Conservation techniques

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Conservation techniques
  Recommended conservation technique  Followings are the techniques applicable for Conservation and restoration of Stone Heritage : 1.   REPOINTING Repointing is best performed by filling the prepared masonry joints with a tinted lime-based mortar mix to a designated level nearly flush with the stone. Lime based formulations are similar to the srcinal mortars and offer greater compatibility with the surrounding masonry than those based on cement. This repointing not only protects the stone from water penetration, but also restores the uniform visual appearance of the masonry surface. 2.   MASONRY REPAIRS: 2.1 Grouting The most effective way to repair cracked stonework is by injection grouting or by applying a resin adhesive (epoxy) where a fragment of stone has completely cracked and detached, and by compensating with an infill of a compatible mortar mix where there is masonry loss. . Repair Techniques 2.2 Indenting: Refacing technique In essence a variation of the complete stone replacement system when individual stones are being considered for either total replacement or refacing. Selection of a suitable new stone is again important, the geological match being critical. The new ‘face’ is usually in excess of 75 mm in thickness and is dependent on the am ount of decay on the ‘mother’ stone. Fixings must be non -ferrous with cramps or pins often set in resin, with a suitable mortar applied to the joints. It is important that any mortar to the rear of the indent is lime rich and does not crate a barrier to moisture transfer. The main advantage of this refacing technique is that it is only a facial repair and less disruptive than having to remove the whole stone from the body of the wall. Some stone features like cornices are tied deep into an inner wall and the consequent structural considerations of replacement are avoided. 2.3 Plastic Repairs The ‘plastic’ refers to the fact that the material is mouldable and can be placed and allowed to ‘set’. Architectural dentistry is perhaps a more helpful analogy, this technique should only be used for small localised repair. The material is itself a mortar and there are a variety of mixes available in order to suit the repair, but the most important factor is the porosity of the mix with the natural ingredients being chosen to provide colour match, etc. Sand cement based mortars used in the past can be counter productive removing srcinal stone as the fall off or hastening decay. They will not be permitted on listed buildings. Care should also be taken with resin based alternatives that they are not too strong for a particular stone.  Lime based repairs are often a better alternative, these require skill but because of their porosity they form less of a barrier to moisture movement. 3.   STONE REPLACEMENT The most effective system of stone replacement was established according to the following protocol:    Missing stones and stones that have been reduced in size to the extent that they are not able to support new stones placed above them should be replaced with a new stone infill. The new stone should be positioned either flush or slightly projected, depending on whether the existing adjacent stones are flush or srcinal (projected). This is due to the need to continue the srcinal surface level already established by the work (where it exists).    . In areas of stone replacement, the rubble core should be completely cleared of soil and debris in order to expose sound masonry and provide a clean surface for better bonding between the core and the new stone.    Hand removal by chisel and hammer should be used with extreme caution to aid in the removal of highly eroded or deteriorated pieces of srcinal veneer blocksstill in place.    It is not advisable to remove sound rubble core from the wall (to minimize vibration and avoid further detachment of srcinal stone units), unless it is to provide sufficient space for stone infill where necessary.    Sizing of blocks and course laying of new stone replacement should follow the srcinal ashlar veneer pattern of headers and stretchers. 4.   Cleaning methods : 4.1 Water Cleaning masonry with water is the simplest, safest and least expensive method. It softens the dirt and rinses deposits from the surface. When water-cleaning, ensure the wall is watertight and mortar and caulking joints are sound, the least amount of water is used, and there are two to five weeks of dry weather before frost. The different techniques are as follows: Hand-scrubbing - using a mild detergent and hosing down when complete. This is simple and effective.    Spraying - using regular water pressure to create a fine mist applied periodically over several hours and hosing down when complete.    Pressure Washing - using mechanized pressure. Great care should be taken on soft masonry and mortar, which can be destroyed if the pressure is too high and spray duration too long. 4.2 Chemical Chemicals are usually used to remove paint. It can, and usually does, destroy the surface of masonry. If contemplated, a test patch should be done to determine the extent of the damage.  The general approach to chemical cleaning involves wetting down the masonry, applying the chemical and rinsing off. The different cleaners are as follows:    Acid - usually hydrofluoric (HFI), is mixed in a maximum concentration of 5%, preferably 1%-3%. Acid should not be used to clean limestone, marble or sandstone.    Alkali - can be used on acid-sensitive masonry such as limestone, marble and glazed brick. It has a potassium hydroxide, ammonia or caustic soda base. Alkali should not be used on stone with high iron content.    Paint removers - are often the only means of removing paint. Reaction with the masonry can vary, therefore a test patch should be conducted first.
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