Hello I'm trying to find some information about black oxidation method, I have a large 33 gallon tank that we are using to blacking long shafts from old style mailing machines that we are rebuilding and I having allot of trouble getting the required finish. It is coming out with a purple/pink type film covering the shaft that I can wipe off or when I wipe my hand across the part the black comes off the part and onto my hand and the finish is really poor on the part and the black colour is almost cloudy.

I have just drained the tank and sanded the sides to make sure there was no containment's on the tank, then I put brand new chemicals in, I used the formula of 1 gallon water to 2 pounds sodium nitrate and 5 pounds Caustic Soda as a guide, I have the temp spot on 140 degrees, before I put any parts in I have giving them a nice sand to make the finish nice and smooth, if there was any oil I dipped them in sulfuric acid that is 60% and rinsed to make sure they are clean as can be.

I have notice that different types of mild steel blacken differently, I have put some parts in and they come out perfect while other parts sitting right next to them haven't even started to turn black yet.
Should I be focusing more on the chemicals I have in the tank or the heat that I have the tank set to? Also what colour should the water be, before I drained the then tank the water used to be almost a cream/orange colour and now that I have brand new chemicals it is a brownish black colour?

Thanks.