Working with Lime
— Barbara Jones
Lime has been used all over the world as a binding material and as a surface protector of buildings for thousands of years. Here in Europe, lime was used in building for hundreds of years before contemporary cement was invented. However, whereas cement is foolproof (in that any fool can use it), lime requires thought and an understanding of the processes involved in the slow carbonation back to limestone in order to use it successfully. The preparation and practice of lime work is simple, but variables in the materials themselves (the sand, the lime, and particularly in the weather during application and drying time) are crucial to the overall durability of the material.
Traditionally, knowledge about lime was passed down from one generation to the next and so built up a wealth of experience based on a sound knowledge of the material. Today, there are very few skilled craftsmen left living (I haven't found any women yet) who worked in those times, and we have to learn as best we can from what we have left. To some extent, that can lead us into an overly technical approach to what was essentially a practical and rather adhoc building practice. We are trying to specify exact lime/sand mixes when most likely what happened on-site was fairly rough and ready, except for the most prestigious jobs.
Limestone and Lime Burning
The raw material for lime mortars and renders is naturally occurring limestone, shells, or coral calcium carbonate (chemically CaCo3). The process of making lime putty from the raw material is relatively simple. Traditional, limestone is placed in a specially built kiln (sometimes a pit or a heap), layered with fuel such as coal or brush, and burnt for about 12 hours. At the end of the burning process, whitish lumps of calcium oxide are left, along with bits of burned and unburned fuel. Overburned limestone appears as black, glassy pieces, and these are removed and discarded.
The material needs to reach a temperature of 2,200 degrees Fahrenheit (1,200 degrees Celsius). At 1,650 degrees Fahrenheit (900 degrees Celsius), carbon dioxide (CO2) is driven off, and 2,200 degrees Fahrenheit (1,200 degrees Celsius) is required for the heat to penetrate through to the center of the stone. As it heats up, steam (H2O) is driven off, and a chemical change occurs. Heat acting upon calcium carbonate (CaC(3) produces calcium oxide (CaO) plus carbon dioxide (CO2). The chemical reaction is usually more complicated than this, due to other carbonates and silicates being present in the limestone, but it's important to understand the basic changes that are taking place at this stage.
Calcium oxide is very reactive and can be dangerous; it is called 'lump-lime' or 'quicklime' and may be left as lumps or ground down into powder. It must be kept dry, as it reacts very quickly with water — even the water in the air or the moisture in your skin — to form calcium hydroxide, which is the first step to reversing the process back to calcium carbonate. Just as making quicklime needed heat, the reverse process produces heat: calcium oxide (CaO) plus water (H2) produces calcium hydroxide (Ca(OH)2) and heat. Quicklime added to water gives us lime putty!
How to Make Lime Putty
Always add quicklime to water. Never do it the other way around. If you do it backward, it can explode!
Recipe: 1 part quicklime to 2 parts water, by volume. Great care must be taken in the making of lime putty. A mask, goggles, and gloves must be worn. The tremendous amount of heat that is generated produces steam and can spit lime.
The process of adding quicklime to water is called 'slaking.' In the method I use, only a portion of the water is used to begin with, followed by some of the quicklime. Two people are needed: one to pour the water and quicklime, and the other to rake and mix. First, water is poured into a metal tub. Care must be taken when placing the bath, as the heat generated can burn any grass or wood underneath it. Quicklime is gradually added to the water, which is immediately raked and mixed; a garden hoe is the best tool for the job. Care needs to be taken as the temperature soars and the whole mix starts to bubble and boil. Water is added as the mix progresses and until all of the quicklime has been mixed.
The purer the quicklime, the faster the hydration process occurs. Raking and mixing is carried out until the lumps have all broken down; the resulting putty (which feels like double cream at this point) is sieved through a 1/16-inch (2 millimeter) grid to take out any pieces of unburned limestone, which will not react. These usually go back in the kiln for burning next time.
Slaked lime or lime putty is best stored for at least three months before use. This is to ensure that all the calcium oxide has hydrated, which can take time.
Do not expose lime putty to the air, or it will begin to carbonate before you can use it as a render or mortar. If it carbonates, then it will not bind with the sand when you mix it. Traditionally, lime putty was stored in a pit in the ground, where it would remain for several months or years before use. In fact, lime putty in pits would often be passed on from one generation to the next. The Romans forbade the use of any lime putty that was less than three years old, and all the old practitioners say that the older the lime is, the better.
Making Lime Render (Stucco) and Lime Plasters
There are two main ways to make a lime render or lime plaster.
