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The Libellus de Alchimia by pseudo-Albertus Magnus describes a rather long process for making minium, or red lead, from lead carbonate.
The quicker way is this.
Starting with white lead carbonate:
lead carbonate before heating
You heat it on the fire until it changes colour:
lead carbonate after heating

Look, red and yellow lead oxide. A mix of red lead, which is Pb3O4, and litharge, which is PbO. This was simply heated on a charcoal fire, without any attempt at temperature control and you can see that it isn’t pure red lead. To get red lead you have to heat it over 340°C, and then if you go over 450°C, not a very high temperature, it turns into yellow PbO.

This could be a bit of a problem, no? The PbO is the most thermodynamically stable form when heated in the atmosphere, and so because the fire wasn’t very well controlled and open to the atmosphere a lot of it was formed. Which means that simply heating lead carbonate in the atmosphere doesn’t guarantee red lead.
So I wonder if the long complex method works better? I shall have to try it out. But as you can see, getting the right alchemical substances isn’t as easy as you might think.
For future reference too, this is the calx of a body, i.e. the burnt stone formed by burning a metal in air. It is very important in alchemical recipes, even if they often don’t tell you the best way to make it.

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