Hot Mixed Lime and Traditional Mortars E-Book

Nigel Copsey

• Herausgeber: Crowood
• Kategorie: Lebensstil
• Sprache: Englisch

Veröffentlichungsjahr: 2019

Beschreibung

Traditional mortars are eminently workable, effectively porous, economic in use and appropriately durable. Used in buildings for thousands of years, these materials are ideal for repair and conservation work. Unlike cement or modern hydraulic lime, their routine use would make a significant contribution in the struggle against climate change. However, despite the 1975 'lime revival' there remains a deficit in research into the most-used traditional mortars. This book seeks to redress the balance. Drawing upon historic literature, material science and industry case studies, topics covered include: a historical overview of traditional mortars; slaking and mixing hot mixed lime mortars; the essentials of pozzolans, aggregates and lime tempering; plasters, lime washes and sheltercoats, and finally, the revival of interest in the use of like-for-like and compatible mortars. A comprehensive book on the true understanding of hot mixing limes and traditional mortars for the twenty-first century. It is fully illustrated with 286 colour photographs and 11 line artworks. Nigel Copsey is an accredited conservator-restorer and works as a Research Associate in the Dept of Archaeology, University of York.

Leseprobe

Hot Mixed Lime and Traditional Mortars

A PRACTICAL GUIDE TO THEIR USE INCONSERVATION AND REPAIR

Nigel Copsey

The Crowood Press

First published in 2019 by

The Crowood Press Ltd

Ramsbury, Marlborough

Wiltshire SN8 2HR

www.crowood.com

This e-book first published in 2019

© Nigel Copsey 2019

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

ISBN 978-1-78500-556-5

Dedication

This book is dedicated to my wife, Vanessa; Emma Michel, my apprentice – soon to spread her wings – and to my particular comrades and collaborators in the ‘Hot Mix Gang’: Ben Gourley, Patrick McAfee, Ivor McElveen, Craig Frew, Bill Revie, Chris Pennock, Richard Jordan, David Wiggins, Oliver Coe and Alison Henry. Also to all those masons and conservators working with traditional mortars once more and adding to our common well of knowledge and experience.

Contents

Introduction

Chapter 1 Why We Should Use Traditional Mortars and a Brief History of Their Demise

Chapter 2 Earth and Earth-Lime Mortars

Chapter 3 Hot-Mixed Lime Mortars – Slaking and Mixing

Chapter 4 Hot-Mixed Lime Mortars – Mortar Proportions

Chapter 5 Pozzolans, Aggregates and Lime Tempering

Chapter 6 Plasters, Limewashes and Sheltercoats

Chapter 7 Hot Lime Grouts and Concretes

Chapter 8 The Problem with Natural Hydraulic Limes (NHLs)

Chapter 9 Functional Behaviour of Traditional Lime Mortar – David Wiggins

Chapter 10 Hot-Mixing for the Twenty-First Century

Bibliography

Index

Introduction

UNTIL VERY RECENTLY THE PREVALENCE of earth-lime and hot-mixed lime mortars within the masonry and timber-framed buildings of the UK, Europe and the Americas has been substantially ignored by the conservation and academic communities alike, although they represent the primary mortars of craft practice and construction over thousands of years. Inadvertently, therefore, the repair of traditional buildings has rarely observed the principle of ‘like-for-like’ repair to which all in the industry subscribe and has often – but not always – involved the use of incompatible and frequently damaging materials. Over the last few years, and with increasing momentum and spread, this situation has begun to change.

Four years ago a group of interested masons and professionals from the UK and Ireland gathered in Edinburgh to discuss hot-mixed mortars. They took confidence from one another’s practical experience of using them, and growing knowledge about them, and set about challenging accepted conservation practices in both countries. They supported this with a growing body of research, prompting – by their promotion of hot-mixing and their reasons for doing it – the commissioning of important research by Historic England and Historic Environment Scotland. Since then the ‘lime world’ has been turned upside down and guidance, as well as on-site practice, has shifted dramatically.

Numerous high-profile buildings around the UK are being repaired with hot-mixed air lime mortars, with and without small pozzolanic addition, where previously NHL, mainly, or lime putty, would have been used. More than 4,000 individuals – masons, plasterers and professionals working in the conservation field, as well as general builders and homeowners – have attended hot-mixed mortar symposia and demonstrations, and the great majority of these have gone on to specify and to use them around the British Isles. Most masons who use a hot-mixed mortar would not choose to revert to other kinds of lime mortar, reflecting very quickly the preferences and priorities of their forebears who constructed the buildings they now work upon with highly workable, useful and economic earth-lime or hot-mixed air or feebly hydraulic limes. Many masons new to the material are making valuable contributions to our common understanding, experimenting and exploring the potential of these materials in a dynamic and empowering process. Hot-mixing is spreading steadily across Europe – particularly northern Europe, in parts of which it never entirely died out – as well as into North America. Hot-mix projects are underway in Western Canada and there are small projects across the USA, all being executed by masons tuned in to events in the UK. This trend is only likely to accelerate in coming years.

