Stone Wall Restoration - Working With Lime Mortar

Our house is at least 300 years old and was constructed from local stone and a few choice pieces of granite 'borrowed' from the local monastery during the French Revolution. It has no foundations but is built directly onto the bedrock from a stone 'sandwich' laid in layers of local clay with a loose rubble and clay infill. This makes for both a solid and well-insulated wall, which starts at a base thickness of around 27", tapering up to 18" at the height of the roof.

Before we came out to live in France, I used to do a little moonlighting teaching maths and IT at my local college. Whilst there I found they had a great bricklaying class and decided to enroll 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!

Now however, in our present house I'm in a whole new ball game with stone but the valuable lessons in using lime mortar are even more apposite for this ancient longhouse.

Over the centuries, including, as we live in Normandy, heavy shelling and bombing in WW2, the building has taken quite a battering and thus there are many different patches of restoration, these are in the main, of local stone and lime mortar or clay..

The object for us is not to destroy any of the charm of the original building (including all honest repairs), which through its 'stone book' tells the story of the people who lived here. For example, the third image above shows our fenêtre meurtrière - literally 'murder window' or rather arrow slit. This not only bears witness to the thickness 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 themselves protected by the walls with a formidable stone shield.

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 instructable.

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 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.

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.

If stones were to be laid directly in contact with each other, any high point on the stone face would be a pressure point and the concentrated load transferred through this point to the stone below would invariably cause that stone to break

Above: an area along our coast, famous for lime kilns, oysters and pirates.

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.

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.

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. For pointing, this is 4 parts sand to one of lime.

That done, I dry mixed them to produce an evenly coloured, lump-free mix.

This stage is important because I want the lime to coat all the particles of sand.

Once satisfied with my dry mix, I add 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 becomes 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.

The mortar should not 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.

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.

Our old walls over the centuries lost their clay mortar and now need that to be replaced. By using the mortar to 'point' the wall, this restabilises it and prevents the possibility of stones slipping out of place.

In some areas of the wall, so much clay has been lost that there are large voids behind the facing stone. These need to be repacked with stone and mortar before pointing.

The first stage in pointing is to rake back i.e. clear out the joints to a depth of about 1" (2.5cm). This is when you find which stones have become loose.

The wall is then brushed down to remove any remaining loose debris and dust.

Once the mortar is mixed, the wall is dampened down, I use a brush and bucket of water.

If there are any really deep holes that have become apparent it is at this stage that I would feed in mortar and then some clean wet stones to fill the void. The joint can then be filled.

I use a large trowel held upside down onto which I place a sausage-shaped piece of mortar. Using a smaller pointing trowel, I press the mortar down so it is thickness of the joint I want to fill. Now comes the tricky bit, using the pointing trowel, I cut a piece of this mortar and in the same motion as cutting, sweep the mortar onto the backface of the pointing trowel. If the mortar is too wet, it will slide off onto the floor! A correctly mixed mortar will sit on the pointing trowel without moving. This can then be transferred into the joint between the stones.

Obviously with natural stones, the gaps between them will vary.

Any mortar which has ended up on the face of the stone, should be removed with a stiff brush, I usually wait until the following day to do this. Stone is a lot more forgiving than brick!

Lime mortar is a wonderful medium to work with and a must if you have an old house or building to work on.

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.