The clay water jar is actually a traditional component of the Japanese tea ceremony. I don't have much more to add about this having not used one myself, but Matt from Korea writes about his jar here: http://mattchasblog.blogspot.com/2008/0 … g-ree.html
Matt speaks in rather unscientific terms, but he claims the jar absorbs unwanted minerals, softening the water. This is contrary to Aaron's take on stoneware jars and kettles, especially purion or the volcanic clay mixture used by Chen Qi Nan. These are said to re-mineralize water in a manner "similar to bamboo charcoal".
Here is a very interesting piece from Lin's purion collection, a 5 gallon water jug with tap. http://www.alibaba.com/product/tw104632 … on_II.html
Yeah - I've seen the Lin's one before; probably pretty expensive to ship, though (and it's not cheap in the first place, either).
Also, I've heard that certain stuff (heavier minerals) may settle to the bottom, and that's why you usually skim the water off the top. I would wonder if having a tap on the bottom like that might not be a bad thing. I'm not sure if it's completely contrary to say that something both mineralizes the water and absorbs unwanted minerals... also, Matt does say in his post "The water is at home cuddling close to the Mool Hang A Ree, absorbing its minerals and natural deposits." I think people have made the claim that certain materials can both add helpful minerals and also remove harmful substances. Whether this is backed up by any sort of hard science, I can't say. :> I have heard a lot of anecdotal evidence of ceramics of various sorts (glazed or unglazed) improving the taste of water at various stages of the game.
Anyone know a US source of glazed or unglazed water jars? I think for the glaze one, probably any sort of large porcelain jar would work; locally there's a shop that sells a lot of reproduction antique Chinese porcelain, but a lot of their stuff is kind of garish. The ones Matt has pictures of are great.... do any of the Korean potters he puts pictures of up have stuff available online (btw, saw an electric brazier similar to, though not as beautiful as, the ones he posted on his site when I was at Hankook this past weekend)?
I have seen the unglazed ones too, but I think the Chinese ones are usually porcelain; I was thinking about this sort:
http://theteagallery.blogspot.com/2008/ … r-tea.html
http://themandarinstea.blogspot.com/200 … water.html
I'm not sure if these are purpose-made for water storage, or if they're just generic vase / jar type things. I have heard that they should be covered with a piece of cloth or something like that.