1. Lime putty mix. Recipe: 1 part lime putty to 3 parts sand, by volume. Ideally, the sand should contain particle sizes ranging from very small (dust) to quite large (3/8 inch or 10 millimeters), and these should be angular, not rounded. The aim is to use only as much lime putty as necessary to fill the void spaces between the grains. The mix is almost always 3 to 1 because the void spaces take up about 33 percent or one-third of the volume of most sands.
The only real difference between a plaster (for inside work) and a render (for outside work) is the fineness or coarseness of the sand used. Render may contain aggregate up to 3/8 inches (1 millimeters) in size in areas that experience lots of wind-driven rain; usually people prefer a smoother finish on their inside walls, and so they would choose a sand with smaller grain sizes.
The longer a lime putty has matured, the more solid it becomes, and the better render it makes. It can be hard to work at first, but by pounding and beating it (in a large boat or on a sheet of plywood) with wooden mallets or posts, the putty soon becomes more plastic and can be worked into the sand. The process can be very labor intensive, but this beating part should not be left out. Because it's so hard to work, it may be easier to mix the sand with fresh lime putty, and then leave this mix to mature.
2. Hot lime mix. Recipe: 1 part quicklime powder to 3 parts sand, by volume. This is probably the most common method used traditionally in the U.K. In this method, the quicklime is added to damp sand and mixed with a shovel. It is raked and mixed continuously, and may not need any extra water depending n the dampness of the sand. Again, it should be left to mature for at least three months.
When the time comes to use a lime mortar or render, it should be beaten and worked to stiff consistency, so sticky that it can be held upside-down on a trowel. It will generally become more plastic with lots of beating! There should be no need to add water to it, which would increase the risk of shrinkage cracks. Traditionally, it was a completely separate trade to be a lime render beater. These days, render can be knocked up in a paddle mill (used by potters) to save all that work by hand. Generally, a cement mixer won't do the job, as the mix stays in a lump and can knock the machine over; the tendency then is to add water to soften it, and the resulting mix will crack due to too much shrinkage.
How to Use Lime Render and Plaster
For straw bale walls, my preference is to apply the first coat of lime by hand (with gloves!) because it's more fun, and because the straw tends to flick the stuff back at you otherwise. The lime needs to be well rubbed in to get a good bond between it and the straw. It's important to encourage the render to cure from the inside out, not to let the outside skin carbonate too fast. Therefore, keep the applied render or plaster moist (not wet). Here in England it takes two to three days before the render feels hard.
I apply the first coat thinly, leaving stubby bits of straw sticking out. It is usually ready for the second coat the next day, unless there are pockets of thicker mix in places. Before putting on the second coat, I wait until the first is hard enough that you cannot push your thumb into it. I wet the walls down with a mister (not a hose) before putting the second coat on. I work the second coat in with hands or a wooden float. The render is kept damp by misting it, unless there is an English drizzle!
Over the next few days, protect the render from direct sunlight, rain, forceful wind, and frost. You can hang sacks from scaffolding and keeping them moist, creating a humid atmosphere close to the lime. Misting is not to add water to the render, but to make sure that carbon dioxide can be carried into the thickness of the layer. It's probable that lime renders in straw bale walls carbonate more quickly than on stonework, because the straw itself is breathable. It thus has access to the back of the render as well as to the surface.
If the render cracks, I rework it (several times if necessary) before the surface hardens, to squeeze and compress the sand particles together. The aim is to compress all the render so that there are no air spaces left. Using a steel trowel on a lime render tends to close up the texture of the surface, preventing humid air from penetrating into the body of the render.
It's essential to understand the chemical change that starts to take place once the render is exposed to air, so as to know how best to care for it. Once render is on a wall, it begins to carbonate, a chemical process whereby carbon dioxide starts to change the calcium hydroxide back to the original limestone (calcium carbonate). Calcium hydroxide (Ca(OH2) plus carbon dioxide (CO2) produces calcium carbonate (CaCO3) and water.
The chemical change happens very slowly. Lime absorbs carbon dioxide from the air and only in the presence of moisture; too much water inhibits the process.
It can take a pure lime putty/sand mix several days to harden, which does not mean that all the lime is carbonated. Some of our renders are hundreds of years old, and still not all of the calcium hydroxide in them is carbonated. The ideal conditions for applying and curing a lime render are high humidity and good ventilation.
