A Brief History of Lime
A bit of lime background
Over the last 30 years, the use of lime for plasters and mortars has been rapidly gaining momentum, to the point where a large minority of people involved in the conservation of our built heritage are aware of it’s presence, even if unsure of the finer details of usage.
Martin Brown from Best of Lime Ltd, sets out a brief introduction to the various historic mixes, with an attempt to contextualise their original usage.
An understanding of the intention behind a certain historic mix will inform us when approaching the conservation of old plasters or replacement with new.
The first and in Martin’s opinion, most important thing to remember, is that any plaster or mortar that is still performing as originally intended, is not an accident. The mixes that we can still see, be they 100 or 600 years old, are the result of 100’s of years of trial and error. We can sum this up as ‘Survival of The Fittest’.
The ultimate in sustainable building materials, although this is a piece about lime, earth as a building material is of great importance and cannot be overlooked. We find earth/clay mixes as cob and clay-lump used as the building blocks, we find it as the daub, for wattle and daub infill, we also regularly find it used as external render and internal plaster.
To this we must add mortar.
It would be easy to dismiss the use of earth simply as a cheap alternative to a better quality, more expensive material and in the case of clay-lump outbuildings that could well be the case, but it doesn’t tell the whole story.
Earth mixes, often without any discernible lime binder, are often found used in high status timber framed buildings.
Martin believes that this has far more to do with good engineering than frugal economics and ‘value engineering’
A building constructed using green oak is going to be subject to enormous potential movement.
Initially, the moisture level in the timber will reduce, this can cause shrinkage across the grain of up to 7.5%, that’s nearly 3/4” off of an 8” sole plate. Add to this potential twisting and the fact that all of the timbers are connected by mortise and tenon. There is then the ongoing seasonal thermal movement, ground heave and anyone that has stood in the attic of a timber frame on a gusty day and felt the building shake will also know the effect of wind loading.
So, the more elastic the materials that come into contact with the frame, the better.
Martin has a slight obsession with the use of clay mortars for the construction of even the largest brick chimney stacks a tradition that he believes is an example of really good engineering. A decent sized chimney could consist of over 20 tonnes, stood in a column, with minimal footings, surrounded by a moving building and subject to heat, cold, sulphates, damp and wind loading, the harder and more rigid the mortar, the more likely a catastrophic failure.
The use of calcium carbonate as an aggregate is rooted in the deepest history of lime plasters, Vitruvius described using marble dust and mature lime putty 2,000 years ago and the Greeks and Mesopotamians before them appear to have used similar ingredients.
Chalk, being the softest and geologically youngest form of calcium carbonate appears to have become widely used in Britain in the 16th Century.
This introduction could have two potential root causes.
Firstly Henry VIII’s most ambitious building project, Nonsuch Palace.
Nonsuch was a timber framed palace on a spectacular scale, decorated with heavy relief stucco, applied by Italian masters, it is highly likely that these Stuccottori brought with them recipes based around calcium carbonate enabling them to effectively create sculpture.
Secondly, evolution through necessity, it isn’t possible to create heavy relief modelling in a clay plaster, the mix is liable to crack and certainly won’t remain weather proof for too long. Sand had historically always been the aggregate of choice when mixing lime plasters, but no matter how well graded, or how much animal hair reinforcement is used, a mix containing sand will be mechanically rigid and brittle. The movement outlined earlier that comes hand in hand with timber framing will, in Martin’s opinion, virtually always lead to the early (less than 15 years) failure of a sand based render. This set of problems led to the birth of high calcium plaster mixes, basically lime and chalk, heavily reinforced with bovine, though occasionally horse hair. These mixes were extraordinarily successful, to the point where most 16th, 17th and 18th century buildings that haven’t been ruthlessly restored by the Victorians or later, will still contain some chalk plaster. It was used externally for very heavy relief modelling and pargetting and internally for very thin skims over brick and daub. A particularly common practise when plastering between ground floor ceiling joists was to nail three or four long laths onto the back of the floor boards above, them apply a single thick coat of chalk plaster. It’s a testament to the mix, that the laths weren’t ‘stood off’ the floor boards with a counter lath, so no plaster folded over to form a key, also continuous deflection from the floor above, yet there must be hundreds of examples in Suffolk alone.
The gradual change to three coat sand mixes
The change from earth plasters and chalk as the aggregate, to sand and lime can be traced parallel to the increased stability in building materials.
As the use of heavy oak framing gradually changed to re-used, stable timbers and slow-grown softwood, the requirement of the plaster to flex with exaggerated background movement diminished.
Along with this change in building materials came the fashion for more formal, classical architecture and decoration, including the use of wallpaper.
From these changes, the standard ‘three-coat’ plastering specification was born.
The scratch coat, containing animal hair reinforcement, often horse and much less than 200 years previously, provides the base either onto lath or brickwork.
The floating coat, this does all the hard work and is used to straighten and level the wall, ready to receive the final, thin finish coat to provide a hard, flat polish, ready for decoration.
We have now entered a new stage in the history of lime as a plaster and mortar.
Not only is it now realised that it is an essential material in the conservation of historic buildings, but with the climate crisis well and truly underway and ‘zero net carbon’ a legal requirement, it is an essential very low-carbon, healthy building material.
At Best of Lime, Warmcote, Rendercote and Limecote have been designed to bring together all that’s historically best, with some innovations, such as insultation, for the modern world.
We can also supply the Pavatex range of wood fibre board, woodwool boards, lath and sheep’s wool insulation
The products are all designed to mix quickly and are easy to use, not only for seasoned plastering contractors, but for homeowners wishing to carry out their own repairs to a historic building.
Advice on most aspects of historic or sustainable buildings is also freely available.
Fireplace & Frieze by Johanna Welsh for Wilderness Reserve.