[Ferric Chloride; Dilute or Not?]

It will probably vary. Depends on how much iron gets dissolved. It doesn't have to be quantitative to work.

[Ferric Chloride; Dilute or Not?]

Make your own  if you  have a little experience playing with chemicals.

All of the safety things you have seen about handling and cleaning up after ferric chloride and Muriatic acid  (hydrochloric acid) apply.  Especially rubber gloves and Eye protection If you have no experience working with toxic chemicals DON'T DO IT!  or get help from someone with  some experience.

 Get some muriatic acid at the hardware store,  In a well ventilated  area, fill a  glass jar 1/4 full of water then add the acid (slowly) till it is half full. Put in a bunch of rusty iron things that  fit inside. DON'T PUT IN GALVANIZED PIPE !!   Let it sit with a piece of plastic screen wired around the top.  DO NOT SEAL THE LID!  ( it gives off fumes that probably wont kill you but are not very nice to  breath. they will also corrode metal things in their vicinity). Store it preferably outside somewhere where it won't be in the way or get filled with rain or snow.  If all of the rusty iron has dissolved,  add more  till it doesn't. Let this evaporate down  till the solution looks dark brown-red.  now you have impure ferric chloride.  You can filter this through a  pant filter and plastic funnel. don't let it touch metal.  All of the safety things you have seen about ferric chloride apply. 

If all the warnings have not scared you 0ff, There are YouTube  vids al over the place that show you how to do this.

I never spray ware for a pit fire.  I get some not too fine saw dust (not sander dust) and use it to absorb a solution of  the unfiltered ferric chloride, until it is damp but but dripping.  I sprinkle this over the pot as it is placed in the pit.  It's not quite like spaying but it gives interesting effects.