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Rare use of Copperas Pigment in External Limewash Paint

Posted on Wednesday 02 October 2013

Between the 16th and 18th centuries copperas was produced in the UK on an industrial scale, this was well before the industrial revolution and its importance to the industrial development of the country is only now being investigated and understood.

The name is extremely misleading, it is actually a compound of ferrous sulphate, produced from iron pyrite fossils washed up on the coast of the Thames, refined through a lengthy and particularly unpleasant process. (A more detailed article on copperas production can be found here).

Though used primarily as a mordant in the fabric industry, over this period and up to the 20th century it was not uncommon to find it used as a pigment to tint exterior limewash.  What is most remarkable about this pigment is that it starts out as a vivid green crystal, but when added to the limewash and exposed to the atmosphere, it turns it a rich honey colour, reminicent of Bath or Ham stone. 

Though historic examples of its use are known to survive (such as at Apsley House, Patrick Baty notes it's use here), we can find no reference to this technique having being applied to any London buildings in recent years.  This has proved something of a great shame, as the revival of this traditional paint to the exterior of this Georgian house has been extremely successful, giving a pale warm stone colour, which shifts dramatically with the light of the day.

Compared to the surrounding buildings (with woodwork and stucco in a bright white colour) the finished house, with woodwork in a brown linseed oil paint from Sweden and railings in dark green, looks far more interesting and convincing.