Problem was it was also very useful as a color and in the early 19th century the French Government offered a prize for anyone able to discover an affordable way of making it. Those who were trying to produce it failed but it was accidentally produced at a Paris foundry, and subsequent investigation revealed the method. It is now produced in shades from yellow and green through blue to violet. The blue variety is the only one of great interest to artists. It is so cheap now that even roofing tiles are colored by it.