This shift is of great significance, not only for the health of our common stock of traditional buildings, but for that of the planet as a whole. The production of ordinary Portland cement is a major contributor to global warming (natural hydraulic lime contributes also, but at a much lower level). Portland cement production accounts for 8 per cent of greenhouse gas emissions: 4 per cent from the release of carbon dioxide from the fired limestone component; 4 per cent from fuel emissions (Olivier et al., 2015; Andrew, 2018).

Only the free lime content of an unslaked or slaked lime can take back the carbon dioxide produced when the original limestones were fired. A typical NHL 5.0 has around 15 per cent free lime; a typical NHL 3.5 has around 25 per cent free lime. A typical air quicklime has at least 96 per cent free lime, all of which, in the absence of pozzolanic addition, will slake to a hydrate or to a mortar; the lime content of this will comprise a similar percentage of free lime, which will ultimately convert to a similar percentage of calcium carbonate, reabsorbing the majority of the carbon dioxide emitted during production.

The readoption of traditional building mortars for new build construction, where appropriate, would make a significant contribution to the reduction of damaging and potentially catastrophic climate change. There is no compelling reason beyond shortterm profit and the deskilling of craftspeople for the construction of domestic dwellings to be carried out in either Portland cement or NHL. Air limes, typically hot-mixed, made feebly hydraulic by the addition of pozzolans (themselves by-products of other polluting industries), or with naturally feebly hydraulic limes, meet all the requirements of international building codes for smaller-scale domestic dwellings. A thorough reassessment of the priorities and assumptions of these building regulations could extend the reach of traditional materials even further into the field of modern construction, as well as leading to a massive reskilling across the world.

It is to be hoped that this book may make a small contribution to this outcome.

Chapter 1

WHY WE SHOULD USE TRADITIONAL MORTARS AND A BRIEF HISTORY OF THEIR DEMISE

The needs of old buildings are simple, but have been complicated and compromised by modern building technology and the mindsets and the materials that go with it. A building of traditional solid-wall construction needs to breathe; which is to say that the materials from which it was built were generally porous. More than this, they were effectively porous. Porosity and ‘vapour permeability’ alone do not necessarily express the effective porosity of the materials used originally, or for repair. This is the critical insight of David Wiggins’s research and the critical flaw in many eco or traditional building repair products, many of which are aggressively marketed and relatively expensive.

Traditional Building Materials

The materials from which traditional buildings were made were quite simple and straightforward: quicklime, earth, sand, animal hair, grasses and the by-products of other economic activity, such as brick dust, wood ash, forge scales, coal ash (in industrial centres). All were readily available, their relationships in use straightforward, and generally inexpensive. Most were locally sourced – most of the time – although the arrival of canals and, more especially, of the railways, saw materials spread further afield more readily from localized markets than had been possible by road or river transport. Even so, most of the materials used remained essentially local in origin, with more specialist materials – such as volcanic pozzolanic additives from Italy or from Germany – brought in as necessary and where they could be afforded.

Wood was a general, though by no means widespread, exception, with softwoods from the Baltic arriving from the thirteenth century at least. Their spread relied upon good ‘natural’ transportation networks – sea and river mainly – but also road where this was deemed essential or advantageous. Baltic lumber was routinely imported into Scarborough, North Yorkshire, from the 1300s onwards. Oak is rarely seen in the buildings of North-East Yorkshire, whereas in West Sussex, where navigable rivers were scarce, oak predominated well into the modern age. Species such as larch were imported to be grown for local markets, and imperial endeavour brought huge quantities of lumber from different parts of the world at different times, the extensive importation of Douglas Fir from the north-west coast of North America being one of the more obvious developments from the later nineteenth century.

Pantiles were initially imported from the Low Countries into North-East Yorkshire, but by the later 1730s were being made locally. Bricks, too, were imported into eastern England from Europe, though soft and very porous bricks were being made around Hull from at least the thirteenth century. For larger projects they were increasingly being made on site, along with the lime, as well as by commercial brick makers.

Malton, North Yorkshire, 1728. Painted by John Settrington. This is truly sustainable vernacular architecture. All of the materials of construction seen here were sourced from the immediate vicinity of the town: stone and lime from local quarries; bricks and pantiles from brickworks within or across the river from the town; earth for mortars from adjacent common lands; sand from the river or from nearby pits; thatch from nearby fields.

Traditional Building Practice

This book is an attempt to reassert the simplicity of traditional building practice and the relative simplicity, good sense and sustainability of traditional building materials, particularly of mortars, the ‘lungs’ and the ‘kidneys’ of any solid-wall construction. It seeks to celebrate craft and craft understanding, both of which came under increasing assault by a variety of often self-appointed ‘experts’, as well as by engineers, from the mid-nineteenth century onwards, perhaps initiated some twenty years before, by Vicat, in France (see alsoChapter 8).

