they can't make a airtight container big enough for weeks of travel, especially not one that can withstand vacuum. Even a day of travel is probably impossible. You need a shipping container worth of air per person per day. Since it needs to have doors it gets even less likely they can make it airtight.
The only technology they had for making airtight ...
If airtightness is the only requirement, then yes, medieval people could create such vessels
By middle ages, creating watertight ship hulls was a very common task. Yes, ships leaked, but that could be successfully mitigated.
Also, diving bells were known since antiquity, and fully isolated "dry" bell is not very different from a spaceship. 1 atm ...
I don't think it's better than the other answers, but I'm adding it for completeness since it was my first thought: pumpkins!
Not only do they shatter on impact, but they follow the Rule of Cool for magic. And if the starting time frame is end of fall, it's plausible that you have plenty on hand.
As someone else pointed out, grenades normally explode (due to being full of explosive with a fuse or detonator and all that) rather than shattering.
However some types of weapon rely on the container breaking and exposing the contents to the air, which seems similar to your use case. Improvised phosphorus bombs, for example. The container used is normally ...
In reality, anything vitreous or ceramic might shatter when it hits something, but you might still need to specify what "like a grenade" means.
Please remember, grenades do not "shatter" and are not interested in contact; they are exploded, from within, by timing mechanisms. To be really picky, that's why we generally speak of grenade &...
Could Prince Rupert's Drops be useful? If you drip molten glass into water it forms tadpole-shaped drops. The internal stress within these drops makes them very strong: you can hit one with a hammer and it won't break. But if you break the tail it instantly shatters into powder. Put magic into these and have a simple mechanism which breaks the tail on impact,...
If it's available, obsidian. Obsidian is glass, so it shatters into very sharp pieces.
The downsides are that the pieces need to be large enough to shatter, and obsidian is rare unless the mountain is/was a volcano.
His magic grenades use runes to hold magic. When
the runes are damaged, the magic stored within
explodes. This means his grenades need to break
(or preferably shatter) on impact.
I don't think your last sentence is accurate, and it's limiting your design space. Grenades are quite sturdy and do not shatter from an impact. They shatter due to the force of ...
I'll agree with others that clay, either fired or just dried, is probably your best bet (presumably a small experiment to see which stood up best to storage and also shattered reliably on landing but not on throwing/hurling/whatever). Slates or sticks sawn to near-failure are probably also a good bet (especially if you start running out of clay). Sticks in ...
I'm not sure how practical this would be, but any riff on this theme would be pretty cool. Prince Rupert's Drops are created by dripping molten glass into cold water. The bulbous end is extremely strong, but if the tail is damaged the whole thing disintegrates explosively. You can find many videos demonstrating their interesting properties. I am not sure if ...
When making ceramics, you first form your object out of clay, wait for it to dry for about a week (give or take a few days depending on humidity), then you bake it in a kiln for anywhere from several hours to several days to make the clay melt together into the glass like substance we generally call ceramics.
While ceramics have been brought up ...
How Low Can You Go?
Wax - from bee hives
Resin - from evergreen trees
Feces - from bears, etcetera
latex - from trees, dandelions etcetera
galls - plant "tumors" grown around insect larva
gourds - or other hollow shelled fruit
animal intestines and bladders
Some combination of the first four ingredients mixed with fibers from plants, animal hair, ...
It's unclear if the rune is an object in its own right held in a delivery device, or merely a symbol on some other object. I'm going to assume the latter, as that seems to more closely match your description. Thus, assuming you can draw/paint/apply a rune to some existing object which needs to break when thrown...
Eggs. Can be produced in rather large ...
Explosions are nice... but a lot of additional damage can be done also by shards of the projectile itself. So you want something, that shatters into many sharp pieces.
When you say "grenade", the first idea is to have it ball-shaped... but runes are usually painted on a surface. If the rune releases its power on the surface, it will ...
An alternative answer: very high carbon steel.
All of the other answers posted answer your question well, but a bit out-of-the box might be what you're looking for.
Forge arrowheads (you said metal, tools, wood?). Make a kiln/furnace and case harden your arrowheads as much as possible (increasing the carbon concentration in iron makes it harder, but more ...
...knowing that when the winter ends, his enemies will be upon him
It's going to be winter until the enemy arrives? Use ice. He surely has water around, and freezing things is within a wizard's capabilities.
If your wizard can make bricks, he can make hollow ceramic bulbs. Bricks are made from baked or fired clay. Via the same process, you can make a vessel of the right thickness to hold when thrown, but shatter upon impact. You could for instance wrap clay around a pine cone or plant bulb or rounded chunk of wood, then fire it in a kiln.
It is on your list! And it shatters great. Have your peeps dig a snowball cellar in case you have some hot days before the baddies come. Then load it up with snowballs.
You can incorporate some yellow snow, to add insult to injury.
Ice grenades would also do the trick.
Egg shells if the wizard wants to go for naturally available materials, or pottery balls if they can go for manufactured materials.
Both are conveniently empty and can be filled with whatever needed, and the filling hole can be sealed with clay/wax. Moreover both easily crack on impact, assuring that the wizard can achieve the intended result.
https://startcaving.com/caving-guides/how-are-caves-formed has a list of how caves can be formed
There are several types of "caves". The most common and largest caves are those cut in limestone by (slightly acidic) water. The only stones that might be of interest to take out are cherts and flints. Some people might find the limestone formations to ...