vivianite, after cleaning, from a wastewater treatment plant
Vivianite is a hydrated iron phosphate salt, which is a by-product of the treatment of waste water. It builds up on the pipes of the wwtp and usually needs to be cleaned off. It is a mineral which is also found occurring naturally, varying from colourless to green crystal.
Used in the 17th century as a pigment, it was known as Blue Ochre, although it was abandoned due to its instability – it changes colour over time, sometimes becoming grey, and sometimes yellow or reddish, a little bit problematic for the painter! It has been found in the paintings of Vermeer, who himself was a Delft based artist and the pigment was most likely extracted from the ground in Drenthe.
In theory we could have taken our vivianite sample to be milled by a paint maker, who regularly does this with pigments, the only problem was that the sample from the Venlo wwtp stank and was totally unacceptable to them!