From roughly the 1850s, the rise of the specifying architect (as opposed to the designing architect) began in earnest. Conscious attempts were being made by architects and other building ‘professionals’ to insert themselves into the building hierarchy over and above the craftsmen and craftswomen, particularly in the choice and specification of materials. This is made clear in many of the ‘expert’ texts of the time (Burnell, 1857; Scott, 1862). This was in part, of course, a response to the increasing complexity and ambition of construction, the increasing availability of less traditional materials such as cements and steel, and increasingly sophisticated transport networks. However, it represents a clear shift in the pattern of building that has continued to gather pace, and the consequences of which, for traditional vernacular fabric, have been generally negative.

Most of the buildings that craftspeople and conservators work on were not built to this pattern. Nor were they built with patented products, or even with ‘reliable’ products whose behaviour might be considered to be ‘always the same’, even when it is not (see The Problem with NHLs below). Instead, they were built to long-tested traditional patterns with mostly proven local materials selected by the builders themselves. The mortars were designed by the mason, bricklayer or plasterer using them, according to principles and preferences developed over millennia and passed from one generation to another – often without record beyond the buildings themselves – as well as to availability. These individuals often did not have scientific understanding of these materials, but they knew what worked and what did not. At the same time, they were able to respond to inevitable variability; mixing their mortars by ‘feel’, not by rote, and according to intended use. In the repair and maintenance of these structures, the building itself remains the most reliable specification, but is too frequently ignored, especially by the twisted mantra of making old buildings into commodities ‘fit for the twenty-first century’. This has seen far too many old buildings destroyed or debased, their essential character and performance compromised or removed in the spurious name of ‘saving’ them.

Modern construction, and its panoply of ‘value-added’ materials, is a function of industrialized capitalist production, not of traditional forms and relationships that have existed since time immemorial. Building conservation, and its associated ‘low-tech’ materials, respects the knowledge and experience of those who came before us. It tends not to assume that we ‘know better’, seeking instead to preserve the achievements and the monuments of the past and make them accessible and available in the present. In this sense, it has always been an anti-capitalist, even a socialist, enterprise, consistent with the political perspective and ambition of one of its primary advocates, William Morris and the organization he founded, the Society for the Protection of Ancient Buildings:

Believe me, it will not be possible for a small knot of cultivated people to keep alive an interest in the art and records of the past amidst the present conditions of a sordid and heart-breaking struggle for existence for the many, and a languid sauntering through life for a few. But when society is so reconstituted that all citizens will have a chance of leading a life made up of due leisure and reasonable work, then will all society, and not our ‘Society’ only, resolve to protect ancient buildings from all damage, wanton or accidental, for then at last they will begin to understand that they are part of their present lives, and in part of themselves. That will come when the time is ripe for it […] Surely we of this Society have had this truth driven home often enough, and often had to confess that if the destruction or brutification of an ancient monument of art and history was ‘a matter of money’, it was hopeless striving against it. Do not let us be so feeble or cowardly as to refuse to face this fact, for, for us also, although our function in forming the future of society may be a humble one, there is no compromise.

(Architecture and History, 1884)

In Malton, documents illustrate not only the pattern of building from the earlier eighteenth century until the end of the nineteenth century, but also the changing relations of their production. An agents’ Memo Book spanning the years from 1733 to 1808 (NYCRO ZPB III 5–2–1) clearly demonstrates that, throughout this period, the masons (or carpenters) controlled the choice and design of the materials to be used. The Memo Book contains numerous small contracts between the estate that owned most of the town and the craftsmen. The work was carried out according to mutually agreed measured rates. Whilst the composition of the team might vary, in all cases the agreement is signed by all of those scheduled to work on a building, indicating that all would be paid the same rate, though some might earn more than others by producing more work on any given occasion. There is minimal specification, most of it relating to the proposed dimensions of the building.

During much of this period the building mortars were still of earth-lime, with lime pointing and finish plastering. The contracts call for the ‘best mortar’, with the lime typically supplied by the estate from kilns that it owned. Other documents from the latter years of the eighteenth century, and not included in the Memo Book, are more thoroughly detailed by the estate agent, but the mortars are not specified in any detail at all. The specifications reflect the general pattern of vernacular construction at the time. It is agreed that if the estate is unhappy with the work (or the mason unhappy with his treatment by the estate), then the work will be inspected and judged by a fellow craftsman on behalf of either. This indicates that the co-operative culture among masons and carpenters in the town, illustrated by the Memo Book, endured at this time. Other documents from this time are proposals from individual masons for the construction of new buildings in the town, accompanied with simple plans and elevations which they have produced.

Malton mason’s sketch for a proposed building. In this period, local masons proffered sketches before commencing work, though the final design might change as works progressed. (NYCRO ZPB III 9–6)

The same building proposed above, as it was constructed.

