Excalibur recipe doesn't work. I had a sapphire, ancient sword, and orb of magic. The thing is that the orb says 1/2 and the recipe too. The wording is confusing.
Ah, I figured it out. You should get the set of 3 elemental orbs and put it with your ancient sword (both can be found in chapter 6) and orb of magic (from chapter 3). Then the synthesis will work.