Potterne, Wiltshire

A downward glance

‘Isn’t that slag?’ The Resident Wise Woman’s mind was more on geology than the elegant but rather forbidding Early English Gothic Gothic architecture of St Mary’s church, Potterne. What caught her eye was the black material among the masonry lining the steps in the churchyard, a by-product of smelting (probably of iron, though one sometimes sees copper slag): a substance that is hard, irregular, dark, and durable.

It’s something we’d noticed before in a different form in the Gloucestershire town of Newnham on Severn, where there’s a house partly built of slag. There, the material, produced during copper smelting, had been poured into moulds while still liquid, so that it set in big rectangular blocks ideal for building. Here at Potterne, though, it’s simply made up of irregular lumps. It’s dark, and more forbidding in its way that the architecture of the church, but has found a useful function.

I don’t know where this slag came from – Wiltshire is not a place I particularly associate with historic ironworking (the Weald and the Forest of Dean were more the places for this kind of thing) although I have seen online references to Saxon-period smelting in the Ramsbury area and Westbury had an iron industry in the 19th century. I’d love to know more about the origin of this unusually located and unlooked-for slag.*

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* Thanks to a reader who has pointed out that the Seend ironworks was not far away: this is a likely source.

Newnham on Severn, Gloucestershire

The colour of masonry

We get used to linking localities with building stone: Cornish granite, East Anglian flint, Derbyshire millstone grit, and so on. In Gloucestershire, the best known local stones are the golden limestone of the Cotswolds and the pinkish sandstone of the Forest of Dean. Between them, in the clayey Vale of Severn, though, there is no dominant building stone and many of the older houses are timber framed or built of brick. So what is this curious dark material that forms part of a house in Newnham, by the western bank of the Severn? None of the above, clearly. The surprising answer is that it’s slag, waste material from copper-smelting, and it must have had an interesting journey to get here.

In the 18th century copper smelting took place at Redbrook, on the River Wye not far from Monmouth, and at several sites in the Bristol area. The best guess is that the slag used for this building came from one of these sites and got to Newnham by boat – Newnham was once a river port. There are several buildings in the locality, and in other Severnside villages, made partly of the material.

Some say that the slag was used as ballast in ships, but the material must have been specifically intended for building because it was deliberately formed into blocks. William Marshall in his Rural Economy of Glocestershire (1789), explained how it was used:

‘Until of late years, it was cast away as useless, or was used as a material of roads only. Now it is thrown, while hot, into moulds, of different figures and dimensions, and thus becomes an admirable building material. It is proof against all seasons, in every situation; consequently, becomes an excellent material for foundations; and still more valuable for copings of fence walls; for which use it is sometimes cast of a semi-elliptical form. It is also used as quoins, in brick buildings; in which case, the blocks are run about nine inches square, and eighteen inches long. It is of a dark copper colour and has the appearance of a rich metal; but flies under the hammer as flint.’

It was relatively easy, when the Severn was a busy highway of cargo vessels, to ship this heavy material upstream and offload it at the various ports and inlets along the river’s course. Ever since there have been a smattering of walls like this hereabouts, adding a dark coppery tinge to Gloucestershire’s architectural palette of pink, and gold.