I'm sorry, I didn't see the question to me come up.
I have had rare success with brass.
When I did have success I did it in a steel box with cardboard under and above the billet.
This allowed the oxygen to burn out and put a layer of carbon between the top and bottom of the box.
I use a pyrometer and if I remember correctly the # you're trying to hit is 1840 degrees.
I hold for 15 minutes and then run in a press.
I have done the traditional method with torque plates, in a charcoal forge with copper, nickel-silver, and steel never iron.
If you use a gas forge it needs to be fuel rich to protect form oxidization.
I don't use flux but have seen people do it with success.
I have never used a kiln,
The kiln process uses a much lower temp with a longer time inside a stainless steel bag with charcoal in it.
The bag has charcoal in it to burn the oxygen off.
As I understand it you are trying to get electrons form both types of material to switch places to create the bond.
If you slightly melt the material and make an alloy you will see a fuzziness on the edge of the bond.
Personally I like both looks.
http://www.aescustomknives.com/docs/tutorial12.htm
This is where I learned the box method.
I learned the regular method from this book.
http://www.guyotbrothers.com/jewelry-bookshelf/mokume-gane-comprehensive-study.htm
I don't check in here a lot but if you have any questions.
please feel free to email me at colorblind72@comcast.net.