[TheForge] Wrought iron patina...

[Andy Vida]

OK, I'm wondering about something.

I just finished a wrought iron cuff for a friend that I made from an 
old railroad spike (there are millions of them all over the tracks in 
places like NJ).  It is a plain cuff, nothing fancy at all.  Given my 
almost absolute lack of tools these days, I ground the finish by hand 
on a stone.  Came out rather nicely.  I was going to etch is for grain,
but my friend wanted a smooth finish, so I tried my nickel patina.  
This produced a really nice deep purple oxide, quite pleasing to the 
eye, much like a rokusho patina on shakudo (shakudo is an alloy commonly
of 4% gold in copper).

Well, I have another little bracelet, in fact from the same spike,
that I thought I'd try the patina on.  The result is a very dull
and dark gray, approaching black.  I'm wondering why the difference
in result, especially given that each object came from the same piece
of wrought iron.  The only difference is that I etched the latter in
HCl.  I'm pretty sure that must be the reason that they are different,
but what I would like to know is why.  Anyone have any ideas?

	-Andy