In historic building terms, Copperas was/is an application applied to buildings which changes the colour of the substrate. Examples exist where it was used to pigment limewash, but it was also applied directly to stone. The applied wash creates a golden/orange hue, which semi permanently changes the aesthetic. I say, ‘semi permanently’, as it fades over long time periods where exposed – likely through UV and being continually washed by mildly acidic rainwater. That said, a number of examples remain in un-restored form, having in the most part, survived since original application and are still notably coloured.
Copperas was used extensively during the latter half of the 18th Century and early 19th Centuries. It was favoured by a number of estates to create a stylistic link across separate buildings and create a homogenous finish to stonework. It is also thought to have been used to raise the status of buildings, as it made them highly visible from a long distance away – creating a stark contrast against the natural landscape.
It was not produced principally for this purpose however and may even have been discovered and adopted by more ‘accident’. Between the 16th and 18th Centuries, Copperas was produced in the UK on an industrial scale for use as a mordant within the textile industry. The term mordant comes from the Latin murdered, ‘to bite’ and it was thought that this added to the dying process acting as a fixative onto the cloth, so it would hold fast during later washes. Queen Elizabeth I granted a Royal Patent to Cornelius Stephenson for the exploitation of Copperas in Kent. This product was used for leather tanning, making black dye and ink, and in the dying process of cotton and wool. His industry supplied London businesses via shipping up the Thames and lasted for well over a hundred years.
The name Copperas is extremely misleading as it is actually a compound of ferrous sulphate. Iron(II) sulphate or ferric sulphate denotes a range of salts with the formula FeSO4·xH2O. It is extracted through a lengthy process from iron pyrite fossils washed up on the shoreline of the Thames and then heavily refined to produce a pigment. What is most unusual about this pigment, is that its natural state it is an intense dark green crystal – quite unlike the finished effect it creates. To use it, it is dissolved in water and bound with lime putty producing a semi clear liquid, before being applied to the stone. Over a period of time (approximately a week – dependent on climatic influences), it oxidises and gradually turns a rich honey colour.
Despite not seeming to be widely understood or recognised, good examples of Copperas application can be found to have survived at a number of large estates, including Blenheim Palace and Stowe House (which is where I first encountered it). Since then, I have noted application to a number of buildings, including St James Palace, which don’t seem to have been documented. Reports I have read from other industry professionals suggest it is acknowledged to have been used at Westminster Abbey, Ham House, Basildon Park and Apsley House to name a few, suggesting its popularity was certainly widespread with instruction from architects such as Vanbrugh, Hawksmoor, Adams and Wren.
Although by no means an exact science, experiments have led me to find the following recipe and method of application the most consistent and effective. Caution should be given however, as if applied with a heavy hand, the effect can be less than subtle.
100 ml sieved lime putty
30g Iron (II) Sulphate – Fe2(SO4)3 hydrated
in; 2 litres of clean water
The lime putty should first be combined with the iron sulphate and mixed by hand until integrated. Only then should it be let down in water to produce a fully dispersed solution. Once in suspension, the liquid is almost colourless, and it is almost impossible to see when applied. Application should be by large brush in smooth even strokes, avoiding unnecessary overlap – as these duplications result in an uneven application and an irregular finish. Trials are essential!