English delftware (c. 1770) from Bristol, Lancaster and Liverpool: A composition study using Raman spectroscopy and electron microscopy

Nine decorated lead-tin glazed earthenwares, colloquially termed 'delftware', produced in c. 1770 in Bristol, Lancaster and Liverpool, England, have been analysed non-invasively by Raman spectroscopy and electron microscopy. The body paste used to manufacture these west coast wares was attained by the blending of highly dolomitic [CaMg(CO3)(2)] blue clay sourced from Carrickfergus, County Antrim, Ireland, with locally sourced clays. Thus, the resulting body fabric of these wares contains significant MgO enabling them to be differentiated from MgO-free London manufactured delftware. The glazes employed all contain arsenic, obtained as a cobalt impurity or by deliberate addition. The presence of this unvolatilised arsenic in the glaze has then reacted with the lead during firing at temperatures approaching 1000 degrees C and then further reacted with calcium and magnesium to form needle-like crystals of lead arsenates in the form of mimetite [Pb-5(AsO4)(Cl,OH)], schultenite [Pb (AsO3OH)], beta-roselite [Ca2Co(AsO4)(2)center dot 2H(2)O], hedyphane [Ca2Pb3(AsO4)(3)Cl], wendwilsonite [Ca2Mg (AsO4)(2)center dot 2H(2)O] and/or adelite [CaMgAsO4(OH)] during high temperature firing.

Automated summary

Nine lead-tin glazed earthenwares produced in Bristol, Lancaster, and Liverpool in 1770 have been analyzed using non-invasive techniques. The study found that the body paste of these wares was made by blending blue clay sourced from Carrickfergus, Ireland with locally sourced clays. The glazes used in these wares all contained arsenic, which reacted with lead during firing and further reacted with calcium and magnesium to form needle-like crystals.