Chemistry 101?

[Landlocked123]

Hi Folks,

 

I want to start this with a disclaimer.  I AM A NOVICE IN THIS AVOCATION AND HAVE LEARNED EVERYTHING I KNOW (or don't) THROUGH EXPERIMENTATION AND MSW.

 

I've read a lot of posts over the last few years about people having problems with chemical blackening of brass.  After a a lot of trial and error I've had a lot of recent success.  I've using Birchwood Casey Brass Black, Acetone, and Sparex.  I first take any brass I want to blacken and soak in acetone for 5-10 minutes to remove any solder flux, lacquer, or finger prints.  I've taken a small piece of window screening and pushed it into a steel can (tomato sauce) to strain the solutions and the pieces being blackened.

 

I make a solution of Sparex (acidic pickling) by putting warm water (125-130 degrees) in a glass container.  Add the dry Sparex in an amount greater than recommend.  After 15 minutes or so stirring the mixture with a piece of wood, pour off the liquid into a jar with a cover leaving the undisolved Sparex behind.  You now have a saturated pickling solution.  Taking the pieces to be blackened out of the acetone, place in the warm Sparex solution for about 10-15 minutes.  Keeping the Sparex warm makes it work faster, but you can use it cold, just give it more time. You should take the pieces to be blackened out of the Sparex and rinse under cold running water, but I just put into a jar with water and shake vigorously.  I then strain into to the can with the screening.

 

I make a solution of Brass Black to water of 1:7.5.  I use a paint pipets, but any way is fine.  After taking the rinsed pieces out of the water I place them in the diluted Brass Black.  I then watch them carefully until mostly black.  I then remove, strain, and rinse vigorously then returning them to the blackening solution. I will generaly do it twice more, following the same process rinsing after each immersion.  I then place on a paper towel to dry for a couple of days.  By following this routine I've gotten deeply blacked pieces without having any of the blackening rubbing off and making a mess of my hands and everything else.

 

Now here comes comes the chemistry question for all my brilliant colleagues out there.  I would much rather darken the pieces with one longer immersion in the blackening solution.  But when I do that, the blackening flakes and gets everywhere making a mess.  I've been thinking about being a small kid in my Grandfather's darkroom.  As I've alluded to, I am dolt as far as chemistry is concerned.  But in a process in developing film and prints, my Grandad would talk about the "Fix" which, I understood was the chemical which stopped the development process.  Given my limited understanding, Brass Black and other blackening processes involve an acidic solution.  Therefore, would a immersion in a base, like a baking soda solution, "fix" the reaction and allow for one blackening run as opposed to a number?

 

I'll try some more experiments and let everybody know if I have a "Eureka" moment, but in the interim, would certainly appreciate any corrections, feedback, or other comments.

 

Best,

John

 

 

[xken]

John, you have hit upon one of the key factors of chemical staining. Another approach depending upon the size of the parts is to scrub the part using pumice and a toothbrush (I save old ones for this) in a bowl of hot water with a few drops of dishwasher soap until clean. You need to do the scrubbing within a bowl to "catch" the excess pumice which if not caught will plug you sink drain trap. Once the parts are cleaned rinse with warm water and set on a clean cloth  to dry. When ready to stain heat the parts under a light bulb and once warmed up then dip the part(s) into the blackening chemical. Heat is a second factor often overlooked in the process that I have had great results with. The product I use is Novacan Black Patina that I purchase at a Stained Glass Supply outlet. Once stained if not pleased try rubbing the part with a clean soft rag and re-stain in the chemical. Sometimes the buffing alone can result in a pleasing finish as well.

 

As you have learned, there are many factors that can influence the results including the alloy used to cast the parts themselves. Next time try heating (warming) the parts and let me know how the heating worked for you.

 

[lambsbk]

Hmmm...it has been a LONG time since my chem days. As I understand the blackening process deposits metal salts from the blackening solution onto the metal being blackened. I think the water rinse stops the process by simply removing the catalysts in the solution.

 

Not sure though.

 

Dave

[Jaager]

As Dave suggests - the acid reaction sounds like a salt forming reaction.  Going longer than

recommended could result in the formation of enough salt that its weight could cause it to

flake off.  Removing the reactants in running water should halt the reaction.  Treating it

with a basic solution could cause either an alternate and undesirable reaction or reverse

the one desired.

[grsjax]

The acid etching results in a surface with a greater area than un-etched surface.  The same is true for cleaning with pumice or other abrasives.  This is a factor in the blacking process because it gives the blacking solution more area to interact with.  I get much the same results using a similar process to that described by Landlocked123 but I use white vinegar instead of sparex.  The white vinegar is acidic enough to etch brass and white metal, it is cheap and doesn't require any mixing.  An important factor is cleaning the metal very well and rinsing all residue from the cleaning solution from the metal.  BTW if you want to make your own blacking solution you can use a super-saturated solution of copper carbonate (CuCO3Cu(OH)2) and regular, over-the-counter ammonia.