Phosphorus was first isolated in 1669. One source I've seen suggested metallic sodium, potassium, or lithium, but as these weren't isolated till the early 1800s...

Hm, plenty of websites taking in each other's washing by mentioning the same lists of flammable ingedients, and the methods of delivery (clay grenades as well as pumps or tubes), but I haven't found anything yet that addresses this stability problem.

Perhaps it ignited on contact with water?