An article originally written for "Rural Wales" the magasine of CPRW

The operation of modern cement and lime mortar is quite different.  Cements set by a non-reversible chemical action to form a solid, non yielding mass. Lime mortars set by the absorption of carbon dioxide over a much longer period of time. An understanding of the chemical action of lime is therefore helpful.

Lime is derived from limestone or Calcium Carbonate (CaCO3), which has been burnt in a lime kiln at over 800 degrees centigrade to produce quicklime or Calcium Oxide (CaO). In this process carbon dioxide and any water is driven off. Quicklime is potentially dangerous having an avid thirst for water. This process creates a lot of heat and produces Slaked lime or Calcium Hydroxide (Ca(OH)2). Builders merchants stock this material having been reground to a fine powder and called Hydrated Lime.

The slaking process releases enough heat to cause the water to boil or spit. For this reason water must NEVER be added to quicklime. The lime must ALWAYS be added slowly into the water in a metal container. For the inexperienced builder or home craftsman, slaking is therefore not recommended.                              Return to Conservation Index

Hydrated lime can be used as the basis of all lime plasters and mortars. It will slowly absorb free carbon dioxide in the atmosphere and revert to Calcium Carbonate. A triangle has therefore been completed converting the calcium back to its original chemical form of limestone. For this reason lime mortars have the potential for continuous recycling.

The first step in the making of a lime mortar is to make a lime putty by dissolving hydrated lime in water and allowing to soak for at least a week, stirring and beating as required to form a thick creamy liquid. The use of a heavy duty paint mixer attached to an electric drill is ideal for this operation.

The dry hydrated lime particles are very fine and the material therefore has a very large surface area. The long period of soaking is necessary to ensure that the surface of every particle is exposed to the action of water. This process can be compared to making a white sauce or flour paste. If dry flour is stirred into cold water the powder does not seem to want to mix, and sits uncomfortably in lumps. If a small amount of water is used then the flour blends more easily to form a thick cream to which more water can be added to obtain the right consistency. It is important to avoid creating lumps of congealed lime particles. For this reason, care should be taken when purchasing hydrated lime to buy fresh material that is as dry as possible. Bags that may have been left on a damp floor in a warehouse will have partially absorbed water to form small congealed lumps that do not dissolve easily.                     Return to Conservation Index

Once a creamy consistency has been achieved the lime putty can be used, but unlike cement, it can also be stored in covered plastic buckets or mixed with sand to form what is called "coarse stuff". Lime putty stored in buckets will slowly settle out leaving a lake of clear "limewater" with a sediment of white lime putty underneath. The longer this putty is stored the more it will improve. Old lime putty that has been kept under water and away from air will solidify, but it can easily be knocked up to become workable again. If you do not want to make your own lime putty in this way, it can now be purchased ready made in large plastic buckets from specialist suppliers. However, for the comparatively little trouble it is to make your own it is a great saving of expense.

While the lime putty is soaking, survey and prepare the walls to be re-pointed. Before starting work remove any wall plants, and clear the footings. An old carpet laid on the ground will collect falling rubble and mortar. It is worth spending some time looking over each wall, as I often say, "give it a good coat of looking at", like giving it a coat of paint. Check how the wall is built. Are the bold quoin stones at the edges of the wall and around window and doorways secure? Check places where water may have flowed from a leaking gutter. Check any lintels for cracks, or ironwork that may be causing damage to the mortar or stones. Many stone walls will use chipped or broken pieces as wedges between the courses. These should not be removed, and if they fall out or seem to be missing in places, then they can be replaced with new matching broken pieces of stone. They should not be hammered in like nails as this could dislodge the structure, but just pushed firmly enough to secure the courses. Walls may have been re-pointed using the wrong type of mortar, and this will have cracked away of damaged the stone. This should be removed carefully, and any old loose lime mortar raked out using any simple tool made from a bent spoon, screwdriver or file. Only rake out lime mortar that is obviously damaged from excessive water action. Do not remove so much that the structure becomes unsafe. If necessary do small sections at a time and re-point before continuing. Generally the use of wedged galleting stones will secure a wall whilst raking out.

When the lime putty is fully soaked, the final mortar can be mixed. Samples of the old mortar should be used as a guide to the composition and texture to be aimed at. Most old mortars are a mix of lime putty, lime sand, and grit. Generally a greater proportion of lime is used for sandstone or sedimentary rocks and a harder mortar use for granite or impervious rocks.

The sort of building sand commonly available from builders merchants to-day was not used. Often rough sand would have been cut from a local source, and a great saving can be achieved by exploring the locality or asking old sages where sand was obtained. In non-clay areas, subsoils can often be sieved to provide a workable aggregate. Grits were often obtained from rivers in which case the particles are of a rounded form. Sharp grits can be used which are a waste product from stone quarrying.                 Return to Conservation Index

Lime sand is not the same as the hydrated lime powder used to make the putty, but is simply ground limestone waste. It is available very cheaply from many lime quarries in North Wales, and the colour can vary from grey to pure white. This material is generally available unbagged by weight, so it helps to bring your own bags and a shovel, or a lorry or trailer if you want a quantity. Lime sand is available in several different grades from course to fine. The finer grade is sometimes called agricultural lime, and is spread by farmers to raise the pH of the land.

All these materials can be intermixed to make up the lime mortar as closely as possible to that which was used before. A few points may be helpful as a guide. Keep the mixture too dry rather than too wet as it is easy to add water but impossible to remove it. The addition of a small amount of fine stove ash will make the mortar sticky and easier to use. If you use a mixing box or bucket and measure each mix carefully then a consistent texture can be achieved over the whole wall. I often use a home made wooden hawk with an extra block of wood along one edge to which some sponge has been glued or pinned. This enables the edge of the board to be held firmly against the rough surface of the wall and the mortar can easily be pushed into the joints without any spillageThe style of pointing chosen is very important. Mortars used in stone work should be slightly recessed from the stone face by about 3mm to 7mm. Aim to leave the mortar surface about 5mm behind the exposed surface of the stones. Raised and trowelled pointing (sometimes called ribbon pointing) should not be used. Very often it is a mistake to copy a pointing style that has been used for a different type of work. Care should also be taken not to create thin feathered edges as shown in the diagram. I generally over fill slightly and then wash the whole wall down after a day or so when the mortar has begun to harden.

Originally prepared as an article for RURAL WALES, the magazine of CPRW.
Written by John Nicholson, Tanrallt, Rhostryfan, Caernarfon, Gwynedd, LL54 7NT

Return to Conservation Index