The man who devised it, Gordon Pearson, is an expert on earth walls and ancient forms of mortar. As surveyor of historic buildings for Hampshire County Council he is responsible for making Odiham Castle safe - a difficult job when flints the size of two fists may fall at any moment from the ruined battlements. 'In the past, the county council tried to make it safe by raking out the soft lime mortar and repointing with cement,' Mr Pearson said. 'But now we are experimenting to see if more of the original can be conserved by hardening the mortar.'
The method of spraying with lime water has been used in the past to harden limestone and lime plaster but not, so far as Mr Pearson knows, to harden mortar.
The present surface of the castle, built in 1214, was never meant to be exposed to the weather. The castle had an exterior wall of dressed limestone and was lined within with the same stone. Between these curtain walls was dumped an aggregate of flints in lime mortar. Later the curtain walls were taken away exposing the aggregate.
The castle was built for King John, who used it as a resting place when travelling between Windsor and Winchester. He stayed there overnight before going to Runnymede to meet the barons and sign the Magna Charta. In 1265 the castle came into the ownership of Simon de Montfort and his wife Eleanor - at that time the most influential family in England.
But, by 1603 it had become a ruin. Local people helped themselves to the stone, which was used to build houses in Odiham.
The lime-water treatment is being undertaken by volunteers from the Odiham Society, which is concerned with local history. Two areas of flint and mortar are being sprayed twice a day for three or four weeks. The lime water (calcium hydroxide) penetrates the mortar where it reacts with carbon dioxide from the air. As a result, calcium carbonate is precipitated within the mortar, hardening it and turning it into a denser form of limestone.
'Ideas of conservation have changed,' Mr Pearson said. 'The lime-water treatment makes use of the chemistry of the mortar itself and strengthens the whole structure. The next stage will be to get contractors to put up scaffolding and spray the whole structure.'