Mortar
Hydraulic lime based mortars are usually used for new building work. For stone repair work and repointing lime putty mortars are often used.
Making Mortars, Plasters and Renders
- Lime putty and aggegates should be thoroughly mixed in large batches using the paddle or roller mixer.
- The proportion of lime putty to aggregate should be 1:2, 1:2.5 or 1:3 depending on aggregate type.
- No water should to be added to the mix.
- The mortar, plaster or render should be left to mature in airtight containers
Pointing Mortars
Preparation
- Remove loose friable mortar and cement mortar to a depth of at least 1″ (25mm) or to a depth of three times the width for narrow joints. Retain as much of the original lime pointing in the joints as possible.
- Flush loose material from the joints and ensure the joints are well wetted.
- Remix matured putty lime mortar to plastic state and add pozzolan if required or if using hydraulic lime then this should be prepared now.
Application: Putty lime pointing
- Pack joints thoroughly using appropriate sized pointing keys and fill joint proud of wall surface.
- Compress the mortar using pointing irons after it has stiffened.
- Shape and texture mortar surface whilst in a ‘green’ state using a churn brush for walling and ashlar and smaller brushes or spatulas for moulded work.
- Ensure mortar cures slowly and does not fully dry for at least three days. Protect curing mortar from sun, wind and rain using dampened hessian. Dampen down mortar regularly in hot weather.
Application: Hydraulic lime pointing
- Pack joints thoroughly using appropriate sized pointing keys and fill to above required level.
- Texture the mortar surface whilst in a ‘green’ state by gently scraping back using metal or wooden spatulas.
- Ensure mortar cures slowly and does not fully dry for at least three days. Protect curing mortar from sun, wind and rain using dampened hessian. Dampen down mortar regularly in hot weather
Plaster
Internal plasters are usually based on putty lime or feebly hydraulic lime. Hair is always added for work on laths and wattle, and sometimes for applications on walling. Traditionally internal plasters are given a fine finish and painted with limewash or distemper.
Making Mortars, Plasters and Renders
- Lime putty and aggegates should be thoroughly mixed in large batches using the paddle or roller mixer.
- The proportion of lime putty to aggregate should be 1:2, 1:2.5 or 1:3 depending on aggregate type.
- No water should to be added to the mix.
- The mortar, plaster or render should be left to mature in airtight containers
Limewater
Limewater is a clear solution of lime which was traditionally used by fresco painters as a binder for their pigments. In conservation it is used for strengthening the surface of fine grained limestones and lime plasters.
Making Limewater
- Place approximately 100mm of mature lime putty (Buxton or Cheddar) in a parallel sided metal or plastic dustbin.
- 3/4 fill the bin with cold tap water and stir vigourously for 5 minutes.
- Place an airtight wood and polystyrene float on the limewater surface and leave to settle and clear for 24hrs.
- Place a wet hessian sacking around the bin to keep the limewater cool. Ensure the hession is kept wet.
Preparation
- Draw off the limewater as required using a patent ‘solo’ spray pump attached to a clear polythene tube inserted 2″ (50mm) into the limewater through a central hole. Alternatively the limewater can be drawn off into a portable garden spray (killaspray type).
- Wet the stone with one application of tap water prior to first application of limewater to aid its penetration.
- Ensure the limewater is water clear.
- Only apply to cool stone. Do not limewater stone that has been warmed by the sun and do not apply limewater in hot weather.
Application
- Apply a floodcoat of limewater starting at the bottom of the area to be treated, moving sideways and upwards.
- Immediately after application sponge off any access limewater from the stonework surface using a clean sponge frequently rinsed in a bucket of clean water. This will prevent white limebloom from forming.
- Whenever possible do not let the stone dry out between applications.
- Eight applications per day is likely to be maximum attainable.
- Apply a total of 40 coats or until the rate of absorption by the stone surface has been significantly reduced.
Sheltercoat
Stone that has been cleaned or is severely eroded is much more vulnerable to atmospheric pollutants and the weather. It is common practice with limestone conservation to work in a protective lime coating to the surface once all other conservation and repair work is complete. This coating is called a sheltercoat and is made up of putty lime, fine stonedusts and fine sand.
Making Sheltercoat
- Blend lime putty and aggregates thoroughly together.
- Mature in airtight containers until required.
Preparation
- Flush loose dust from stone surface.
- Dampen stone down several times using limewater.
- Thin sheltercoat mortar down to a thin creamy consistency with a 50/50 mixture of water/skimmed milk. This must be done no longer than one hour before it will be used otherwise the binding effect of the casein may be lost
Application
- Apply to the stone using a soft short haired brush working the sheltercoat well into the surface paying special attention to eroded areas, cracks and fissures.
- As the sheltercoated surface turns matt (as water is absorbed into the stone) work in the sheltercoat a second time with a clean dry brush. Timing is imperative – if the sheltercoat is too wet it is merely moved around the stone surface, if it is too dry it is brushed off. This second working in is comparable to the compacting of putty lime mortars as they stiffen after application. When sheltercoat is applied properly the brushstrokes should be barely visible.
- When the sheltercoat has completely dried a small brush can be used to remove any excess that may have built up in undercut areas and fine detail.
- Protect sheltercoated surfaces promptly after an area has been completed. It is essential that sheltercoat dries slowly to ensure maximum strength is obtained.
- Dampen down sheltercoated areas each day for three days after application.
Limewash
Limewash is the lime equivalent of paint. In its pure form is finely sieved lime putty diluted with water. Pigments can be added to produce the broad range of creams, buffs, pinks, yellows and reds which adorn so many traditional buildings. To increase the durability of limewash linseed oil, tallow or casein can be added.
Making Limewash
- Mix lime putty with clean water to the consistency of thin cream.
- Pour the mixture through a fine seive.
- Add pigments by first mixing powdered earth pigment with a little warm water in a screw top and shaking well. Add this liquid to the white limewash and mix thoroughly.
- Thin mixture again to the consistency of creamy milk.
Limewash is a traditional paint for decorating stonework, renders and plasters. In its raw form it is white or off-white but is often coloured with earth pigments to give colours like pink, warm grey, buff, cream and yellow. Limewash is moisture permeable and allows stonework and plasters painted with it to ‘breathe’. It is made from either lime putty or slightly hydraulic lime.
Preparation
- Brush down and flush loose dust from stone/plaster surface.
- Dampen stone/plaster surface thoroughly.
Application
- Apply using a large bristle brush working the limewash well into the surface paying special attention to eroded areas, cracks and fissures.
- Apply up to four coats allowing one day curing between each coat.
Protect limewashed surfaces from premature drying as it is essential that limewash dries slowly and remains damp for at least four hours to ensure maximum strength. Limewashed surfaces may be dampened down with fine mist sprays if early drying is a problem
