I just take a beautiful crown and throw it against a brick wall untill the desired effect is created.
Just kidding.
I cut the wet porcelian .5mm deep or close to it with a razor and paint some paste opaque in the fissure. Then fold it back together and fire. Play with that technique. I have never heard of anyone doing this, but I tried it untill I got the proper result. Sometimes I use B1, sometimes I swirl a little brown stain into A4 opaque.
It's easy!
The obvious way most do it is to finish a crown, then cut with burr and paint stain, then try to cover it up with porc.
I think that looks fake close up and wanted better....
Scott
