POINTING OF WALLS
Posted: 26 May 2026, 01:20
POINTING OF WALLS
5 April 2006
All this started with a personal gripe against Time Team (A programme of archaeological digs on Channel 4 TV). A few weeks ago they did a dig in the middle of Manchester on an old Arkwright mill and the programme quickly confirmed my impression that all the presenter Tony Robinson is interested in is Romans and gold artefacts, Saxon is interesting but industrial is boring. At one point in the programme one of the ‘experts’ (Prior) woke up to the fact that he had ‘discovered’ a new way of dating parts of the building by the type of mortar used in the joints.
In a word, phooey! I refrained from doing anything about it, it’s Somebody Else’s Problem (SEP). However, I noticed something yesterday that started a train of thought about pointing and made a connection that will become obvious below.
Dating a construction by the composition of the mortar. The first thing to say about this is that my bible on pointing is a booklet produced by the Society for the Protection of Ancient Buildings (SPAB). If you have any pointing to do anywhere in the world get hold of a copy and follow it religiously, you can’t go wrong if you do.
So how do you do the dating? Take Barlick as an example. The classic mortar from Roman times onwards is Putty Lime and sand. Putty lime is produced by digging a pit, filling it with freshly burned lime (quicklime) which is violently hygroscopic, that is, if you pour water on quicklime there is an immediate exothermic reaction. The lime and water mixture reacts, produces enough heat to boil the mixture and eventually it settles down after a couple of days to a layer of hydrated lime covered by a layer of water. The water was drained off the top and the putty lime (it is just like soft putty) is taken out as required and mixed with sand and sometimes horse or cow hair. It can be identified by the fact that after it has set it is almost pure white and the early quicklime was also contaminated by ash or charcoal which show as black flecks. Straight putty lime will eventually set rock hard but the addition of sand and other ingredients softens it and makes it more porous.
Putty lime was superseded by ash lime mortar which was still being used in the 1950s. This is mortar made by grinding clinker and ash from industrial coal fired boilers with quicklime and water in a mortar pan. The pan is a heavy cast iron saucer with a single or double roller mounted on a central pillar which is driven by power, very often off the end of a lineshaft at a steam-driven mill as the ashes and power were present on the site, all that was needed was the lime. This grinding produced a very slow-setting coarse grey mortar which can be kept for days before use as long as it is kept damp and ‘knocked up’ again before use to regain plasticity. Ash lime mortar is dark grey, gritty and very durable. It’s final hardness and porosity can be controlled by the proportion of lime added. The more lime, the softer and more porous.
In Barlick mortar-based dating is simple and the same principle applies to every other early building. If the mortar is white lime sand mortar the building was almost certainly constructed before steam boilers arrived in the community. If it is dark grey ash lime it was made after the boilers and steam power arrived. In Barlick this can be dated to 1830.
I was asked a question a few years ago about the composition of the mortar in a very old mill that was being refurbished under supervision of English Heritage. There was doubt about what mix to use for the replacement mortar in the re-pointing. EH had commissioned incredibly complicated chemical analysis which whilst precise, told us nothing in practical terms. I pointed out that at the time the mill was built transport was the biggest consideration when building. Therefore the lime would come from the nearest kiln and the sand from the river behind the mill. They tried this and hit the exact composition. The point of this is that the best way to decide in a case like this is to use common sense and historical knowledge.
If you have been reading carefully you will note that I keep mentioning porosity. Finding the ingredients is one thing, knowing what the best mix for the job is is an entirely different matter. We have to go back to basics and ask why we use mortar at all. Most people will immediately assume that it is used as an adhesive to bind the stones or bricks together. Not so, the mortar is used to bed the bricks or stones to each other, the weight of the construction gives it the strength as long as the elements of the wall are in intimate contact. My evidence for this is that when a chimney is felled the first bricks removed from the base have to be pulverised to get them out. Once the weight has been removed, the bricks surrounding the gap can be easily removed whole.
There is another reason for using a weak mortar. It is more porous and will allow water to escape from the structure through the joints. If the mortar is impervious it prevents water from flowing in the structure to the outside where it is evaporated. Not all bricks or stones are impervious and the natural course of water in the construction is to descend by gravity and migrate to the surface where water is being dried off by evaporation. Anything that hinders this progress holds water in and increases the chance of frost damage in winter.
