falconium

I have received already baked terra cotta tile. My reading on the subject indicates the red color comes from oxidized iron in the soil. Any suggestions on a solution that would turn the red tile to yellowish-white after it has been manufactured? Out of thousand tiles we get 100 or so that are like this naturally, but no one knows how to control it. I've been told excess heat during firing can bake out the color, also iron deficient soils. But what about after it is made? Any ideas?