3rd Augt 1799. I agree to build a tenement in the cow pasture in Malton Fields according to the plan hereunto annexed or with such alterations as may be made therein before the building is begun at the prices within mentioned […] 6s per rood for walling stone; 1s 6d per rood for mortar without lime; 11s 9d without pointing; 12s 6d if pointed. (NYCRO ZPB 9–6)

This situation continues into the second decade of the nineteenth century, when architects begin to be more involved in the design of prominent buildings, but not in their specification. Building accounts survive which show the material and labour costs for particular buildings. The architect simply signs off the work. The first documentary evidence for the detailed specification of works and materials by the estate is 1846, by an architect (John Gibson). By the 1870s such specifications are, for the first time, being put out to competitive tender by the appointed architect. Without exception, the contracts are awarded to the lowest tender. Similar patterns may be seen in most archived building accounts around the UK, wherein specifications are scarce and rarely found before the 1860s, although where the workers are directly employed by rural estates (for example, Newburgh Priory), the traditional ways continue for much longer and most works are detailed mainly in daybooks.

Architect John Gibson’s drawing of seventeenth-century Hunter’s Hall, Old Malton, prior to repair and conservation. Gibson called for the retention and repair of all structurally sound stone, called for repair over replacement and specified lime mortars throughout. His father had been the lead carpenter in Malton. (NYCRO ZPB(M) 7–49)

The buildings produced mainly by the masons and carpenters themselves, according to traditional patterns of construction, remain. The greatest threat to their survival, and to their proper performance, has not been the materials used in their construction, nor the practices of those who built them, nor the passage of time since. It is the imposition upon them of modern, incompatible materials, very often specified by expert professionals and applied by individuals disconnected from the general understanding and traditions of their forbears, whose minds have become clouded by the modern preference for hardness and apparent solidity and the over-rapid turnaround of any investment. All of them have been trained in a pattern of building technology that has only existed for around 100 years, the principles and materials of which may rarely be applied to buildings of traditional construction without detriment to the health of their fabric and to that of their occupants. Even if twentieth- and twenty-first century buildings, and the technology that designed and made them, were ‘better’ and more substantial, this would be a moot point in the conservation and repair of traditional buildings not built in this way, or with the same materials. This book is about some, at least, of the materials with which these buildings were constructed, and with which they should be repaired by those wedded to the principles of like for like and compatible repair, as well as carbon reduction.

Traditional v Modern Mortars

Traditional buildings need to perform as their builders intended (Oxley, 2003). This means that the mater ials of repair and conservation should be compatible. There is little better way to achieve such compatibility than to use the same materials that were used by the original builders, and to process them, where practicable, in the same way. In the case of mortars, to use the same materials, processed in the same way and used to the same or similar ends, will mean that we can have similar success with the buildings we work on. This will sometimes require patience and determination, and at other times entail partial or abject failure.

As will be demonstrated in the course of this book, most above-ground masonry construction, historically, was executed in earth-lime or in a pure or nearly pure, typically hot-mixed, lime mortar, rich in binder content, either in combination or discretely.

The ingredients of both mortars were locally sourced, relatively ‘weak’ and both met the essential criteria for building mortars. These were well known to masons and others and set down by Boynton and Gutschick in 1964: ‘Mortar strength […] is often greatly over-emphasised to the detriment of other essential mortar properties, such as workability, water retentivity, and bond-strength […] those builders who strive for high or maximum mortar strengths usually obtain inferior mortar for normal, abovegrade masonry construction […].

‘Before the advent of Portland cement in the United States (from 1886 onwards), all masonry mortar was a straight lime-sand mix that inherently possessed very low compressive strength. True, some of the lime produced was derived from impure limestone that had varying (but usually faint to moderate) hydraulic qualities; other pure limes were mixed with crude, unwashed sand containing clay that acted like a mild pozzolan with lime. Whilst both of the latter types of mortars possessed slightly more strength than the pure (‘fat’) lime-clean sand mixes, all would be regarded today as extremely weak in compressive strength (ranging between 50 and 300 psi (0.34–2.0 Mpa) in 28 days).’

Whilst some more energetically hydraulic lime mortars were used locally, in the absence of alternatives, most were used underwater and underground. Even in these cases predictable fat limes and pozzolan were more common as hydraulic lime mortars struggled to meet the same criteria unless they had unusually high levels of free lime. On top of this, hydraulic lime mortars were routinely made on site, from quicklime, producing a material of different character and performance, the inherent variability of which could be responded to and adjusted by the craftsmen.

Mortars of good workability, good water retentivity, tenacity and bond, as well as extent of bond, allow for best practice and good workmanship, and guarantee the longevity of structures they are part of. These properties are generally absent from modern cement or NHL mortars and ingredients that might be added to somewhat ‘improve’ workability will tend to compromise bond strength and extent of bond. Harsh-working mortars do not encourage – and sometimes do not allow – best practice.