On the whole, it is a straightforward matter to use a lime render directly onto a mud plaster backing; this is traditionally what we did with our cob houses. However, there have been some failures, usually in the form of cracks developing around corners of buildings, and occasionally whole sheets of lime work falling off. There are two problems occurring here. The first has to do with different rates of expansion and contraction in the mud and the lime. Mud plasters generally have greater flexibility than those of lime, as in conditions that would cause expansion and contraction — such as significant changes in temperature or humidity — the backing coat (mud) may move more than the finish coat (lime), and cause the later to crack. This problem would be accentuated at corners where the render coat is in tension.
Solutions would be to: 1) make corners curved rather than angles to reduce tension; 2) add lots of fiber (chopped straw) to the mud and hair to the lime, to increase the tensile strength; 3) make sure the mud coat is wet and sticky before applying the lime.
The problem of the lime coat dropping off in pieces from the mud is caused by a poor bond between the two materials. This can be solved by: 1) making sure the mud coat is wet and sticky; 2) leaving the mud coat rough to provide a good key for the lime; 3) alternatively or also, applying a coat of limewash to the walls before rendering; 4) rubbing the lime well into the mud backing.
Dry Hydrate Lime Mixes: Type N and Type S
The bags of lime bought at a builder's supply store are almost always hydrated lime, which is quicklime that has been factory slaked only to the point that a powder is formed and not a putty. Hydrated lime is far less reactive and dangerous than quicklime and usually does not have the same properties as lime putty or hot lime mixes. In the U.K., our hydrated limes are made from quite pure limestone, otherwise known as high-calcium lime. In the U.S., due to differing geology, much of the limestone contains proportions of magnesium. It can still produce a good material.
Limes containing magnesium were often less favored when slaked outside of factory conditions, due to the fact that the magnesium component took longer to slake. However, this is no longer the case with Type S hydrates. Type S hydrates are autoclaved, ensuring that all the magnesium oxide has been slaked, as well as the calcium oxide. Although Type S dry hydrate can be used right out of the bag, it improves when made into a putty and gets better still if left to remain even longer in the putty stage. Type N hydrate is only partially hydrated; only the calcium portion of the lime is combined with water. Type N hydrate needs to be made up into a putty and aged before using to make sure that all the magnesium oxide has slaked.
When you buy powdered hydrated lime it is difficult to know anything about the lime. Usually you won' know what type of lime it is, how it was burnt, or how it was slaked. However, you may be able to find out how long it has been on the shelf and that, in the long run, may be the most important thing to know. If the dry hydrate has been in the bag too long, it may have already begun to absorb moisture from the air and begun to carbonate, thereby declining in quality. Some manufacturers date their bags. If not , you can always ask when a shipment arrived.
How to Make Lime Renders From Dry Hydrate
1. Always use fresh hydrated lime, less than one month old if possible. Look for the date of manufacture on the bag.
2. As far as possible, check the production process and buy from a reputable company --although this still doesn't guarantee the quality of the product!
3. Make up the hydrate into a lime putty by putting it into a bucket and adding water. Stir well, and only add enough water to make a very stiff mix. Leave it for 24 hours or longer, and then make up the render as described previously.
4. Use 1 part lime putty to 2-1/2 parts sand, by volume, to compensate for the extra aggregate if you think your dry hydrate is not quite pure calcium hydroxide. If you have reliable information that your hydrate is pure, then stick to the original 3-to-1 mix.
5. Once mixed up with sand, use in the same way as any other lime render.
Care of Lime Renders and Plasters
With all lime renders, most problems such as cracking or powderiness can be solved by a coat or two of limewash. Limewash is basically a more dilute form of lime putty, often with small amounts of other substances, such as casein, added for greater durability. Lime renders have the capacity to 'self-heal' (that is, cracks tend to close up over time) because they are slowly carbonating. As long as a lime render is limewashed as needed, there should be no need for other maintenance, unless something else is causing problems (for example, a broken gutter or an overgrown garden).
Lime renders should be limewashed as least every five years — every two or three is better — and it may need to be once a year on the most exposed wall. We had a tradition in England of limewashing our houses on May Day each year. Limewash is much quicker to use than ordinary paint because it's very watery. If you do need to patch in a section of render for any reason, the patch will adhere well to the rest of the render. Old render can be pounded up and used in addition to or instead of sand for aggregate in a new mix.
Barbara Jones has trained for over 20 years in carpentry, cob building, and limework in England and the U.K. She runs a (mostly)) free information and advice service, acts as consultant on large contracts, teaches on-site, and acts as an enabler for people to become involved in the building process themselves.