There is another point to take into consideration as well. If you look at the construction of houses in Cumbria that are built of the local stone which has a very narrow bed and is more like slate, you will see that the gaps between the individual elements aren’t pointed to the surface, the bedding mortar is perhaps two inches back into the wall. This never weathers as the effects of the weather are broken by the jagged face of the edges of the thin stones on the face of the wall. Many old walls in the NW of England built of squared masonry are ‘watershot’. Each course is set so that the face of the squared block faces slightly downwards. This ensures that any water on the face of the wall or below the surface naturally moves outwards and then drips down. These walls are not damaged by the pointing being weathered or set back, indeed the correct way to point them is to cut the old mortar back at least one inch and point to just below the face of the wall. The classic bad pointing is ‘strap pointing’ which should never be allowed.
Rubble stone walls are perhaps the only construction where pointing is crucial. Because there is no coursing they are best maintained by attending to the pointing but keeping it back from the face. Many ‘expert builders’ use strap pointing on such a wall because it makes the coursing look more regular and dressed stone is a higher status construction.
When we come to modern sand and cement mixes for mortar we find that almost every builder uses river sand and cement in varying proportions. Usually four or five parts sand to one of cement. These mortars have one thing in common, they are virtually impervious which is bad for the wall. The cure is to incorporate hydrated lime in the mix. 1:4:1 lime/sand/cement is acceptable but I have often used 2:3:1 to get more porosity. Peter Tatham my old steeplejack taught me this. One more tip if you have set a contractor on to point a wall. Make them do a test patch which shows at least three quarters of an inch cut back on brickwork and more on stone. (Only allow cutting wheels on the horizontal joints, the verticals should be done with cutting-out chisels to avoid damage to the arris of the bricks.) Point half of the test patch to the result you want, making sure that it is pointed back below the surface. Make it part of the contract that any work not done to this standard must be pulled back and re-pointed at the contractor’s cost. They won’t like it and it’s expensive but this is the proper way to do the job, anything else is cosmetic only and a waste of time and money.
Here’s the bit of wall that got me going on this. This a new wall made with porous concrete blocks and pointed with a straight sand/cement mix. Notice how the construction has dried out after heavy rain everywhere except over the joints where the impervious mortar has trapped the water. This is a classic example of the way that non-porous mortar affects a construction. Right! Sorry about that but I needed to get it off my chest. If it helps to date one building or improves one pointing job it will be worthwhile.
5 April 2006
5 April 2006
All this started with a personal gripe against Time Team (A programme of archaeological digs on Channel 4 TV). A few weeks ago they did a dig in the middle of Manchester on an old Arkwright mill and the programme quickly confirmed my impression that all the presenter Tony Robinson is interested in is Romans and gold artefacts, Saxon is interesting but industrial is boring. At one point in the programme one of the ‘experts’ (Prior) woke up to the fact that he had ‘discovered’ a new way of dating parts of the building by the type of mortar used in the joints.
In a word, phooey! I refrained from doing anything about it, it’s Somebody Else’s Problem (SEP). However, I noticed something yesterday that started a train of thought about pointing and made a connection that will become obvious below.
Dating a construction by the composition of the mortar. The first thing to say about this is that my bible on pointing is a booklet produced by the Society for the Protection of Ancient Buildings (SPAB). If you have any pointing to do anywhere in the world get hold of a copy and follow it religiously, you can’t go wrong if you do.
So how do you do the dating? Take Barlick as an example. The classic mortar from Roman times onwards is Putty Lime and sand. Putty lime is produced by digging a pit, filling it with freshly burned lime (quicklime) which is violently hygroscopic, that is, if you pour water on quicklime there is an immediate exothermic reaction. The lime and water mixture reacts, produces enough heat to boil the mixture and eventually it settles down after a couple of days to a layer of hydrated lime covered by a layer of water. The water was drained off the top and the putty lime (it is just like soft putty) is taken out as required and mixed with sand and sometimes horse or cow hair. It can be identified by the fact that after it has set it is almost pure white and the early quicklime was also contaminated by ash or charcoal which show as black flecks. Straight putty lime will eventually set rock hard but the addition of sand and other ingredients softens it and makes it more porous.
Putty lime was superseded by ash lime mortar which was still being used in the 1950s. This is mortar made by grinding clinker and ash from industrial coal fired boilers with quicklime and water in a mortar pan. The pan is a heavy cast iron saucer with a single or double roller mounted on a central pillar which is driven by power, very often off the end of a lineshaft at a steam-driven mill as the ashes and power were present on the site, all that was needed was the lime. This grinding produced a very slow-setting coarse grey mortar which can be kept for days before use as long as it is kept damp and ‘knocked up’ again before use to regain plasticity. Ash lime mortar is dark grey, gritty and very durable. It’s final hardness and porosity can be controlled by the proportion of lime added. The more lime, the softer and more porous.