The design and specification of modern mortars is driven by scientific analyses evolved to inform modern construction but which have little in the way of constructive comment about buildings of traditional, solid-wall construction. These err in favour of mortars of excessive compressive strength and low effective porosity, both of which will be damaging to the historic fabric and compromise its necessary performance:

Palmer, as long ago as the early ‘30s, completely discredited (laboratory freeze-thaw tests as a measure of real world durability), claiming it was misleading and unrealistic. He contended, in fact, that the most frost resistant materials are usually the most dense, but that they tend to remain excessively wet in the wall. ‘The most weather-resistant wall is one that remains relatively dry even though the materials composing it have poor records in laboratory freezing and thawing tests […] the analogy between a freezing saturated mortar cube and a monolithic wall structure is ridiculous.’ (Boynton and Gutschick, Durability of Mortar and Masonry, 1964)

Traditional buildings were engineered very differently to those typical after 1919. The significance of this is unfortunately lost on many specifying professionals, and also on the designers of numerous ‘breathable’ but ineffectively porous ‘eco’ and ‘conservation’ products. These proclaim themselves ‘capillary closed but vapour open’, or set far too much store by claims only of ‘vapour permeability’, thus fundamentally misunderstanding the actual and necessary in situ performance of porous and hygroscopic building materials. David Wiggins’s chapter will detail the essentials of his game-changing research into effective porosity, the essential veracity of which is confirmed by the observation and experience of craftspeople across the UK.

This error has in some ways been compounded by the ‘lime revival’, which, in the best of faith, has been driven by mistaken assumptions about the binders used historically, as well as by false assertions about the nature and the deployment of hydraulic materials in the past. Putty lime, drowned during slaking and mixed in too lean a lime proportion, was often a problematic material (though, in capable hands, not always). It was frequently too sacrificial for comfort and prone to premature failure, prompting frustrated professionals and craftspeople to turn instead to the almost ubiquitous embrace of hard Natural Hydraulic Lime binders with low free lime content, low workability and low water retentivity. The bond and proper performance of these might be readily compromised by the lack of on-going hydration during the progress of their long-term set, as well as by the addition of water-repellent or air-entraining additives, and their properties might vary, not only between ‘brands’ but also between batches of the same brand.

In the case of mortars, only very few conservators or mason-conservators, most of them working in the best of faith, have practiced this philosophy since the onset of the ‘lime revival’, despite proclaiming a philosophy of ‘like-for-like’ repair. This book will offer a narrative of mortar use historically that will inform an already initiated regeneration of the ‘lime revival’. It will hopefully open numerous new avenues of research and good practice that might begin to re-empower craftsmen and women around the world, as well as professionals and specifiers, so that they may carry such endeavours forward in a non-hierarchical spirit of co-operation and mutual respect for the benefit of old and new buildings alike.

In most cases over the last forty years, the choice has been presented as having been between putty lime or NHL as the primary binder, both mixed at too lean a lime-to-sand proportion in comparison to historic practice and understanding. Earth-lime, hot-mixed air lime or traditionally slaked hydraulic lime was rarely considered. Whenever hot-mixing was mentioned, even by its apparent advocates, it was coupled with assertions of excessive risk and ‘danger’, or impracticability. This discouraged experimentation by many of those concerned that NHLs were incompatible but who were working in parts of the country where putty lime mortars, as above, had proved themselves too weak.

John Ashurst had mentioned ‘quicklime mixed with sand’ mortars in 1988, and even advertised their superiority over putty lime mortars, but any exploration of these was quickly overtaken by the embrace of NHLs and the assertion that these had ‘always been used’ in the UK and in France (in fact, before the later nineteenth century they had only been used underwater, or underground, and very rarely in the air). This was all the more surprising given the preliminary conclusions of the Smeaton Report (Teutonico et al., 1993), which had demonstrated the successful use of lime putty, sand and brick pozzolan mortars in site trials at Hadrian’s Wall, as well as illustrating the variable performance of trialled NHLs. Phase 2 of the Smeaton Report has never been published.

Hot mixes were mentioned by Holmes and Wingate in Building With Lime (1998), and occasionally specified, but the focus of that book was mainly upon lime run to putty as a primary binder, as well as (and importantly) upon feebly hydraulic limes, which remained available in the UK at this time. Gerard Lynch had written a seminal paper in 2007, The Myth in the Mix, building upon his earlier insights into the hot-mixing or sand-slaking of mortars (1994), which had pointed out the discrepancy between modern 1:3 mortars and historic 1:1½ mortars and explained how this may have come about, demonstrating that, historically, lime mortars had generally been mixed at 1:3 from quicklime, which would expand upon slaking. He did not go on to advocate hot-mixing, seeming content with putty lime and modern NHLs for modern use. In his other writings, and in his training, however, Lynch had kept the flame of hot-mixing, as well as of sand-slaking, alive in England and elsewhere. Most observant practitioners, of course, and all mortar analysts, already knew that old mortars were lime rich and that to mix NHLs at historical proportions would have been to generate mortars of even greater hardness and rigidity. Modern putty lime, unless of significant age and appropriate density, would have produced a slurry, not a useable mortar, when mixed to historical proportions.

