Can arrows with glass heads be effective and optimally implemented in an early medieval war setting?
I'd imagine it could be cheaper and faster to produce than metal heads, since the shape of the head doesn't matter, and the entire point of a glass head is the fact that it explodes in a cloud of glass upon impact with armor and does well on flesh too.
Therefore it doesn't really matter if it's actual glass, any crystal would work. all it takes is a piece of glass/crystal and a stone
In principle, glass arrowheads would be highly effective, at least against unarmoured opponents. According to this study, glass shards need to travel in excess of ~15 m/s to deeply penetrate flesh, and the greenman longbows site rates longbows as propelling arrows with a velocity of around 170 feet per second, or around 52 m/s. Glass arrowheads can - with some practice - be formed in the same manner as flint (or other stone) arrowheads, by knapping.
The question then is not whether glass arrowheads are effective, but whether glass is economically viable as a material. Glass was manufactured and used in ancient times, as well as in the medieval period, but not typically for arrowheads. There are two possible reasons for this: the skill of knapping stone (or glass) arrowheads was not widely known, or glass was too expensive to be used in such a disposable fashion (at least relative to iron). Information presented here suggests that glass was very expensive to produce.
So in the questioner's world, use of glass (or a crystalline rock that can be similarly knapped) for arrowheads would depend on it being common enough a material that it would be used in preference to e.g. flint or iron.
Glass arrow heads are effective and deadly if made well. These are the issues that I would consider for a medieval war scenario...
It takes much more skill and time to create good knapped arrow heads than it does to make metal ones.
Metal is more durable than glass so that arrows are more likely to be usable again after they are retrieved
Metal arrowheads are much more effective at punching through armor. Glass arrow heads would do very poorly and the damage from broken glass would be negligible.
I think that lack of access to metal would be the only plausible reason for armies to use glass heads.
I make wooden self bows for a living and have made arrow heads from glass and obsidian. I'm not an expert on knapping but I know a fair bit from experience and research.
http://raregoldnuggets.com/?p=5497 Sure you could have glass arrowheads. Obsidian is volcanic glass and makes fine arrowheads. The Amerinds did this for millennia.
Would someone make arrowheads out of glass if they knew about metal? Absolutely they would if metal were scarce and valuable and they had plenty of glass and were good at making arrowheads. Save the metal for your kitchen knives and armor and shoot the cheap glass on your arrows that you may or may not get back. It makes sense!
One important aspect is the huge durability problem that such weapons would pose, besides being less economically viable than metal arrowheads.
You have to take into consideration that medieval campaigns would often last months, if not years, most of which was spent traveling. In that respect, the last thing you'd want is brittle arrowheads that will break at the slightest shock and probably even damage each other as the marching bowmen knock the arrows around in their quills while marching.
I can't even start to imagine the logistical nightmare it would be.
For a one-off mission however, like an assassination where you'll get the chance to shoot only one arrow at your target anyway, such an arrow might be interesting against unprotected, high-value target. You could even imagine a hollow glass arrowhead designed like a vial containing a poison, that breaks up and releases the poison upon penetration.
Is a glass arrowhead possible? Yes. Using modern glass, an arrow has nearly the same penetration as a modern steel broadhead arrow, when measured using gel powder.
However, in our world, this is not very practical. Forest glass was the main type of glass made in Europe in the medieval ages, it was low quality, prone to chipping and cracking. Transporting, storing and marching with glass arrowheads would damage them, making them less useful than metal ones.
The other problem is making them. Glass making was expensive. Cheaper to use bog iron, melt down old metal items, or dig iron out of the ground than go through the trouble to make glass.
If people were low on metals, they would turn to natural glass like obsidian and flint. They're cheaper, easier to make, and not as fragile as the man-made glass of the time.
