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13 minutes ago, Moab said:

I just used one from BlueJacket that worked very well

May I ask what prep you did on the pieces? I haven’t tried the method in the thread above yet (and I should) as I’m not having great results yet (I think I may have learned today that I was polishing a little too hard as well...).

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I just thoroughly clean the brass/copper with fine steel wool to the bare metal and dip it in acetone trying not to touch it with my fingers. Afterwards I dip it in Birchwood Casey Brass Black for a few minutes. I noticed that if I keep it there for too long, the efect is undesireable, blotchy and uneven. I then wipe it with a cotton rug and dip in water.

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As has been mentioned here before, take care with this stuff .All these preparations contain selenium dioxide (or selenious acid when it is in solution- same stuff in a different form). This stuff is  highly toxic so wear gloves and ensure good ventilation.

 

Now, what is happening here is what chemists call a Redox Reaction.  Modellers certainly do not need know any chemistry but there are a couple of important consequences of the chemistry.  Firstly the metal (or actually the copper in the brass) is etched away by the selenious acid and goes into solution as copper ions.  At the same time the selenious acid is converted to selenium metal (which is black) and this is deposited as a tiny black spec in place from which the copper was removed.  

 

So notice that the Selenium metal is not chemically attached to the metal in any way so it can be removed by mechanical action.  Also although others have warned about the flaking that is a consequence of prolonged action a second effect is that more of the metal surface is etched and surface detail is compromised.  Obviously this is not important for an eyepin but more significant for a cannon, say.  In the minute or so that it takes to achieve a good result this would be barely noticeable but if one were persist for half an hour there would be noticeable pitting and significant loss of detail.

 

All of these products also contain a mineral acid (nitric in Jax Black, hydrochloric in Jax Pewter Black, and phosphoric in Birchwood  Casey) and they also usually contain some copper sulfate, which is there to moderate the reaction. This, of course, is the blue colour of the product

 

The Birchwood  Casey product also contains molybdate  which acts in a similar way but with the zinc in the alloy.  So you possibly get a deeper black since both components of the brass are being blackened.

 

After the initial blackening most people polish the surface to remove any excess flaking.  So the black stuff, which ends up on the cloth (in dvm27’s post for example) is Selenium metal which is also toxic and worse still it is a dust, which you can breath in.  I would not be polishing with a power tool but if you must, wear a mask and gloves.  If you get black on your fingers don’t go and eat your lunch with out washing it off with copious soap and water!

 

John

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VTHokiEE,

I have what may be a dumb question... between "baths" are letting the brass dry?  I wipe dry using a paper towel or two after taking my brass out of any solution except the blackening.  Even after wiping, I set it aside on dry paper towel and let it finish drying.

 

I only ask because the first time I blackened, I went from acetone, to vinegar, to water, to blackening and didn't dry.  I wasn't a pretty sight.

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I use a small plastic sieve to contain my parts and use Sparex to clean off the contamination.  So the sieve containing the parts goes into the Sparex for a couple of minutes then into a bicarbonate bath to neutralise and wash under running water.  Then into the blackening reagent (no drying).  Again into the bicarbonate to quench the reaction then wash again in running water.  Only then do I dry the parts before gently polishing with a micro-fibre cloth. I used to use an acetone step but no longer find it necessary. If I did I would allow to dry before the next step.  But acetone dries quite quickly.

 

John

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