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You really have to see this for yourself, the camera is not enough. But hopefully you can get an idea of this quintessentially alchemical operation.
All you do is, take your sulphur, put it into your cucurbit, place a vessel on top, and heat the sulphur until it melts and vapourises.

Then you get this:
sulphur in cucurbit bright red in fire

round bottomed flask with sublimed sulphur in it
This is the round bottomed flask when sulphur has condensed inside it. Mind you it is a little hard to get the sulphur out of it through the narrow neck.

Then when you stop heating it, the sulphur vapour falls back down into the cucurbit in a ribbon of sulphur:
sulphur falling back into cooling cucurbit

Quite an extraordinary thing to see, and hard to photograph because of the glass in the way. Well worth doing though.
At the end of the day the sulphur in the bottom of the cucurbit is red and solid, basically it’s been polymerised into a kind of plastic, probably with oxygen trapped inside as well.
Why do this? Well for starters, the sulphur that condenses out at the top is more pure than what is left below. And some of it becomes fixed, in this case probably oxidised, or at least turned into a polymer that looks different. Fixing sulphur is one of the most important operations in medieval alchemy and there were many ways of doing it.