Give Dale Edmonds a call at (816)444-2040. I spoke to him earlier this week, as I am sending him a set of barrels to do for me. His prices are very reasonable. He has done a lot of Parkers.
We had a rather long conversation about the process. He started doing it after he read Oscar's articles in the DGJ. He spoke on the phone to Oscar several times.
He has worked out his own unique set of methods. He is very willing to share the knowledge he has. A couple of things he told me. He plugs the barrels with rubber stoppers. The type used by chemists. They have holes in them. He runs a tube up and out of the bath to allow the barrels to breathe. He also said that if he didn't do that, they act like a pressure cooker and the stoppers eventually pop out.
He feels that the blackening process is not an exact science. That as, Jim mentions in his post, there are a lot of factors that come into play. One of the things that he mentioned, was that he feels alot has to do with the type of steel used in the barrels. He feels it may have something to do with the carbon content and perhaps the trace elements that were present when the steel was forged. It takes him about 2 weeks to do a set of barrels.
Like I said, give him a call. He has much to share.