In the light of a DCMS funding been awarded to The Natural History Museum, to tackle the poor performance of the biggest of the western range roof of the Waterhouse building, the NHM Petrology Collection and museum scientists have come to the project’s aid to address one of the first hitches that occurred.
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The western range roof of the Waterhouse building that requires intervention is the one above the gallery formerly housing the hall of Human Biology and as part of its renewal project, the stone roofing tiles, that have come to the end of their expected life, will be replaced and new glazing will be installed.
One of the first problems encountered was the lack of historic information regarding the type of roofing stones that were originally used. Heritage consultants have tried several archival routes in the hope of finding indications of what material was used on the roof, in order to replace the original tiles with similar roof stones, but not satisfactory information was found.
Here is where the NHM Petrology Collection and museum scientists come in, helping to identify the roofing stone tiles that were originally used.
One roofing stone was removed from the roof for identification and the first eye inspection of the old used tile, confirmed the roofing stone as slate as opposed to tiles produced from limestone and sandstone, also very commonly used and a significant feature of many historic buildings.
What are slates and which ones were used?
Slates are fine-grained, metamorphic rocks that are created by the alteration of shale, mudstone or volcanic ash by low-grade regional metamorphism. Their foliation gives them defined lines of breakability allowing to split them into thin sheets, this makes them ideal for turning into roofing stone tiles. Slates are popular as roofing stone, also for their aesthetic appearance and their durability. In fact, if well maintained, can last for at least a century and possibly much longer.
There are many different types of slate with various naturally occurring colors, these are the result of the rock mineralogical composition that can vary from one location to another.
Archival research suggested that the architect Francis Fowke, Alfred Waterhouse‘s predecessor, had considered the use of a particular type of Welsh slate commercially known as Bangor and sourced at the Penrhyn Quarry in North Wales. With this in mind, two Bangor slate samples from the NHM Petrology Collection, along with several other modern slates, were analysed and results compared to the analysis acquired on the slate originally used for the roof of the NHM Waterhouse building.
Small fragments ( ̴3-4 mm) of the slates were sampled and powdered using an agate mortar in order to acquire X-Ray Diffraction (XRD) analyses. The investigations were carried out in the NHM xrd laboratories , by Petrology curator Dr Epifanio Vaccaro and X Ray Lab Manager Dr Jens Najorka.
This investigative method was performed by directing an x-ray beam at a finely powdered sample and measuring the intensities and scattering angles of the x-rays that were diffracted by the material irradiated. The result was a xrd diffraction pattern characterised by peaks that are effectively the fingerprint of the way the atoms are arranged in the material investigated. From the xrd pattern it was then possible to determine what crystalline phases and how much of each crystalline phase was in the powder.
XRD analyses of the original roofing stone that was removed from the roof for identification were acquired and the xrd pattern revealed the slate’s mineralogical assemblage and the abundances of the minerals making up the sample. The mineralogy of the sample analysed was exactly what it is expected for a typical slate, consisting of Quartz, clinochlore, muscovite, calcite, pyrite, dolomite , albite and orthoclase.
XRD analyses were also carried out on several other samples both from the NHM Petrology Collection (BM.1985, E1711 and BM.1985, E8502 commercially known as Bangor) and modern slate samples commercially known as Burlington Blue Grey, Westmorland Green along with sample used on a previous roofing project. Their xrd patterns were then compared to xrd pattern of the original roofing stone that was removed from the roof for identification.
The results have indicated the following:
The Bangor slates, suspected of having been used for the roof of the Waterhouse building, have shown a slightly different mineralogical assemblage to the original roofing stone. Both Bangor xrd patterns are in fact characterised by the presence of minerals such hematite and rutile not present in the original roofing stone. The presence of hematite also explains the slight purple colour of the Bangor slates, not obviously seen in the original slate of comparison. These results have therefore suggested that it was unlikely that Bangor slates were originally used for the Waterhouse building roof.
How did the other slate samples analysed compared?
The slate Westmoreland Green showed a similar high chlorite content compared to a slate used on a previous roofing project, this explains their green colour, which is not characteristic of the original slate, therefore both these green slates were also considered not a match to the original slate.
Among all the slates analysed Burlington Blue Grey slate is the one that showed a remarkable similarity to the slate originally used for the roof of the Waterhouse building, both in terms of mineralogical assemblage and mineral abundancies. Both xrd patterns are characterised by the presence of minerals such as dolomite and pyrite in comparable quantities, mineralogical phases not seen in the other slates analysed.
Whilst the similarity of these two slate samples it’s a good indication, it doesn’t necessarily mean that the two sample were quarried in the same location and are definitely the same rock. It is in fact very difficult, by analysing a sample, to establish the exact location where the original slate was quarried. However, in this instance the striking similarity of the mineralogical assemblage and mineral abundances of the Burlington Blue Grey to the original slate, was considered satisfying to choose the Burlington Blue Grey as the slate to use for the western range roof renewal of the Waterhouse building.