Are you sure that 0.4 lbs is enough to hold 4 lbs of water? This here is "large" but takes only 1.1 litre, about 2.5 lbs of water: https://www.etsy.com/listing/501655255/large-hand-crafted-authentic-medieval
I made some quick calculations assuming it was cylindrical and got a capacity (or volume) of little over 5 lbs of water, or about 2300 cubic centimetres. No idea why capacity seems so low compared to that.
Anyway, I seriously doubt this would be so light, though I might be wrong. 0.4 lbs is the weight of a hamster or a baseball...
This is a really hard topic to search for. Apparently leather is graded in ounces, based on its thickness per square foot. That's not very useful to understand how much it would weigh.
It seems that a rabbit pelt (with fur) is about those 0.4 lbs or a bit less. Would one rabbit hide be enough for a container like this that takes a bit over 4 lbs of water? Would you need thick leather for it? Won't the pitch or beeswax for waterproofing kind of add to the weight? How much leather would you need to wrap around a 2 litre bottle of soda?
You are correct about a number of things here. 5lbs of water is a little over 2 litres so the skin would be about the size of a pop bottle. In terms of holding the weight some fairly thin leather would be fine for it - it's the waterproofing that's the problem.
Sources mention a couple of things. One is the use of a stomach or bladder from a large animal. One is using rawhide as wet rawhide doesn't let much water through it. Of course that involves drinking rawhide flavored water.....
Another interesting one is the use of a goat hide flesh side out hair in with apparently no additional waterproofing reputably used by the romans. Now, that climate is much hotter and drier so maybe they simply accepted some water loss or the water evaporated at roughly the same rate as it soaked through. This would have the side benefit of keeping the water inside cooler. Or maybe goat skin is more watertight than other skins - like seal skin is.
The Sami made theirs out of reindeer hide. Maybe this shares the characteristics of the aforementioned goat hide. Or, they waterproofed it.
In terms of waterproofing pitch would do, as would pine resin or bees wax. All of these would make the skin quite stiff. They would add a bit of weight, too, but I don't know if it would be significant. Birch oil would be possible, too. It's made as a step in making birch tar and birch tar is something we know was around. Birch oil would effectively make "Russian Leather" that is was sold as a valuable export for the very reason it was waterproof.
All in all, I think I might have to experiment and see what I can come up with. I'm not drinking out of a raw hide skin, though.
- Shane