In Barlick mortar-based dating is simple and the same principle applies to every other early building. If the mortar is white lime sand mortar the building was almost certainly constructed before steam boilers arrived in the community. If it is dark grey ash lime it was made after the boilers and steam power arrived. In Barlick this can be dated to 1830.
I was asked a question a few years ago about the composition of the mortar in a very old mill that was being refurbished under supervision of English Heritage. There was doubt about what mix to use for the replacement mortar in the re-pointing. EH had commissioned incredibly complicated chemical analysis which whilst precise, told us nothing in practical terms. I pointed out that at the time the mill was built transport was the biggest consideration when building. Therefore the lime would come from the nearest kiln and the sand from the river behind the mill. They tried this and hit the exact composition. The point of this is that the best way to decide in a case like this is to use common sense and historical knowledge.
If you have been reading carefully you will note that I keep mentioning porosity. Finding the ingredients is one thing, knowing what the best mix for the job is is an entirely different matter. We have to go back to basics and ask why we use mortar at all. Most people will immediately assume that it is used as an adhesive to bind the stones or bricks together. Not so, the mortar is used to bed the bricks or stones to each other, the weight of the construction gives it the strength as long as the elements of the wall are in intimate contact. My evidence for this is that when a chimney is felled the first bricks removed from the base have to be pulverised to get them out. Once the weight has been removed, the bricks surrounding the gap can be easily removed whole.
There is another reason for using a weak mortar. It is more porous and will allow water to escape from the structure through the joints. If the mortar is impervious it prevents water from flowing in the structure to the outside where it is evaporated. Not all bricks or stones are impervious and the natural course of water in the construction is to descend by gravity and migrate to the surface where water is being dried off by evaporation. Anything that hinders this progress holds water in and increases the chance of frost damage in winter.
There is another point to take into consideration as well. If you look at the construction of houses in Cumbria that are built of the local stone which has a very narrow bed and is more like slate, you will see that the gaps between the individual elements aren’t pointed to the surface, the bedding mortar is perhaps two inches back into the wall. This never weathers as the effects of the weather are broken by the jagged face of the edges of the thin stones on the face of the wall. Many old walls in the NW of England built of squared masonry are ‘watershot’. Each course is set so that the face of the squared block faces slightly downwards. This ensures that any water on the face of the wall or below the surface naturally moves outwards and then drips down. These walls are not damaged by the pointing being weathered or set back, indeed the correct way to point them is to cut the old mortar back at least one inch and point to just below the face of the wall. The classic bad pointing is ‘strap pointing’ which should never be allowed.
Rubble stone walls are perhaps the only construction where pointing is crucial. Because there is no coursing they are best maintained by attending to the pointing but keeping it back from the face. Many ‘expert builders’ use strap pointing on such a wall because it makes the coursing look more regular and dressed stone is a higher status construction.
When we come to modern sand and cement mixes for mortar we find that almost every builder uses river sand and cement in varying proportions. Usually four or five parts sand to one of cement. These mortars have one thing in common, they are virtually impervious which is bad for the wall. The cure is to incorporate hydrated lime in the mix. 1:4:1 lime/sand/cement is acceptable but I have often used 2:3:1 to get more porosity. Peter Tatham my old steeplejack taught me this. One more tip if you have set a contractor on to point a wall. Make them do a test patch which shows at least three quarters of an inch cut back on brickwork and more on stone. (Only allow cutting wheels on the horizontal joints, the verticals should be done with cutting-out chisels to avoid damage to the arris of the bricks.) Point half of the test patch to the result you want, making sure that it is pointed back below the surface. Make it part of the contract that any work not done to this standard must be pulled back and re-pointed at the contractor’s cost. They won’t like it and it’s expensive but this is the proper way to do the job, anything else is cosmetic only and a waste of time and money.
Here’s the bit of wall that got me going on this. This a new wall made with porous concrete blocks and pointed with a straight sand/cement mix. Notice how the construction has dried out after heavy rain everywhere except over the joints where the impervious mortar has trapped the water. This is a classic example of the way that non-porous mortar affects a construction. Right! Sorry about that but I needed to get it off my chest. If it helps to date one building or improves one pointing job it will be worthwhile.
5 April 2006