In Scotland the SLCT (Scottish Lime Centre Trust), from the early 2000s under the direction of Pat Gibbons, had bucked the national trend and advocated the use of gauged quicklime – NHL mortars in imitation of traditional feebly hydraulic lime mortars – and this experience fed directly into the revival of hot-mixed mortars outside of Scotland, though usually without hydraulic lime addition.

Elsewhere, masons and others began working with hot mixes, evolving their understanding and competence with these mortars and, over time, demonstrating in their practice their durability and performance, which much exceeded that of putty lime mortars. For most, this was a reaction against NHLs; the eminent workability, ease of mixing and use of hot mixes, as well as their enhanced bond strength, self-evident breathability and economy was simply an unexpected bonus. The methods of mixing that were evolved also demonstrated something that a preoccupation with the sand-slaking, banking and later mortar-mixing method could not: that hot-mixing was an entirely feasible and practical method of mortar production within the context of a modern conservation or repair project. It also showed that the logistics differed little from ordinary Portland cement or NHL mortar mixing and were at least as efficient, if not more, that mortars of eminent utility could be produced either by hand or by pan mixer, and, moreover, that the health and safety hazards of hot-mixing were as readily managed as those attaching to other commonly used alkaline binders.

A Summary of Traditional Mortars and their Primary Uses (as indicated by material science, historic literature and experience)

Earth mortars Clay-bearing subsoil, improved or otherwise and well tempered. Typically 12 per cent clay, high volume of silt and of generally fine texture. Used in masonry construction, plastering, daubing, floors (often with additives). Cob, adobe and rammed earth construction.

Earth-lime mortars Typically 10 per cent lime (sometimes more) to a clay (and silt) bearing subsoil; usually added as quicklime or freshly slaked lime. Used in masonry construction, especially of stone buildings across the UK, until at least 1800, base-coat plasters. Usually pointed with limerich mortar and finished internally with lime-rich or pure lime, haired finish coat and limewash. Exterior finish plasters in association with limewashes or beneath haired, lime-rich finish coats, as internally. Rammed earth construction. Some mortars in adobe construction. Some adobe construction (Norfolk and elsewhere).

Hot-mixed air or feebly hydraulic lime mortars General building with stone and brick, repointing, plastering (except top coat), external renders. Feebly hydraulic limes in water works local to their sources, with and without pozzolanic addition, some air lime mortars with small volumes of pozzolanic addition, to speed set and enhance carbonation. Pozzolanic mortars for underwater and underground use. Concretes and floors.

Putty lime, slaked with minimum necessary volumes of water, diluted somewhat and sieved Used on its own as a mortar for gauged brickwork, very fine stone ashlar, fine plaster finish coats. Very rarely as a binder before twentieth century, when it was gauged with cement or gypsum to compensate for perceived weakness in binding properties.

Hot-mixed or sand-slaked moderately or eminently hydraulic lime mortars Either artificial (air quicklime and pozzolans) or NHL (typically Blue Lias in England and Wales, locally and then somewhat more widely after railways). Used in underground or underwater works (docks, quays, sea walls), concrete footings and floors. Some above-ground construction late nineteenth century and for brickwork, earlier twentieth century, or where only locally available quicklime. Some gauging of hot-mixed air lime mortars with NHL. Swiftly displaced by cement-lime mortars for general use, and by Portland cement for concretes. Rarely used for above-ground construction or repair until it was embraced by the conservation industry in the UK and then elsewhere after the late 1990s.

Natural Cement (After 1796). Waterworks, quays, cast mouldings, fortifications (alone or as a gauge for hot-mixed common mortars), external renders, some repointing over earlier mortars.

Chapter 2

EARTH AND EARTH-LIME MORTARS

General Discussion

THAT EARTH BUILDINGS ARE WIDESPREAD across the world – whether of rammed earth, cob, ‘mud and stud’ or adobe – is widely acknowledged and understood. Less widely appreciated is the enormous geographical and historical spread of earth-built masonry structures. Whilst the use of earth mortars in masonry construction was the pattern in the UK during Neolithic times, it would seem fair to assert that the majority of stone buildings constructed across the UK and Ireland before the nineteenth century were built with earth or earthlime mortars, generally pointed with lime-rich or pure lime mortars, often haired to increase tensile strength, and were originally plastered with a simple system of earth-lime backing coats overlaid with quite thick, haired lime finish coats. This was probably also the case with exterior walls in earlier times. This system of masonry construction was most likely introduced into the British Isles by the Romans, continued after their departure, and was reasserted by Norman builders.

