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Saint Mary the Virgin Gamlingay
The Church Tower 2009
As most of you will know the Church Tower is at last receiving attention. For some 15 years the Church’s architects have been concerned about the state of the pointing of the stonework to the South face of the tower. The problem arises because of work carried out some 40 years ago. At that time the mortar between the stones was raked out and refilled. The problem is that cement mortar was used.
I will try to explain the difficulty. The stone of the Tower is a form of sandstone that is often called Carr Stone. This stone can be found widely from as far North as Hunstanton and Southwards towards Woburn. Locally there are deposits under Gamlingay and to the West of the village.
Almost everyone knows that water rises through the ground during the winter and falls to a lower lever during the summer. This action of course occurs everywhere. What makes Carr Stone areas special is the combination of two further factors. The first is that the soil has to be of a light sandy, almost gritty texture. The second factor is the presence of traces of iron dissolved within the ground water. Over thousands of years the ground water carries traces of iron into the sand as the level rises during the autumn but leaves some of the iron behind as the level falls in the spring. Eventually the iron binds the sandy particles and Carr Stone. begins to form. Concentrations of about 4% of iron are quite common and it is this that gives the stone its red (rust) colour.
Unfortunately the process is reversible. Thus if Carr Stone is exposed to rainwater the iron content can be washed out and just sand is left behind. The builders of the Church were well aware of the problem so they topped all the walls with Limestone which is much harder and does not dissolve in driven rain. The limestone caps keep the walls reasonably dry. Even so it is essential that the Carr Stone walls are not kept wet.
All walls get wet from time to time but this normally does not matter because they can “breath” and dry out by evaporation. Unfortunately cement mortar prevents this natural drying out because it is virtually impervious to water. Thus the cement mortar is actively harmful because it traps water inside the Carr Stone walls and then the iron content begins to leach away. This is just what was beginning to happen to the Tower. The surfaces of the Carr Stone blocks were beginning to return to sand!
To make matters worse the sand can trickle down inside the wall to fill in any small voids that are there. When winter frost freezes this wet sand it expands forcing the blocks of the walls apart. This is the possible cause of two cracks appearing just above the Nave roof on the East front of the Tower.
The present work includes stitching the cracks with stainless steel rods buried within the wall itself. Repointing consists of chipping out the cement mortar and replacing it with mortar made from lime. This sounds easy but there really is a lot of wall involved.
When I visited the works the architect was discussing with the contractor stones of a suitable colour to serve as infills. They were also discussing the exact colour of the lime mortar. This will have a critical influence upon the final visual outcome. It was decided that there was a need for more rusty red in the mix so a different source of sand was decided upon.
Of course whilst the scaffolding is up the contractors are undertaking associated works. There are a surprising number of “holes” in the wall. These must be filled and any of the old stone blocks that have eroded by more than 2 inches from the surface need to be replaced.
The cost will be in the order of £ 40,000. The money is available from The Friends of Saint Mary’s. It will however more or less exhaust the Friends reserves so there will be an urgent need to raise further sums. Anyone willing to help with any subscription or in organising an event should contact.
The Friends of Saint Mary’s Gamlingay
care of Philip Gorton
at The Emplins Gamlingay Nr Sandy Bedfordshire SG19 3ER
Telephone 01767 650581
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