The use of arsenic in the green glaze has some basis in history. Green wallpaper was very popular among the affluent in the late 1800s, but it was eventually discovered that arsenic was leaching from the dyes into the air and slowly poisoning those who live in the rooms in which it was used. Though the arsenic on the outside of a vase wouldn't likely be much of a hazard, colorful pottery, has been notorious, even in recent years, for heavy metals leaching out of the glazes and into food that comes in contact with it.