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I was browsing the images the Wellcome collection has put online, when I came across this 15th century one:

L0032812 Various types of furnaces, 15th century

From here

and began to wonder about it. I suspect it has simply been copied from older and worse drawn pictures; someone has spent a lot of time getting the perspective right but there are a few features I don’t like.
For instance at the bottom left, you can see flames playing at the top of the cucurbit and onto the alembic, which is bad practise and doesn’t help the distillation along. From which one deduces that it might be supposed to be in a water bath. Or else it should have a proper cap on the fire so the heat can’t get up and out. Or else the person doing the distillation is really bad at it.

The one in the middle with a chimney coming out either side, that is an old fashioned ‘eared’ furnace, commonly seen in 14th century and earlier drawings, somewhat less so in the 15th, and the ears should be at the top of the sides or corners, not some way down them.
And whilst the other drawings show furnaces, they seem a little lacking in practicality, shall we say? The one at the top left, what is heated and where do you insert it? The one at the top right looks accurate enough at least, but the others are lacking information as to where the things to be heated go.

Mind you, having written that, this 16th century illustration has a similar problem:
L0047981 Vessel and furnace.
Found here

if you were doing this distillation for real I’d expect it to boil and distill over rapidly and you’d lose most of it because of a lack of cooling. Bear in mind most distillations were of water or alcohol solutions, and if it was a higher temperature dry distillation you’d expect it to be covered in lute and you wouldn’t be using that shape of vessel anyway.

And if you look closely at the large tower furnace in this one from the same volume as the first illustration we find:
L0072751 Manuscript on alchemical processes, by Raymundus Lullius
From here

That the collection vessel is located directly over one of the flues for the fire below!
Having said that the cucurbits and alembics above it are an interesting sort, I haven’t seen their like before which makes me wonder about them, and the tower furnace itself is accurate in general. So maybe the artist was working from descriptions or rough sketches of them.

Either way, you have to be careful how you look at illustrations in alchemical books. Some are just jottings by readers or transcribers trying to imagine how a furnace looked, others are integral to the text as a whole (for an extreme example see the Ripley Scrolls), or else are there for a different meaning than that related to precisely how to build and use them.

I’ve seen some very nice illustrations in various MS, but they are not yet scanned and publicly available, and some of the best, such as those from Norton’s Ordinal, are being kept back to use as cash cows by the BL. I suppose I can’t blame them for that, but it is still a little irritating.

Hopefully this has aroused your interest in furnaces and drawings thereof; it’s an interesting area, especially once you have experience of trying to make things work using a wood or charcoal powered furnace.