It looks like modern clear glass is pretty much impossible without magic or machinery
glassonline.com/infoserv/history.html
And reaaly good glass could be a declaration of a trade war with Venice. Breedin Monsters in the caves has nothing on that...
A very good point.
A nice link - even got a Ravenscroft in its pages!
We got 50 years before Venice enacts an Ordinance against us. 
The Ravenscroft Societates in Ex Misc is quite extensive after all. 
Formulating a formal stature doesnt rule out that it might in fact even have been a policy prior to that point - only now perpetuating it in writing...
Even if you had the following of an part of a simpleton non-Roman House I will still torch you for falsely putting words in my name that were not mine to begin with!!! 
You'll be torched before you can spell to Creo Ignem you fool of an Ex Miscellanea!
Those flambies, always torching things.... So rude....
Xavi
I deny any accusations of word-putting by a Twilight-prone Flambeau. 
I'll flame you for those insolent words!!
Consider yourselves BoAFed 
Sure sure. Make sure to bring some sausages to the event, so we can have a nice BBQ
Xavi hides in a lake regio until Furion reaches the 100th post and goes into final twilight. Only 80 posts to go....
Cheers,
Xavi
I'm assuming that this has come from something like City & Guild, as I've not come across these crafting rules. So I'll comment on my feelings around general principles rather than anything else.
Strikes me that if Rego can't create clear glass, then a Creo spell should be able to. Creo after all enjoys the broad scope of healing, improving and perfecting. However, whether this might require Vis I wouldn't know really, and that could make this potential solution a bit of a pain!
Correct, with Creo, you can create perfect things.
It'll require Vis if you want the glass to exist on a permanent basis, as with all Creo magic.
I think the root of Fruny's argument is partly that rego can only achieve what is possible by a mundane craftsman and his tools - which IIRC is the wording in Covenants. Two things then play a part. A) that you might argue that clear glass is beyond the reach of ME craftsmen (as it was to medieval craftsmen according to the link cited by Andrew) and thus also of rego magic, and/or B) that clear glass is only possible by alchemy and thus not possible for a mundane craftsman and thus also impossible for rego spells.
Fear my Ex Miscellanea sock of water summoning!
Damn, my original post wasn't clear. What I wanted to suggest was whether a Creo following a Rego might be able to perfect the Rego'd glass, rather than create it from scratch. CrTe L20 ritual "Perfection of the Crystal Vial" permanently removes all flaws, bubbles and other imperfections from within the covenant building (L3 +2 Voice +3 Structure -1 Glass is being healed not created outright).
Aha, interesting point! Perfection rather than Creation.
Ravenscroft you scare me! 
I was about to say somthing about how water can boil (which also goes for anyone incidentally hiding in a lake, Xavier!), but your mighty picture just can't be topped. Especially in a pre-Geneva Conventions world where there is no restriction on chemical warfare!
The fact that making clear glass is an alchemical process indicates that it isn't a perfected form of glass, but rather another substance entirely (like lead and gold...) This would indicate that using a Creo on obsidian-type glass (the non-clear sort) would mend fractures, but not make it clear.
It would probably be most useful to have a real glassmaker come to the covenant to make the glass, and use craft magic to shape the glass...
Alchemical process? You mean, like, melting something and skimming the impurities off? Pretty complex...
"Clear", window/optical quality glass didn't show up until the Renaissance, approx circa early 1400's. It wasn't a matter of "changing" the glass, but removing the impurities that fogged it up. (That would naturally be a greenish color; chemicals were added to make it "colorless", and that is an alchemical process.) Quality control, higher temperatures, steel tools, and nobles who didn't mind throwing money at a given problem all contributed to the advancement.
Interestingly, for sheets, they would melt the glass onto a pool of molten lead, which, being a liquid both hot and much heavier, would both support the glass and give it one almost perfectly smooth side while it was handled and then cooled. (Not very healthy, but effective, and a technique that lasted well into the 1900's.)
As for "wood ash" requiring an Herbem req... sorry, but - phhh. At some point, a fallen tree turns to mulch, and then becomes part of the soil, which is clearly the realm of Terram. You wouldn't require Herbam to effect "rich soil", even tho' plant (or animal) material is exactly what ultimately makes it rich. Nor require Corpus to remove gravedirt, even tho' there is a strong element of humanity within it. Charcoal, which is as yet not completely burnt, might still be considered Herbem, but by the time it's been burnt to pure ash it has, indeed, undergone an alchemical change into a different substance.
However, iirc, the point is moot, for it's not "ashes" that are added, but the chemical soda-ash (aka sodium carbonate, or "trona"), tho' it's not inconceivable that both were used at various times.
ecu.edu/glassblowing/oldrecipes.htm
unc.edu/courses/rometech/pub ... Glass.html
The "best" glass makers were found in the blossoming city-state of Venice, which banned outside glass not because it was better, but because a monopoly is a desired state for any merchant environment.
Huge standing rewards were offered for any glassblowers who would defect and teach their secrets abroad, and likewise bounties put on the heads of any such who would dare cross the Guild.
Just pointing something out:
There is magical alchemy and there is mundane alchemy. Both are acknowledged in 5th ed. and Alchemy itself in the 1200s refers to a wide array of processes attributed to alchemists - specialized fermentations, distillations, making alloys of metals ... all of these things adn much more are mundane alchemy. So making clear glass, making proper mirrors, good paints and fancy inks all have the word alchemy thrown about in the 5th ed. writing because all of those things were considered such in the day.
Hermetic and Vulgar Alchemy, etc. are the obvious magical examples, but in the Mysteries Revised it clearly makes mention of large numbers of mundane, ungifted, alchemists practicing the 'science' of alchemy (in the Order of the Green Cockerel writeup). IMHO this is likely a carryover from the 'guilded alchemist' of 4th as there is no good reason to have all those guilds and danks basement labs vanish suddenly 
Thus I would contend there are many non-magical 'alchemical' processes going in in Mythic Europe. If Rego Craft Magic can replicate a mundane process, it should be able to create clear glass if such a thing is possible (I grant it may not be ... but not because Alchemy is always magical as that is only one interpretation seeminly contradicted by published materials).
It should at least be able to create clear-er glass or various colors of glass at higher finesse difficulties as someone else mentioned above (IIRC).
I know that the process in historic reality wasn't about actually changing the glass, but during medival times, turning sand into a transparent glass would have appeared pretty amazing - pretty much changing it from one substance to another. And since we're in Mythic Europe, rather than Historical Europe, I would argue that the change is in some ways supernatural.
And by clear glass I understand all transparent glass...
Perhaps a Perdo requisite to destroy all but the pure material?