food grade

When it comes to ingredients 'grades', which is best for bath bombs? I would have assumed pharmaceutical grade would be better because of purity, but is it? Is it even safe? I was reading somewhere that some food grade ingredients are better suited as opposed to the cosmetic grade of the same...

I just switched from the Lye I purchased at the hardware store, which was packaged for drain clearing, to "food grade, high quality NaOH beads". These beads are almost 4x larger than the drain cleaner. I am also noticing that the water is not clean and clear after adding the lye and dissolving...