Preservation Of Stones
Preservation of stone is essential to prevent its decay. Different types of stones require different treatments. But in general stones should be made dry with the help of blow lamp and then a coating of paraffin, linseed oil, light paint, etc. is applied over the surface. This makes a protective coating over the stone. However, this treatment is periodic and not permanent. When treatment is done with the linseed oil, it is boiled and applied in three coats over the stone. Thereafter, a coat of dilute ammonia in warm water is applied.
The structure to be preserved should be maintained by washing stones frequently with water and steam so that dirt and salts deposited are removed from time to time. However, the best way is to apply
preservatives. Stones are washed with thin solution of silicate of soda or potash. Then, on drying a solution of CaCl2 is applied over it. These two solutions called Szerelmy's liquid, combine to form
silicate of lime which fills the pores in stones. The common salt formed in this process is washed afterwards. The silicate of lime forms an insoluble film which helps to protect the stones.
Sometimes lead paint is also used to preserve the stones, but the natural colour of the stone is spoilt. Painting stone with coal tar also helps in the preservation but it spoils the beauty of the stone. Use of chemicals should be avoided as far as possible, especially the caustic alkalis. Although cleaning is easy with chemicals, there is the risk of introducing salts which may subsequently cause damage to the stone.
In industrial towns, stones are preserved by application of solution of baryta, Ba(OH)2 - Barium hydrate. The sulphur dioxide present in acid reacts on the calcium contents of stones to form calcium sulphate. Soot and dust present in the atmosphere adhere to the calcium sulphate and form a hard skin. In due course of time, the calcium sulphate so formed flakes off and exposes fresh stone surface for further attack. This is known as sulphate attack. Baryta reacts with calcium sulphate deposited on the stones and forms insoluble barium sulphate and calcium hydroxide. The calcium hydroxide absorbs carbon dioxide from the air to form calcium carbonate.
Ba (OH)2 + CaSO4 ¾¾¾® BaSO 4 + Ca(OH)2(Barium sulphate)(Calcium hydroxide)
Ca(OH)2 + CO2 ¾¾¾® CaCO 3 + H2O (Calcium carbonate)
The question whether or not stone preservatives should be used on old and decayed stone is a difficult one. Real evidence of the value of various treatments is most difficult to assess. The treatments, if carefully applied under favourable circumstances, may result in an apparent slowing down of the rate of decay. However, the rate of decay of stone is so slow that a short period experience is of very little value in establishing the effectiveness of the treatment. Also, there is some evidence that treatments which appear to be successful for few years, fail to maintain the improvement. In fact, the value of preservatives is not yet proved, and they may actually be detrimental if judged over a long period.
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