I too had a big problem w/blackener rub-off in my first plating attempts. Since I was not convinced that the actual zinc plate was very good, I wasn’t sure if the problem was w/blackener in general, or due to poor plating.
As you can see, most of my problem was due to use of brightener. But even w/out brightener, I still get some black coming off, particularly at sharp corners, such as on a bolt head. It’s not as bad as your bolt, but still noticeable.
I leave it in the blackener for a long time for a couple of reasons:
1) Caswell instructions are inconsistent. The plating manual says “immerse until uniformly black”, yet the bottle of solution says “immerse for 2-5 minutes”. And I have a recollection of reading somewhere (although I can’t find it again!), to immerse for 1-2 minutes per 0.1mil of plate thickness. Since I am plating to 1 mil, that works out to 10-20 minutes, so I split the difference
2) In my tests, after 2 minutes there was still quite a bit of silver showing. Like you, I kept trying for longer periods, and have found it to be more durable w/ longer immersion times. So I’m working on the assumption that longer immersion = thicker black layer.
I’m interested in your voltage/current curves for one primary reason: Your bolts look FAR better than mine did using Caswells power brick. w/12” of parts, my power brick was only providing 0.4-0.5v @ 750-800mA. This was completely out-of-sync w/instructions, and comes out to 62-67 mA/sq-in. I considered the resulting plate to be poor, and likely contributing to poor blackener adhesion.
I refer you to my earlier post concerning my problems:
http://www.caswellplating.com/bbs/viewtopic.php?t=2630
After receiving much advice here and elsewhere, I have settled on a few rules of thumb which work best for me:
1) get the voltage above 0.5v. Anything less will not result in good plate
2) plate at 75-100 mA/sq-in current density. Any less is sub-optimal. Any more will burn.
3) to get your voltage up, a) keep the electrolyte cool (I run mine at 65º now), and b) get more separation between your parts and the anodes. In a 2 gal bucket, w/anodes on either side, you’ll only get 4.5” of separation. I now put both anodes on one side of the bucket, and parts on the opposite side, giving 6-7” of separation. This seems to work better, but I rotate the part 180º every few minutes.
I have my suspicions that the blackener will never be very durable.
FFI, see my other test results:
http://www.hogheaven.com/hobby/plati...c/cctest1.html
http://www.hogheaven.com/hobby/plati...c/cctest5.html