At the British Museum, the controversy surrounding the use of Anstrude Roche Claire limestone centered on its similarity to Portland Base Bed. Its weathering properties were never a specific issue because in that job it is covered over and not exposed externally.

However, your report (NSS October issue) that it is being considered for tracery at Lincoln Cathedral raises the question of its durability in a harsher weathering environment.

Anstrude Roche Claire is a stone with a moderately high porosity and a very high saturation coefficient (ie a lot of the pore space consists of very fine pores that hold tightly onto water if the stone is wetted).

Some years ago, staff at the Building Research Establishment published a version of the attached graph (redrawn from Honeybourne and Harries 1958 in Proceedings of the 10th Colston Research Society), which summarised some work done in the middle of the 20th century.

About 200 limestone samples for which these two properties were know were exposed to natural frosts for a varying number of years. Some remained sound, whereas others failed sooner or later. As can be seen, the most frost-susceptible stones were those that fell on the right-hand side of the scatterplot - ie they combined higher values for both these parameters.

Anstrude Roche Claire was not tested in this BRE study, but petrographic study confirms that it is a stone with a high microporosity.

Another BRE study (see Honeyborne, 1982, The Building Limestones of France, page 67) quotes overall porosities in the range of 20.5% to 24.3%, and saturation coefficients from 0.74 to 0.89 (the shaded zone which has been added to the graph).

These values would suggest that the frost susceptibility of Anstrude Roche Claire might be suspect in an exposed setting and that great care should be taken in the selection of individual stones if costly repairs are to be avoided some years down the line.

Dr Tim Palmer
Institute of Earth Studies, University of Wales

NSS: We sent a copy of this letter to Nicholas Rank, the surveyor of the fabric of Lincoln Cathedral and an architect with Buttress Fuller Alsop Williams. He replied: "Thank you for your email. We researched Anstrude at length, including having our own tests undertaken at UMIST. I have forwarded on your email to our consultant engineer for his comments."

We also sent a copy to Hugues Duflot of Rocamat in France. He said: "Thank you for this letter. I will certainly comment on it because measurement of the coefficient of saturation is not recognised in France (because it is not accurate in itself).

However, the freeze-thaw cycling test performed according to French norm and similarly performed now under new EU norm, gives satisfactory results and allows Anstrude to be used on appropriate places that people of Lincoln Cathedral are aware of. So there is no controversy to introduce. Furthermore it is Rocamat\'s responsibility to offer stone for its suitable use."