Shadow captured within glass has many uses to other crafters and practitioners of Magic, for ghost-touch armor and weapons, and other makings and castings. And that is the true secret of CaraVela’s visit on this particular day, not just as a tourist, but to obtain from the glassmaster some carefully packed flasks of Shadowstuff for a lowlands buyer.
Sun, Moon, Shadow, each of them representing the three aspects of the Equinox eclipse.
Whether or not it ends up happening, that's pretty clever
Quill - To help resolve whether it can happen, here are some suggestions:
Apparently the idea of bottling Shadow can work, though the application in
this example is different from the one I suggested for the glass master at the lake.
One requirement to enable the glass master’s flask to bottle Shadow could be that it has a property for Shadow equivalent to the SRD
Ghost Touch property, which requires CL9 and Plane Shift – which could give the flask a Planar attunement to Shadow. I would think that the glass master would be L9 at least, anyway, if he’s that good at his craft.
In D&D4E the Shadowfell, where the Shadow bottling metal is mined, is itself the equivalent of the Plane of Shadow. So it may be that the glass master can only make Shadow bottling glass twice a year during the six minutes of the Eclipse, when Shadow falls across the lake. Perhaps the glass master makes the flasks by casting a Plane Shift into the furnace, pouring hot glass into a mold made of
Force Sculpture to allow many to be made at once, and an assistant uses Magic to cool them (that’s the easy part); then the glass master Dispels the Force mold and other assistants quickly use the flasks to bottle the ambient Shadow present at that time, perhaps by closing a metal, stone, or wooden stopper onto the mouth of the flask by a L1
Hold Portal spell from previously prepared Wands for efficiency. Then lesser assistants wrap and pack the bottles securely so that the process is not slowed down by that task. Like his assistants, the glass master can repeat his part in the process quickly by similarly using items to store spells prepared in advance for the day; the Force Sculptures, for example, last long enough that the molds could even be set up in advance an hour or so ahead of time.
So it may be that the glass master is also on the boat out in the middle of the lake to maximize his crafting time, with a magically heated glass furnace and a small group of assistants, one of which might even be CaraVela, helping the glass master’s team to wrap and pack in exchange for the goods she is taking downriver.
If you like this idea, it’s a way for you resolve allowing it to be done.
What does the riverboat look like? Riverboats are usually shallow-draft, and if they are intended for cargo as well as passengers, they are broad-decked as well. With cargo, they need sail power, especially to go upriver; poling would be hard, and rowing not easy either. A large square sail was a common design, even from Roman times. The ancient Egyptian riverboats of the Pharaohs even had multiple decks with different purposes, staterooms, and canopied areas.
With Magic, a windmage could earn a nice portion of the trade by keeping the sail taut in the direction the ship needed to go; the role of oars and poles as backup measures for doldrum days would be much less needed.
Here is what the river traffic might look like, for ships that ply the river shoreline. We could be on the large one toward the left of the picture.
Shallow draft riverboat, sail powered by windmage