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When looking through alchemical works and examining the illustrations, it always pays to ask, is that real equipment and methods I see in use, or are they just making it up?
So ever since I realised there was something odd smack in the middle of one of the illustrations from Thomas Norton’s Ordinal, I have been looking out for more evidence for it, or not.
Here it is, in a free copy of the picture taken from the, IIRC, 16th century copy in the Ferguson collection in Glasgow University:

Ferg191_41 nortons furnaces

See the cucurbit, with the flat plate on top? What do you call that? What is it for?
The only bit of likely evidence for it that I have, apart from that illustration, is that in the Voarchadumia of Giovanni Panteo, (on page 84 of the English translation published by Adam Maclean), there are the words “It is then fixed in a vase sealed with iron plates and subjected to intense heat for a further 14 hours.” with regard to the manufacture of cinnabar. Which does rather match the illustration.

The potentially problematic assumption here is that Norton’s illustrations are accurate and truly reflect apparatus in use at the time. It is generally held that the copy of it that the illustrations are from was written under his instruction, the issue is how much you think he knows what he is doing. I and I think some others reckon he does. So many other parts of the illustrations match known alchemical equipment and practise that it would be odd if these rather obvious things didn’t. In the case of Norton I am certain that most of the rest of the illustrations are accurate, even taking into account the illustrators inability to draw hands and deal properly with perspective. And without a comprehensive examination of alchemical drawings (A task which needs to be undertaken, shame there’s not enough money in it to live on) I can’t say conclusively similar items don’t appear elsewhere.

Moving on, the question is what use would such an arrangement be? I have a suspicion that given the material limitations in those days, it would actually be easier to get a good solid seal about the top of the cucurbit using this method, because of the joining area and the weight of the iron plate bearing down onto the uneven and not totally flat or round, rim of the glassware.
But why not use a glass stopper like we do now? Mention is certainly made of them in John of Rupescissa’s work on the Quintessence for instance. However that was in dealing with a liquid, which could easily be poured in and out of a small hole. If you want to deal with a larger mass of solid or scrape it from the side of the vessel, you need a large diameter neck to the vessel. Which is most easily stopped by a plate.

You can see a related solution in this picture from Harley MS2407 in the British Library:
harley 2407 from book cucurbit
It seems to be of an actual cover for a wide mouthed cucurbit, used in the later 15th century.
In search of the plate on a cucurbit, I looked round the internet, some of the pictures available online, and some offline, including Adam Maclean’s compendia of images, also the Wellcome collection and couldn’t find any. Nor was there anything appropriate in the Summa Perfectionis.

A possible source is Arabic alchemy. I have a copy of “Chemistry in Iraq and Asia in the tenth century AD” by Stapletone, Azo and Husain (1927 print, with a bookplate inside apparently for a Robert E. Schofield, who seems to have been a real historian of science, and who retired some time around the late 1980’s)
Of course it is a translation and there are no accompanying illustrations, yet mention of “Replace the alembic by a cover” {page 386} or “In place of the alembic we may have a cover properly adjusted on the head of the aludel. It should have a hole large enough for the head of a large needle to enter. In this (hole) is placed a woollen lamp-wick, with one end of the wick hanging down into the dish, so that all the moisture that is in the mercury may be distilled.” {page 385}
(Of course what this all shows is that access to manuscripts and copies of MS and printed works etc etc is limited even now with the digitisation projects that are going on)
I do see plenty of glass vessels stoppered with small bulbous glass cucurbits. So I think this is a definite piece of alchemical apparatus, that went out of use after good glass blowing became more widespread. These stoppers can also be seen in the Ripley scroll, and in Harley 2407, suggesting that they were commonplace in later medieval times.
Ripley scroll Yale version glass stopper in Pelican

Harley2407 from book stoppers

Ultimately I say yes to the question in the title of the post, and am certain that such a method of stopping a wide mouthed flask was used, but not very sure for what or how often. Which is so often the way with historical studies.