Modifying my longbow to +11 and higher

I have a +10 Longbow, and I want to modify it to +11.

I read that I need Titanite Chunks; I have 4 of them, so I went to the blacksmith. But I can't seem to upgrade my bow.

Did I miss something to reinforce my longbow above +10?


Solution 1:

You have to give the Very Large Ember to the Blacksmith Andre of Astoria.

It is found in the New Londo Ruins, under Firelink Shrine.

It is the same process than to upgrade from +5 to +6 ; only this time, the ember is very large.