Lime For Conservation
Traditionally, buildings constructed of lime have been repaired and maintained using lime. In the 19th century with the advent of the harder Portland cement the use of lime went into decline. However time has shown that dense, hard, grey cementitious mortars do not benefit stonework, brickwork or plasterwork in the long term and can often be damaging. The conservation of the west front of Wells Cathedral was a watershed in the revival of the use of lime where a method of conservation known as the 'lime method' was used. Over the last 20 years this has become the standard repair technique for limestone on historic buildings in Britain.

Lime has many conservation and repair functions and can be used as a:

• mortar for repairing damaged or decayed stonework,
• mortar for repointing joints,
• grout for filling voids and cracks,
• surface consolidant,
• protective and decorative surface coating

Repairing Stonework
Decayed or damaged areas of stonework can be repaired using a colour and texture matched lime mortar. A typical repair mortar is composed of lime putty to aggregate in the proportion of 1:2.5. The aggregate is usually a mixture of crushed stone and sand which should be clean, well graded and angular. Often crushed stone of the type being repaired is included in the repair mortar to aid colour and texture matching. The addition of 10% pozzolan to putty lime repair mortar to increase its strength is common practice. Hydraulic lime can also be used for repair mortars although care must be taken not to make the mortar harder than the stone being repaired.
Using stone repair mortars.

Pointing
Traditionally stone and brick buildings were built and repointed using lime mortar. The soft and permeable qualities of lime mortar allow for any movement in the structure and ensure moisture in the walls can freely evaporate.

Unfortunately over the last 150 years or more Portland cement based mortars have commonly been used for repointing joints that were previously filled with lime mortar. Strong cement mortars are invariably too hard for limestone, they can cause damage and being grey are drab and unsightly. Wherever possible they should be removed and replaced with lime mortar which allows the wall to 'breathe'.

The typical cream, buff, pale brown, pink and white lime mortars which reflect the local sand colour, are much more in keeping with the colours and textures of stone and brick. Putty lime mortars or feebly hydraulic mortars should usually be used for repointing brickwork and limestone. However areas susceptible to greater weathering and in exposed positions may require repointing using moderately or eminently hydraulic lime mortars. Whichever type of lime is selected it is essential that well graded, angular and washed sand is used to provide strength and be free of salts.
Using pointing mortars.

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. Casein in the form of skimmed milk is added as it reacts with lime to from a binder for the fine aggregate. Sheltercoat should dry the same colour as the clean weathered stone being treated. Colour trials are essential to obtain the correct colour otherwise the appearance of the treated work can be ruined.
Using sheltercoat.

Limewater
Limewater is a clear solution of lime (calcium hydroxide) which was traditionally used by fresco painters as a binder for their pigments. It is only slightly soluble - at 20 degrees centigrade 1.4 grammes will dissolve in 1 litre of water. In conservation it is used for strengthening the surface of fine grained limestones and lime plasters. It is not a scientifically proven technique but many conservators have experienced success with it.
Using limewater.