Thursday, 26 November 2009

Stone repairs

Many years ago, 1989 I think, I drew up and specified repairs to window mullions in an Oxfordshire church. They were split where the ferrementa (the iron framing used to support larger leaded windows) had corroded and had previously been repaired with iron straps, which had also corroded and caused further damage. The internal sections of the mullions had several hundred years of grafitti and were to be retained and consolidated. The new mullions were in Taynton limestone and the ferrementas were repaired with stainless ends to prevent future corrosion. Richard Noviss did the work.

Thursday, 19 November 2009

Lime training III

One of the training events was held at Low Luckens Organic Resource Centre. Great location; lots of old stonework and pointing and a tatty bit of wall on which we could practice pointing and limewashing.

Wednesday, 18 November 2009

Lime training II

Also did this one on limewash.


There are many different ideas about limewash mixes and whilst some people advocate a mix as thin as skimmed milk and in many coats (as much as twenty) I have seen mixes like thick cream, applied in a single coat that still work well. It seems that preparation and aftercare are more important. Though it is unlikely that this was often achieved traditionally it will minimise the risk of flaking or dusting.

If pigments are to be added these should be kept to a single colour as it is very hard to maintain consistent colour between batches.

The surface should be wetted and allowed to soak in so that the surface appears damp but there is no standing water. Apply the limewash with a natural bristle brush (use a cheap one because the limewash rots them very quickly) working it well into the surface to fill irregularities and cracks. A fresh coat will be translucent but as it carbonates and dries it will become more opaque. Limewash may dry at different rates because the water content varies within the substrate. Limewash sets by carbonation through absorbing carbon dioxide. If it dries too quickly it cannot carbonate and becomes dusty, however if it is too wet it will remain translucent. If it dries too rapidly it should be lightly wetted or covered with dampened hessian.

Limewashing should be avoided in very hot or cold weather. I suggest that the best time is at the end of summer and early autumn when the weather is cooler and damper.

Assuming that the existing surface is old limewash this should be brushed down to remove any loose areas and dust.

Old render that has never previously been limewashed should be inspected to check the cement content. If it absorbs water readily then it may be a relatively soft mix or even contain some lime. Limewash will not bond to a hard cement mix. This will require thorough cleaning to remove moss and lichen, especially if it is roughcast or harling. If one applies a thin limewash it will kill moss and lichen and it is possible to leave this for a few weeks, months or even a year before brushing it down and limewashing properly.

If there is modern masonry paint this should be scraped off or removed with a pressure washer or the Stonehealth DOFF system. However great care is needed as all pressure washers, including the DOFF can cause great damage if used carelessly. Limewash can be applied over small patches of paint. It will be patchy but this will help seal edges of the paint and as it is very alkaline it will accelerate its decay.

If further coats are required it is important that the previous coat is allowed to carbonate thoroughly before applying the next, otherwise the brush will lift the previous coat. The surface should be wetted again before applying the next coat.

Lime training

Recently did a couple of lime training events for Cumbria Green Build. I like to do this sort of thing spontaneously, partly because I seldom get time to prepare properly, but at the last moment I wrote up some handouts.


Lime mortar is a mix of aggregate and lime putty. There are many mixes that are gauged with materials such as cement to give an hydraulic set. This adversely affects the plasticity and porosity and in my opinion should be avoided.

A typical mix will be one part of lime putty and 2.5 to 3.5 parts of aggregate. 1:3 is always a good starting point but you may require a stronger mix (1:2.5) for a sharp sand and weaker (1:3.5) for a soft sand. When mixing, there should be just enough lime to cover each particle of aggregate. If there is any less the mortar looses plasticity, if there is too much the risk of shrinkage is increased. Lime mortars must be mixed thoroughly and when properly mixed they become very plastic and will stick to a trowel even when held upside down. Proper mixing is essential to ensure the optimum vapour exchange. Lime mortars set by carbonation through absorbing carbon dioxide. If mortar dries too quickly it cannot carbonate and becomes friable, however if it remains wet it will not set at all. If it dries too rapidly it should lightly wetted or covered with dampened hessian.

Traditional mortars appear to use any available sand and I have occasionally seen mortars with no obvious aggregate, just fine silt. It is not unusual to find mixes that have large lumps of unslaked lime. In theory these mixes will always fail though they clearly work in the long term.

Avoiding failure in modern lime work is a difficult issue, especially as the specifier may have professional or contractual liabilities. A cement mortar or render can be thrown onto the wall and will usually look good for many years, despite the problems developing underneath. However a lime mix may show dampness or spall and crack in the first winter. This is not failure, it is the result of using a softer and more breathable material, which in the long term is beneficial for the building. The specifier should resist the temptation to design a gauged mix or a complicated application procedure as this may give an inappropriate finish. It is very important to educate the client so they understand the limitations of lime.

If one makes a mix with a sharp sand, with a maximum particle size of about 5mm then this can be used as a general purpose mix for pointing, bedding, render base coat and haired plaster base coat. If the pointing is to be in thin joints of ashlar stone then the particle size should be 2mm max. Hair is used in plaster base coat to reduce cracking when applied onto laths and if cracks do appear then the hair will help retain strength. For a plaster top coat you will need a fine and soft sand, for example silver sand and a strong mix, possibly as much as 1:1.

Natural hydraulic lime (NHL) is becoming more popular in the UK. This is available as a dry powder much like cement and can mixed in any site mixer. It does require 15to 20 minutes to give a thorough mix, this will ensure the optimum vapour exchange. NHL is usually available in three grades of strength, 5, 3.5 and 2. This describes the rate at which set occurs and eventual hardness. NHL 5 is rather hard and should be reserved for structural work only.

Vapour exchange is the rate at which moisture evaporates through the mortar and is measured in g air/m²/hour/mmHg. Soft mixes perform better and hard mixes, like 1:3 OPC, are in effect waterproof.

Matrix Ratio VEx
Lime putty 1:2.5 0.71 approximately
NHL 3.5 1:2.5 0.68
OPC/hydrated lime 1:1:6 0.23
OPC 1:3 0.15 approximately