I believe the regular shards take regular weapons to +5, large shards to +10 and then chunks to... +12? and then slabs after that, or possibly the chunks straight to +15. either way you can't do that til you get the large ember from the chest next to the guy with the meat cleaver in the depths. (after beating the Capra demon)
There are Four different Blacksmiths in the game. The one that uses the Ember you just got is Andre, right near the Undead Parish church.
The mage blacksmith is in New Londo, right near the beginning in a cage. He does Magic and Enchanted weapons.
The Fire blacksmith is a skeleton in the graveyard area, he does Fire and Chaos.
The Divine Blacksmith (Andre) does Normal (+15), Divine, and Occult.
The Lightning Blacksmith in Anor Londo does Lightning, Crystal, and Boss Soul Items.
Note that each blacksmith has 2 or more embers to find and give to them. Each Ember is pretty dang well hidden. Explore well... --- http://www.youtube.com/watch?v=-TsEpDS7-Mw See quote for my gamertag, steam name, and youtube :D