This pattern of masonry construction is also evident in Spain, France, the Czech Republic and across Europe and Asia and, whilst it already prevailed in pre-conquest America, was exported there by European settlers, beginning many centuries of cross-fertilization.

Only during the eighteenth century did it become more common to use lime-sand mortars for masonry construction, although these already had been used in some – but by no means all – high-status buildings and were adopted by bricklayers earlier than by stonemasons in the British Isles.

Colonial-era adobe church, earth-plastered, Taos Pueblo, New Mexico. After a brief dalliance with cement-stabilized earth plasters on the current church, the Pueblo community was quick to learn the error of others’ ways and returned to traditional patterns.

Colonial-era adobe church, Pecos, New Mexico, showing the pernicious effect of cement-stabilized earth. The darker red patches on this building are the remains of such a render, which has accelerated decay of the earth blocks behind the ‘protective’ coating, which then falls away, leaving a powdery, disrupted surface.

Until very recently the prevalence of earth and earth-lime mortars in masonry structures has been largely ignored by both the conservation community and the ‘lime revival’. The same is true of the demand for the use of truly compatible and eminently breathable earth-lime and lime mortars for the repair and conservation of said structures. Whilst the use of putty lime mortars for this can do no harm to the fabric of such buildings, the use of cementitious or even hydraulic lime mortars may be seen as offering essentially incompatible options. There is increasing evidence that NHL mortars, low in the free lime content necessary to achieve effective porosity (Wiggins, 2015), with much greater compressive strength than was possessed by the typically hot-mixed air, or very feebly hydraulic lime mortars traditionally used in association with earthen materials, led to an ongoing accumulation and entrapment of received moisture. This is to the detriment of appropriate thermal performance, will lead to the unnecessary decay of embedded timbers as well as stone, and may ultimately lead to collapse or other less exaggerated structural failure.

Moorish-period rammed earth wall with probable lime inclusion, Chite, Granada, Spain.

Moorish-period rammed earth house wall, Chite. Note the typical 4in (100mm) increments in which the earth was rammed into the formwork, representing the thickness at which earthen materials may be optimally compacted. Stone quoins laid in earth mortar.

Hot-mixed and applied aggregated limewash, garden front. For all that the ‘grand arch’, added c1690, was built with one of England’s finest stones – Hildenley limestone, local to Malton – and the west range refaced in the same, the façade was always limewashed: originally an off-white, as here; in later years a rich copperas-pigmented wash, as to other elevations. York House, Malton.

Copperas-pigmented sheltercoat, reflecting later colour schemes, north elevation. All treatments and colours were informed by the archaeology of the building itself, as were all works to the fabric, responding to discovery and analysis, not to a pre-ordained specification. York House, Malton.

As the routine modern use of around 3 per cent quicklime addition for soil stabilization during road construction demonstrates, a small volume of quicklime has a disproportionately efficient effect upon the tenacity, cohesion, water management and load-bearing capacity of clay-bearing subsoils, as well as ‘locking in’ surface fines (Minke, 2006), reducing friability and creating a well-consolidated surface layer. Whilst carbonation of the lime addition may ultimately occur, it is primarily the chemical interaction of the clay and calcium oxide (or calcium hydroxide) ions that offers this improvement in performance:

Lime’s reaction with soils is twofold. First, it agglomerates the fine clay particles into coarse, friable particles (silt and sand sizes) through base exchange with the calcium cation displacing sodium or hydrogen ions. Next, it provides a cementing (hardening) action in which the lime reacts chemically with available silica and some alumina in the raw soil or with pozzolanic additives, forming complex calcium silicates and possibly aluminates. (Boynton, 1980)

Boynton, although writing about soil stabilization, lists the ways in which the addition of lime to soil alters its properties: as much as a three-fold decrease in the plasticity of the soil; an increase in the plastic limit and a decrease in the liquid limit of the soil; reduced shrinkage and swell; a potentially dramatic increase in unconfined compressive strength of up to forty times and a consequent increase in load-bearing capacity; an enhanced water-shedding capacity (Boynton, 1980). All of these are as relevant to the use of earth as a mortar and all offer constructional benefit.

Earth-mortared, c1690 stone window surround, the window stones fine-jointed with lime putty. York House, Malton.

Oldest floor in York House, 24in below seventeenth-century floor level, excavated by Diedre Ford. Limestone cobbles in earth mortar. York House, Malton.

Original earth-lime mortars to c1690 addition. York House, Malton.

Lime-rich earth-lime mortar over lath on ceiling beam; in this case, much more than 10 per cent lime addition was used to enhance bond. York House, Malton.

Observation would suggest that quicklime was the most common form in which lime was introduced into otherwise earthen mortars, but this may have been ‘hot lime’ run to a thick paste just prior to addition on occasion and depending upon customary lime slaking procedures locally. It may have been slaked dry hydrate, or even putty, or ‘milk of lime’.

