These buildings were assembled by hand with skills developed over the generations and nurtured through the trade apprenticeship system. They were practically detailed to exclude and shed water and, viewed from today’s perspective, the process was almost entirely benign: everything was sustainable, renewable and carbon emissions in construction, if not in use, were negligible. The system, developed over centuries, worked in ways we are only now beginning to understand.
Paradoxically to the modern mind, this is because in traditional building, the materials and the construction as a whole were, in relative terms, soft, weak, flexible and permeable. Rainwater was shed or otherwise carried away from roofs, but the walls, protected by overhangs, cornices or drip- courses, were permeable. Moisture absorbed in driven rain penetrated the outer surfaces of the walls, but evaporated out again when the rain stopped and the sun came out. And, of course, dry walls always were – and are – warmer than wet walls.
It is well known that cement is incompatible with traditional building and that the use of cement mortar and the application of strong and impervious finishes are likely to make walls wetter. This is because moisture from the ground, minor leaks and fine surface cracks – which are almost inevitable when a brittle material is applied to a soft substrate – is prevented from evaporating out through the external wall surfaces. Attempts to exclude moisture with damp courses and impermeable membranes only exacerbate problems by causing wetness to build up.
It is perhaps less well-known that most stone and brick decay is caused by the crystallisation of soluble salts, as moisture evaporates from the surfaces. If the joints are impermeable, this process is concentrated in the immediately adjacent surfaces. Hard mortar causes sacrificial damage to the stones, while permeable mortar is less susceptible and more easily repaired.
Modern construction is the absolute opposite of traditional building. It employs materials which are manufactured, processed, transported and assembled by machines with a minimum of labour and skill. The materials – steel, concrete, glass, ceramics and plastic – are also hard, strong, inflexible and impermeable. They depend on waterproof and airtight membranes, so that water is excluded. In terms of moisture management, it is the difference between a tweed overcoat and a plastic mac!
Lime is the essential ingredient of traditional mortars and surface finishes. There are different sorts of lime and different ways of preparing and using them. At its simplest, what
90 Listed Heritage Magazine January/February 2020
Pointing and thrown harl on Lindisfarne Castle
is called ‘air lime’ is made by burning pure limestone in a kiln, driving off carbon dioxide. As the mortar cures, the carbon dioxide is re-absorbed. This so called ‘lime cycle’* is what makes lime mortar in its pure form ‘carbon neutral.’
For better or worse, the selection, specification, preparation and use of lime mortars requires knowledge and skill, which not every builder possesses. For the DIY owner, there are courses where the selection, preparation and use of lime mortars can be learned. For the careful and conscientious owner, the skills exist almost everywhere and, with a little trouble, can be found.
It will come as a surprise to most owners to learn that the more lime mortar there is on the surface of a wall, the better it will perform and the drier the wall will be. Full covering with a thrown harl or a floated plaster finish may be best of all. The recessed or ‘hungry’ joint, derived from 19th and 20th century Ancient-Monuments ruin-consolidation practice, has no place in traditional masonry and brickwork. Walls were commonly finished flat, or nearly so, and the word pointing referred not to ‘joint-filling’, but to the use of a pointed tool to strike joint lines on the finished surface!
In the end, however, besides the practical function of lime finishes in keeping walls dry and houses warmer than they might otherwise be, they can be utterly beautiful!
*For those with a basic knowledge of chemistry, the so called lime cycle – the burning of limestone to give quicklime; the slaking of the quicklime to produce mortar; and the setting (that is the drying and carbonation) of the mortar in or on the wall may be represented as follows:
1. CaCO3 – CO2 = CaO 2. CaO + 2H2O = Ca(OH)2 3. Ca(OH)2 + CO2 = CaCO3 + H2O
ABOUT THE BUILDING LIMES FORUM The Building Limes Forum exists to encourage expertise and understanding in the appropriate use of building limes and education in the standards of production, preparation, application and after-care. The Building Limes Forum is a Scottish Charitable Incorporated Organisation (SCIO) No SC033659 with over 400 members in the UK and overseas, the majority being actively concerned with the repair of historic buildings or in the use of lime in new builds.
James Simpson OBE HonDSc FRIAS FSAScot Consultant to Simpson & Brown, Architects Chairman the Building Limes Forum
www.buildinglimesforum.org.uk
Images © Simpson & Brown
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