Lancenigo di Villorba (TV), Italy
Well, PLASTER OF PARIS is an ancient pygment coming from French as its name suggests. Really, it has been named beacause French was the country using the largest amount of this pygment. POP IS A WHITE PYGMENT BASED ON LEAD CARBONATE, e.g. onto CERUSSITE. MAY IT RESULT POISONOUS ONE?
Well, POP results poisonous by direct assimilation once it has been manufactured, e.g. Fresh Pygment instead old peinture which don't.
PLASTER OF PARIS means a jelly-mass (PLASTER) having a white sludge. However, most effective artisans in ancient europe aren't dutchmen despite frenchmen ; in Netherlands, manufacturers learnt that LEAD-WARES HAD TO ENTERS IN PITs FILLED BY MOIST AND ORGANIC MATTERs.
So, bacteria activities will ferment organic stuffs and BACTERIA ASSURE THE CONDITION LOOKED FOR :
-) aerobic bacteria breath air and it diminute the availability of Oxygen ;
-) aerobic bacteria slow down and anaerobic bacteria ferment organic stuff ;
-) fermentation gives off Carbon Dioxide which saturate the pit.
De-aerated conditions allow a late corrosion of Lead-wares forming its Basic Carbonate
2 Pb(s) + CO2(g) + O2(g) + H2O(aq) ---> Pb(OH)2.PbCO3(s)
which is on the basis of POP.
POP had been involved in phenomenon of BLACK MARY, e.g. the religious pictures which turned black.
Indeed, ancient candles came from Animal Fat which burn leaving of Sulphur-rich Fumes. Well, the Sulphur-rich gas react against POP to form the LEAD SULPHIDE
Pb(OH)2.PbCO3(s) + 2 H2S(g) --->
---> 2 PbS(s) + CO2(g) + 3 H2O(aq)
having black colouration.
I hope this helps you.