In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: [email protected] marked for Barry’s attention.

Q: We wish to erect a single block of limestone to commemorate an event and have it last several hundred years. What characteristics should we be looking for when identifying suitable limestone?

Limestone is often chosen for monuments to be sited in the open air because it can be relatively cheap (compared with denser stones such as granites and marbles) and is easier for a mason to carve and cut inscriptions into.

The downside of limestone is that it might weather more than other stones. There is a variety of reasons for that.

The choice of which limestone to use relates to how much you have to spend. The more crystalline and dense limestones typically cost more. However, there are other issues to be considered, such as block sizes and shapes, that also come into play.

Ideally, a local stone ought to be used. As you advised that you are in the westerly fold of the Cotswolds, the Cotswold stone ought to be considered first.

This material is often used for roofing, paving and block walling because of its splitting properties. Large blocks may not be available. You might ask local suppliers about available block sizes, but do not spend too much time on it if you are told the size you need is not available. The same question should be asked of potential stone supplies wherever you are based.

Moving further afield, you basically have Bath Stone and then the Portland and Purbeck stones.

These stones have been used for hundreds of years and, with buildings such as St Paul’s Cathedral in London constructed of Portland, have demonstrable longevity – although Portland stone does lose about an inch of surface in the course of 300 years or so.

It is difficult to prevent natural weathering but in a rural setting away from acid rain and other pollution issues, the main worry will be frost, particularly after rainfall when the stone, which is porous, may be saturated.

The important thing with all limestones is to make sure it is well drained and protected from rising damp.

Of the stones I have mentioned, I would choose Portland, which has proved successful in service throughout the British Isles as well as overseas for monumental and construction uses.

Purbeck limestone is almost like a marble, but there may be difficulty in obtaining the size you require because the bed heights are typically quite small. Before dismissing stones not renowned for larger blocks it is still worthwhile asking about them as larger blocks do occur.

If you want to look at other limestones, I suggest you need to settle first on a colour, as this will lead you in quite different directions.

Basically there are light creams and buffs, blues and greys and oranges and browns, which do have quite different properties.

Most people would choose the lighter colours, which tend to have all the potentially nasty minerals already weathered out of them.

Once you have decided on a colour, you would then be looking at the properties of absorption and strength and, if available, results from salt crystallisation testing and maybe also frost testing. You are looking for lower absorption material, higher strength, and lower losses in both salt crystallisation and frost testing.

But a large chunk of any well-drained limestone should last several hundred years without too many problems.