Preservation of stones

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Though stones are durable, preservation of stones is essential to prevent its decay. The durability of the stone also increases if it is preserved properly.  Preservation of stone means making the stone strong enough to face the adverse effects of the atmosphere.

Factors causing deterioration of stones

The factors which can cause deterioration of the stones are as follows:

• Filling up of pores present in the stone by water or chemicals.
• The use of stones containing carbonate of lime in industrial town.
• In a structure, laying of limestone and sandstones, magnesium limestone and granular limestone close to each other cause mutual decay of the stones.
• Use of binding material which has adverse effect on a particular stone affects the durability of the stone.
• Growth of plants and trees on the stone.

*For detailed discussion on “Deterioration of stones” – Click Here

Methods of Preservation of stones

            The method of prevention varies from stone to stone. In general, preservation of stone is done by coating the surface of dry stone with paraffin, linseed oil or light paint.  However, this treatment is not permanent.

           Washing the stone surface frequently with water and steam removes the deposited dirt and salts. This ensures the preservation of the structures. Using preservatives for washing the stone is the best way. Examples for preservatives are sodium silicate (or potash), calcium chloride.

            At times, Lead paint and coal tar are also used for preservation of stones.  But, the use of such materials spoils the natural colour of the stone.

            The use of chemicals make the cleaning of stone surface easier. But, the use of chemicals may introduce salts resulting in damaging the stone. In industrial towns, Barium hydrate solution helps in preserving the stones effectively.   

What is a preservative?

            A preservative is an oil, paint or solution which when applied on a surface, protects it from the direct attack of some deteriorating agencts.

Characteristics of an effective preservative

An effective preservative must possess the following characteristics:

• The preservative must be easily applicable and must penetrate into the pores easily.
• The preservative must be unleachable. It must remain effective for a long time after drying.
• It must be economical.
• The preservative must not allow moisture to penetrate the stone surface.
• The preservative must not develop any objectionable colour.
• It must harden sufficiently so as to resist effects due to various atmospheric agents.
• It must be non-corrosive and harmless.

            A single preservative which satisfies all the above criteria do not exist. Hence, the preservatives are chosen carefully based on the chemical composition of the stone and their location in the structure. 

Commonly used preservatives

 Some of the preservatives used for the preservation of stones are

• Coal tar – The application of coal tar on the stone surface produces objectionable appearance when coated. Surface coated with tar also absorbs heat.
• Linseed oil – Raw linseed oil does not disturb the original shade of the stone, but requires frequent renewal. Boiled linseed oil makes the stone surface dark and also lasts for longer period.
• Paraffin
• Paints – The application of paint changes the colour of the stone. The paint can also be applied under pressure if deep penetration is required.
• Baryta solution (Barium hydrate) – When baryta solution is applied over the stone surface, barium hydroxide reacts with calcium sulphate present in the stones and forms barium sulphate. The barium sulphate formed is insoluble and is least affected by atmospheric agencies.
• Szerelmy’s solution – Szerelmy’s solution is a mixture of sodium silicate and calcium chloride.
• Alum soap solution – Alum soap solution is a mixture of 0.75N alum and 0.5N soft soap dissolved in a litre of water.