what is the pale red coating on hydroton?

[RW]

Does anyone know what the pale red coating on hydroton is made from and if it's harmful in any way? 

I rinsed my hydroton and got most of it off before adding to the media bed, but there's still some hanging around and on my fish tank bottom and walls.

[TCLynx]

Probably some sort of oxide from the process in the kiln.  Not harmful though depending on what kind it might affect the pH a little in the very short term.

[RupertofOZ]

Hydroton is, according to the manufacturer... both pH and chemically inert...