Mixing and using lime mortar and why it's a great idea to take a natural building diploma!

Whenever we take a trip to the Pointe d'Agon to collect oyster shells for the chickens we pass by an area called Les Fours au Chaux - 19th century lime kilns, these are dotted all over this coast and were apparently mainly for manufacturing lime for agricultural use. Ironically lime production for building work historically used oyster shell. For an idea of how long construction and lime have been associated, the first lime used for building was in the Ancient Kingdom of Anatolia, the region that is now eastern Turkey and the foundations studied date from around 14,000 years ago.

The oyster beds from the Pointe d'Agon 

Once the food of the poor, the now rare deep sea oysters are harvested here only for a few weeks at the end of November and kept 'in captivity' before ending up on the New Year's dinner tables of Paris.

Introduction

When I moved back to England to study for my doctorate we bought a 1930s house in great need of repair some of which was to do with the masonry of the building. At the time I was also supplementing my research grant with some teaching at a local college (Maths and IT) and it was here that I found that there was an evening course in bricklaying for D.I.Y. Keen to acquire new skills, I enrolled for this and over the next eighteen months or so I became acquainted with some of the practical techniques of this fascinating trade. Our instructor was a professional builder and the tips and suggestions he made to me and my classmates regarding our own specific home projects were invaluable. Although Natural Building per se was still in its infancy or rather rebirth, we were very lucky in our instructor. One of the most important for me and avant garde for a mainstream building course, was his his stressing of the use, where possible, of lime mortar. We used this in class and became well acquainted with the medium. On finding out that I wanted to replace an ugly 1960’s fireplace with a cast iron Art Nouveau one he immediately cautioned me not to use the very expensive commercial  fire cement but to opt for a ‘soft’ lime mortar mix to fit the new fire-back, Following his advice I was able to undertake the job at a fraction of the cost through using his recommendation and it worked perfectly. In fact the money I saved through not having to purchase the expensive refractory cement more than paid for the evening classes!

If you have a project that requires bricklaying or stonework and you want to do it yourself then I would highly recommend contacting your local colleges or schools to see if such a course exists. Nowadays you may be lucky and find they have an actual designated Natural Building Course or Workshop, or at least a sympathetic tutor like ours, you won’t be disappointed and it may be a start of a brilliant new career.

Whilst my knowledge of lime mortars is in no way as extensive as I would like, I have never found that the mortars I have mixed and used have done anything detrimental to the structure of the buildings. For further information on specific uses of lime mortars, I have included three of my favourite sites at the end of this post.

I’ve used lime mortar a lot in our present house firstly because it is obviously appropriate to a 300 year building, as it is sympathetic to the original  build of stone laid on a bed of clay mortar, secondly because in the mortar mixes I have used it will never be stronger than the stone of the house and thirdly when used in wall construction, the wall is able to ‘breathe’, essential for the 70cm (27”) thick walls that we have here. Clay mortars were very popular in the 15th century and were still used in vernacular houses and buildings in the 18th century.

Battle scars Our two workshops showing the variety of 'make do and mend' pointing that the house front has suffered over the centuries as it has weathered the various bellicose attempts of man. To us this is all about the persona and age of the house, so unless the repairs have been of later date and thus of cement, we have left them alone. In keeping with tradition and harmony, Sue of course laid our front garden stones on a bed of lime mortar.

Fenêtre meurtrière - literally murder window or rather arrow-slit window, not only bears witness to the width of our walls but how much an 18th century French farmer's house was his castle. These windows allowed the occupants a wide field of view and thus fire, whilst being protected by the walls with a formidable stone shield.

A Word About Natural Hydraulic Lime

Sea shells, chalk and limestone are all forms of calcium carbonate, when these substances are crushed and heated in a kiln between 800ºC –1 000ºC the carbon dioxide in the compound is driven off leaving solid calcium oxide(quicklime).

Adding water to the cooled medium results in an exothermic reaction and produces calcium hydroxide which subsequently, if left exposed to the air, will absorb carbon dioxide to form once again calcium carbonate. Adding sand to the calcium hydroxide produces a mortar known and used for thousands of years. If the limestone is not pure but contains impurities of  clay then during the heating process the calcium will react with the clay to form compounds called silicates which will set with the addition of water. Not all of the calcium will be in the silicate form, there will still remain some calcium hydroxide, which will set by the carbonation process. Thus mortars of different strengths and setting times can be produced. depending on the level of the impurities.

There exists a classification for Natural Hydraulic Limes which describes their performance due to the level of impurities. Namely: feebly hydraulic NHL 2, moderately hydraulic NHL 3.5 and eminently hydraulic NHL 5. The higher the number, the faster the setting time and the harder the resulting mortar.

Mixing the Mortar by 'Feel'

If I was to use the word Art in connection with lime, it would no doubt conjure up an image of High Renaissance fresco but in effect my meaning here is much more prosaic and refers to the mixing of mortar. Or rather it has become so, as over previous decades we have been taught to devalue what was an art or at least artisan, down to the level of manual labour. There is as much of science and art in getting mortar right as in any other endeavour, which is why no Mediaeval cathedral ever carried a ten year roofing guarantee.

From my experience with fitting the fireplaces I knew that a weak mortar mix would be suitable for the temperatures the storage heater wall was to encounter and I decided to use a natural hydraulic lime in the mortar (NHL 3.5). mixed with a sharp sand in the ratio of 6 parts sand to 1 part lime.

Because my quantities of mortar were small I mixed it by hand in a mortar mixing tub which could then be carried into the house.

The first thing to do was to accurately measure the correct proportions of sand and lime.

That done, I dry mixed them to produce an evenly coloured, lump-free mix. This stage is important because I wanted the lime to coat all the particles of sand.

Once satisfied with my dry mix, I added water a small amount at a time and with each addition, continued with the mixing.

The water needed to be evenly distributed throughout the mix so it was important to make sure that the lime/sand mixture in the corners of the mixing tub were moved to the centre.

As the quantity of water was slowly increased, the mortar became more homogeneous and the feel of it on the mixing spade changed. It was at this stage that I stopped and tested the mortar with a trowel.

You do not want the mortar to be like a slurry, but one ought to be able to slice a sausage-shaped piece with two cuts of the trowel which can then be lifted onto the trowel, it should still retain its shape and yet have an elastic appearance when shaken (this and all the steps above are shown in the first film below).

Frankly, I find this to be a very subjective operation and this ’feel’ for a good mortar was one of the first things I learned in my evening class. Nevertheless, the too runny, slurry stage would be self evident.

Bear in mind that the mortar is there to keep the stones apart so it must have some ability to retain a shape but be sufficiently workable that when the stone is laid upon a bed of it, it will form a joint and support the stone.

In Conclusion

Lime mortar is a wonderful medium to work with and a must if you have an old house or building to work on. In the following two films you will see a demonstration of me mixing the mortar, as in the above paragraph, by 'feel' for my recent storage heater project and in the second film I'm also pointing the wall of our barn.

Now if you'd like to sit back and watch the films.


I would certainly recommend the following sites:-

http://www.buildingconservation.com/articles/articles.htm#limemortars for more information on the use of lime mortars in restoration work,

http://www.naturalhydrauliclime.net/FAQs.html for a contemporary review of the applications of the medium and

http://www.stastier.co.uk for comprehensive technical information on the performance of mortars made using lime-based products.

If you've enjoyed this article and found it useful please feel free to share it or to comment and/or make observations. All the very best and until next time,

Andy

© Andy Colley 2014

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