Hello all!

Hereunder is a sentence the structure whereof is obscure and incomprehencible to me:

The whole paragraph:

«If a glass be cracked, then wet a linen cloth in the white of an egg beaten to water, and lay upon it, an upon that presently while it is wet, sift some unslaked lime and press it close with your hand. When that is dry, lay on another cloth thus wet as before and on it sift more lime.»

The strangest fragment is the following: «... an upon that presently while it is wet...»

Any help in translating it will be appreciated.
Where did you get that?!! Great! I'm not a poet but an is a mistake and should be and. That part (in more comprehensible English) would be: and after that soon while it is still wet. Upon or on often means 'soon/immediately after'. Perhaps Nona the Brit or somebody else can decipher your instructions and 'translate' them into plain English.
Oh, thanks!

I am aware of that meaning of "upon" (and "on") but I in no way could suggest "an" should be "and"... So I was getting "upon" as being a noun and thus having troubles.

That's an old (al)chemical text, "The Art of Distillation" by John French.
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This sounds like 17th or 18th century instructions to repair a mirror.

Roughly: If the mirror is cracked, take a piece of linen and soak it in a mixture of water and beaten egg white until it is wet. Then place the cloth on the mirror [on the back, I presume]. Whilst the cloth is still wet, sift [with a sieve to remove lumps] some unslaked lime [this is a kind of mineral - look it up in the dictionary - which was used especially in painting] onto the cloth and press it down firmly with your hand. When the cloth and lime mixture has dried, put on a second layer of wet cloth and lime.

«... an upon that presently while it is wet...» - should definitely be 'and' not 'an' - means 'and put [the lime] on it straight away while it is still wet'
Cool Breeze: Sorry, I replied to you as Anonymous, so it'll appear a bit later. I don't want to double the post.

Lil' Ruby Rose:

Thank you, but I understood everything after Cool Breeze's reply. The only problem was with the "an(d) upon" part. I quoted the whole paragraph to make the sentence in question easier to understand.

«This sounds like 17th or 18th century instructions to repair a mirror.»

First, a mirror repaired this way would assume a rather anattractive look.

Second, in the old times mirrors were quite rare and they were considered magickal implements. To shatter a mirror was a very bad sign. Even a worse sign was to see your own reflection in a broken mirror!

Even at modern days in Russia has remained the following custom. When someone dies, all the mirrors (and icons for some reason) in the house are covered by a curtain.

To satisfy your curiosity, the text is about fixing a glass vessel for distillation.
Ah! I see. I knew it couldn't be a drinking vessel, and judging by the language, it was a time when 'a glass' was used for a mirror. I would never have guessed it was a distillation vessel - like an alembic or something, then? What's the date of the source, just for curiosity's sake?
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Lil' Ruby Rose: 1651. Being not an alembic in modern sense, the device is like that.