exhibit A:
I really like that effect and would generally paint the back sides with black, just for aesthetic reasons. After a while, I decided to just clear coat the back sides of the pieces (sometimes polishing backside first, sometimes just clear coating over the milscale if it was something being hung inside).
Generally, I never really paid my attention to the back of pieces after torching considering it wasn't something people would see. As I did more and more pieces with the torch coloring, I realized it made for a pretty cool effect on the side that wasn't polished, but for whatever reason it never really clicked as to what was causing it.
exhibit B:
It finally dawned on me that the effect is being caused by whatever contaminants were on the steel after being taken out of the acid/water solution (i.e. milscale, flash rust, etc). So after playing with a few variables I figured out how to do the marble-ish effect
exhibit C:
Hopefully this will be beneficial to some of you on here. I can't always contribute to the file sharing, but hopefully trying to share some of the finishing techniques will make up the difference
If you have any questions, feel free to PM me or post them in the thread. I have an "original" piece I'm doing where I'm hoping to do the marblized effect as the sky/cosmos, stay tuned 
side note - all three have been clear coated prior to taking the picture, so the effect is a little different due to gloss.
