thing is clear, though, and that is that few, if any of them, bear much resemblance to most historic mortars. 

John [54:18] These recent developments have also been accompanied by an evolution in mortar advice. In the past, statutory organisations like English Heritage promoted repair with materials that were, quote, 'like for like', unquote. This may have been a useful mantra at the time to combat the use of cement mortars. A change of attitude is embodied in the conservation principles of English Heritage, published in 2008. This promotes a values-based approach to conservation decision making centred on the relative significance of a historical feature or building. And in our book Mortars, Renders and Plasters, which Allison and I co-edited, there is great emphasis on the understanding on the condition of fabric and its environment as a basis for specifying the most appropriate repair rather than offering prescriptive mortar mixes. 

Alison [55:21] OK, so we hope that this has given you an overview of the complexity of historic mortars. The past two centuries of mortar development, in particular, have been defined by several distinct holy grails, specific to the needs of their periods: reliable hydraulic mortars from the 18th century, natural and artificial cements of the late-18th and early-19th centuries using British materials, standardisation of cement binders, the first lime revival replacing cement mortars for historic buildings and now hot-mixed lime-rich mortars trying to replicate traditional mortars more closely than very strong NHL mortars. 

John [56:07] So the goal posts still do continue to change. This means that conservation professionals need to be aware of changes in materials and recommended practice and avoid blanket prescriptions. Thank you. 

Alison [56:21] Thank you. 

Alice [56:29] Right, thank you very much, Allison and John. I wanted to ask the audience if you have any questions, please do type them in the chat box. I do have one question that came from Fiona [Deaton?] for Allison and John. What about prompt natural cement that can be applied in particularly wet areas such as bridges or harbours, rivers or canal walls? 

Alison [56:51] Shall I do that? Shall I tackle that one, John? Yeah, John's on mute, so I'll answer that. Yeah, that's absolutely-- 

John [57:01] Yes please, yes please, yeah. 

Alison [57:03] That's absolutely fine, particularly if you're dealing with something that was built or lined with natural hydraulic cement, but it comes back to understanding what you're dealing with because of course, there might be certain circumstances where that modern natural cement would be too strong. But our next webinar coming up in a few weeks' time is going to be looking at that process of deciding what are the appropriate materials? But yeah, natural cement is one of the things that is still available. 

Alice [57:39] Another question just in. A few questions are popping in. That's great everybody. So this one is from Michael [Dubrovsky?]. Why is damp such an issue with using modern cement on older buildings? I thought lime was the key to halting this, but it seems that many different types of mortars have been traditionally used. Any comments on that question? 

Alison [58:00] Well, again I can sort of respond to that. Yeah, lots of mortars were used in the past. They weren't always successful in different scenarios, and I mentioned that natural cement had been used for repointing and that it was pretty disastrous for that, so it's really about understanding, yes, what was used in the past, but whether actually replicating that is sensible. It may be that that's the right thing to do, but you've really got to look at how the material performed as well. I don't know if that answers it. Yeah. 

Alice [58:43] That's great. Sorry, just trying to read in the questions. Thank you, Allison. I think a lot of it comes down to sort of what you've mentioned before, Allison – understanding the materials that are there or ready and what has been used in the past to repair. I have another one from Clyde Whittaker. His question is, we are aware of an elevated house with a highly exposed south-west facing gable wall of solid sandstone, which has recently been repointed with non-hydraulic lime putty. The wall now takes in water. Would repointing with moderately hydraulic lime be a better option? 

Alison [59:20] Well, probably worth listening to the recordings of the conference that we did before Christmas, which is available on the technical advice web page, because we talked about the problems there of driving rain penetration. It's probably-- Well, it is too complicated to try and tackle just in a quick answer, I'm afraid. But yeah, bizarrely we often do find that pointing and repointing sometimes can cause more problems, and traditionally so many buildings were rendered, but as I say, that's sort of another topic in itself. So it's possible that using a slightly stronger material might be appropriate, but so often the failures that I see are actually down to workmanship – cracking and crazing – and very small cracks can let in huge amounts of water, so I think it would need to be looked at very closely, yeah. 

Alice [01:00:24] OK, that's great. I'm not sure if this-- This feels like it could be a longer question from [Haddas Ricks?]. He's asking if there's a possibility for you to explain when to use which-- when is NHL 2 versus hydrated lime recommended where the original material failed? But I feel that could be a much deeper conversation. 

Alison [01:00:46] Well, that's actually a webinar in itself, and that's the next one on 23 March. We're going to be talking about specifying-- Oh no, sorry, no. That's the summer. 

Alice [01:00:54] Here we go. 

Alison [01:00:54] No, no, sorry. That's not the March one. The March one's analysis, but there's going to be one on specification in the summer series, so yeah, we couldn't cover everything in one webinar today. But we will do that. 

Alice [01:01:07] Great. So [Haddas?], if you could just wait for the next webinar, and we'll be able to answer that question more fully. 

Alison [01:01:10] I think it's going to be in June. 

Alice [01:01:16] And I think that's the end of our questions at this point. We have a few comments within the chat that are, sort of, everybody's encountering issues with failed lime grouting and pointing. And Tom [Nisbett's?] asking if there's any further research being conducted that might help, of the understanding of its use for hot-lime grouting and pointing.  

Alison [01:01:40] I mean, we are doing research on all these things, but in most cases, there isn't a simple answer. There's a combination of reasons why things fail and why they perform well, so we are doing research on all those issues. Well, partly as a-- 

Alice [01:02:00] It's not an easy answer is it. 

Alison [01:02:01] Well, no. There is no easy answer. We've been researching these things for years now, since the 1980s, and I'm afraid we still don't have all the answers. 

Alice [01:02:11] Yeah. Well, thank you very much. I think that will draw us to a close, so thank you, Allison, and thank you, John. I would ask you both to go on mute, and thank you to our participants today and our guests. You have been a great audience. I'm sorry that there were a few technical issues for a few people. I would ask you please to sign out now and thank you very much.