dye "by using indigotin, otherwise known as woad." A dye known as in Aramaic is mentioned in the Babylonian Talmud. Celtic blue is a shade of blue, also known as in Welsh, or in both the Irish language and in Scottish Gaelic. Julius Caesar reported (in Commentarii de Bello Gallico) that the Britanni used to colour their bodies blue with , a word that means primarily "glass", but also the domestic name for the "woad" (Isatis tinctoria), besides the Gaulish loanword (from Proto-Celtic ). The connection seems to be that both glass and the woad are "water-like" ( is from Proto-Indo-European ). In terms of usage, Latin is more often used to refer to glass rather than woad. The use of the word for the woad might also be understood as "coloured like glass", applied to the plant and the dye made from it. Due to this and other Roman accounts of them painting (or possibly tattooing) their bodies, northern inhabitants of Britain came to be known as Picts (), meaning "painted ones" in Latin. Gillian Carr conducted experiments using indigo pigment derived from woad mixed with different binders to make body paint. The resulting paints yielded colours from "grey-blue, through intense midnight blue, to black". People with modern experiences with woad as a tattoo pigment have claimed that it does not work well, and is actually caustic and causes scarring when put into the skin. It has also been claimed that Caesar was referring to some form of copper- or iron-based pigment. Analysis done on the Lindow Man did return evidence of copper. The same study also noted that the earliest definite reference to the woad plant in the British Isles dates to a seed impression on an Anglo-Saxon pot. The authors theorize that vitrum could have actually referred to copper(II) sulfate's naturally occurring variant chalcanthite or to the mineral azurite. A later study concluded the amount was "not of sufficient magnitude to provide convincing evidence that the copper was deliberately applied as paint". Woad was an important dyeing agent in much of Europe and parts of England during the medieval period. However, dye traders began to import indigo during the sixteenth and seventeenth centuries, which threatened to replace locally grown woad as the primary blue dye. The translation of vitrum as woad may date to this period. Medieval period onwards Woad was one of the three staples of