There are real practical advantages in adding quicklime, however, as the earth mortar – improved as necessary by the addition of sands – may be mixed beyond the liquid limit to fully engage the clay content and then be brought down below this now lowered limit by the addition of a small volume of quicklime. This addition also improves the workability and ‘elasticity’ of the earth mortar, reduces shrinkage in use (particularly when the clay content is above the optimum level), and improves cohesiveness and durability.

Many earth mortars contain a multitude of ‘lime lumps’; others far fewer and with streaks or swirls of carbonated hydrated lime. Analysis of these mortars – particularly where no obvious under- or over-burned, or simply unmixed and subsequently carbonated lime lumps are present – may misleadingly indicate the use of putty lime, as quicklime added to a very wet earth or earth-lime mortar will slake to ‘putty’ during mixing. Even where lime putty may have been used, it will have been slaked traditionally to a breaddough consistency – the necessary water added to the lump lime, not the other way around – and will have been mixed immediately after slaking was complete, while still very hot, or soon after a short period of rest. Alternatively, and particularly when the lime addition was significant, the mortars will have been made by the ‘ordinary method’: a basin being formed of the earth aggregate, the quicklime placed within this and then just sufficient water or a slight excess of water added to the lump lime to effect the slake before earth and slaked lime were mixed together to form the mortar, and before the addition of more water, ample beating and tempering.

Earth-lime mortars; original bedding, dubbing out and base-coat plaster to earliest masonry fabric. York House, Malton.

Nagtsho Dorji and Dierdre Ford earth plastering within original central hall space. York House, Malton.

Haired, lime-rich pointing to the exterior, with faded copperas wash evident upon the stonework after cleaning away of sulphation soiling. York House, Malton.

Earth-lime plaster with hay addition, although the hay is not obvious. York House, Malton.

Cross-section through the same earth-lime plaster. Remnant angular lime inclusions confirm the lime addition. Straw was not commonly used, probably because it would impede application of the plaster with floats. York House, Malton.

Haired, earth-lime plaster, rare in Malton, but not uncommon elsewhere. York House, Malton.

Despite the apparently near-universal use of earth and earth-lime mortars for construction across the British Isles, it is very rarely mentioned or discussed in the majority of historical texts dealing with mortars. This is because it was either mysterious and unknown to most ‘men of letters’, considered beneath their attention, or seen to be entirely within the realm of the building crafts, unamenable to improvement other than by wholesale displacement with ‘better’, more ‘wholesome’ materials. Their manipulation is discussed even less, with only Henry Best describing this in 1641 (see below).

Cross-section of earth plaster and lime finish coat. Although seemingly without lime addition, naturally occurring limestone aggregate is clearly visible in the earth base coat. York House, Malton.

Cross-section of earth-lime plaster and lime finish coat; lime lumps in the earth backing coat are small, which may indicate that the quicklime was initially slaked to a thick, dough-like putty or even to a ‘milk of lime’ for addition to the earth. York House, Malton.

In Moffat in the later eighteenth century, the Duke of Buccleuch, who had hitherto lived and been educated in Edinburgh, prevailed upon his tenants to demolish their earth-mortared houses for them to be rebuilt with sand-lime mortars instead, so horrified had he been to discover such base materials in use upon his estate (Telford, 1838). Only two earth-mortared houses survive in Moffat as a result (pers comm Niamh Eliot).

In most cases, however, such mortars went unnoticed and were simply overlaid with lime plasters, renders, limewashes or pointing. They are mentioned in Roman texts, acknowledged by Alberti, and identified in several seventeenth- and early eighteenth-century texts. Later writers in France, however, in common with British ‘improvers’, took earth building much more seriously and throughout the nineteenth and into the twentieth centuries engineers and others wrote as much about adobe and rammed earth construction, as well as associated earth and earth-lime plasters, as they did about lime-sand and plaster of Paris. The latter had been routinely used, although by no means exclusively, as a building mortar and as an interior and exterior plaster in the Paris region for centuries, despite the relative dampness of the climate, and despite exterior renders not being expected to last more than fifteen to twenty-five years before replacement (Rondelet, 1803). Loudon (1833) is the most encouraging of the use of earth, often presenting it as an equivalent, sometimes a superior, alternative to stone. Beyond these, very few authors discuss earthen mortars in masonry construction, and yet earth and earth-lime mortared masonry buildings can be found as readily in the British Isles, France and Spain –in limestone regions as much as in those with little or no easy access to lime – as they are around the world.

Alder seed head within earth plaster, indicating the gathering of material from the nearby riverbank or from the river itself. York House, Malton.

Original earth-lime mortar, rich in added hay. York House, Malton.

Original earth-lime bedding mortars to the west front, probably dating from changes to this elevation made c1690. York House, Malton.