How can I get a darker finish with farric acid ?

[flatblackcapo]

I threw my Kershaw leek in 100% ferric acid left it in for total of 40 min ,taking it out every once in a while to check the finish. I rinsed with hot water saw it was not as dark as I wanted put it back in. I am doing the scales,blade and clip.Any trick you might know to get it dark please let me know.I will try to get some pics up soon if it would help.