Traditional Masonry Archives

Building Stone Walls

There is a limestone quarry in County Kerry in the South West of Ireland near the village of Lixnaw which in Gaelic is Leac Snámha meaning the swimming flag or stone. I had the pleasure in recent times of running a workshop there in the techniques of traditional stone walling and lime mortars.

Swimming stones is an expression I have come across before in the course of building stone walls. We normally do not associate swimming and stone as having any relationship with each other unless it refers to the level of proficiency a particular swimmer might possess. However we are not talking here about potential Olympic champions but rather stones in newly built walls that like to swim about a bit.

In Ireland we don't have to be told that it rains nearly everyday in winter, also spring, summer and autumn. And when its not raining it is often quite damp.

Problems occur when building mortared walls with wet stones, after awhile the stones at the lower levels begin to swim about. This can be ignored for a while and progress can be made by laying more stones overhead, but not for long. The swimming stones develop from a mere dog paddle to a full-blown Australian crawl. At this stage usually attempts are made to push these lower swimming stones back into place, but they won't have a bar of it, all that happens is that they begin to achieve greater versatility in their new found freedom. You see they like to swim once they start and there is no stopping them. Prayers are often said at this stage such as "please don't fall down just for my sake" but no, they are not going to accept this pathetic grovelling for one minute. Anyone who was stupid enough to lay them to begin with must suffer the consequences.

There is unbelievable guilt in building stone walls, sometimes your mistakes instead of taking centuries to manifest themselves, when you are safely gone and forgotten, suddenly confront you head on. You always recognise the stone that starts to move first, the slightly unbalanced one, or the one not quite as thick on top as at its base, in particular the one not bedded with its longest length into the wall. It comes back to you in a flash, you knew it was a trouble maker from the start, but for expediency or laziness or whatever you chanced it and hoped for the best. You cannot fool the stones, they have been around a lot longer than you have. Most of us who build mortared stone walls experience this (less so, hopefully as we get older) it is both embarrassing and costly because the only answer is to take the work back down and start again. If we don't do this, by pretending it doesn't exist then we may have to jump very quickly to get out of the way or be entombed within our mistake. I can hear you say that we should not work with wet stones, and didn't stonemasons in the past stop working between late Autumn and early Spring anyway and return to the warmth of their homes like any sensible person would do in a cold damp climate. Possibly the swimming stones know this better than we do and are trying to jump out of the wall and get home ahead of us, I don't know.

We know that wet stones contribute to swimming but so do modern mortar mixes based on sand and cement, they are unworkable if stiff and shed water too readily if wet, carrying cement down the face of the work and causing staining. They also create other problems I wont go into here. Dry, stiff mortars are part of the answer but they are very hard work on the wrists.

All of this is overcome by using a mortar as old as time itself, namely a hot lime mortar. Hot lime mortar is made by mixing quicklime and sand (usually coarse sand for rubble work) together with water, the result is a thermic reaction within the sand from the quicklime and quite high temperatures (Note: this is a dangerous procedure which requires training and safety precautions before attempting).

It is generally thought and accepted that it is best practice to allow this material to sour out (an Irish expression meaning to lie in a wet state under cover, the lime and sand) for a period of months before use, but was this always done in the past? I think not. It is possible to mix and use this mortar while still hot and in a plastic state so that it will spread easily. The stone is laid and in a matter of a very short time with evaporation from the heat, the mortar naturally stiffens and those wet damp stones find it much harder to start their devilish movements. A rather primitive mortar most would agree but an exceptional one to use as it readily sticks to everything including vertical stone surfaces, a crucial element in the proper jointing of stones.

Traditional stone walls were solid and generally varied from two feet in thickness upwards. With non-hydraulic mortar mixes and thick walls, evaporation of water from the mix was more important than carbonation.

Evaporation causes stiffening of the mortar, thereby reducing the tendency of stones to swim and allowing work to proceed vertically. Carbonation is a rather slow process requiring access by air (difficult in the center of thick walls). It is more crucial that effective carbonation occurs in mortars near the face of the wall to prevent washing out by rain than in their centers.

To have hot lime mortars available, lime kilns would have to supply quicklime, not exactly on a daily basis but at least once a week because the quicklime will air-slake and become useless if exposed to the air for extended periods of time.

What better than a hot lime mix to create evaporation and therefore stiffening. Also how pleasant it must be for the stonemason to feel the warmth of the mortar on a cold damp day.

Patrick McAfee, stonemason, author of Stone Buildings, The O'Brien Press, 1998, and Irish Stone Walls, 1997, works as a consultant, trainer and lecturer throughout Ireland. He has also lived and worked in Australia and for short periods of time in a number of European countries. He runs regular workshops in stone and lime at a medieval castle in Dublin, Ireland.

TM