THE BLACKEN-IT TRIALS

[Erebus and Terror]

The Problem:

 

Anyone who has followed my HMS Terror scratch build may remember my issues with blackening brass fittings for the stern assembly. To briefly summarize, I immersed the parts in a standard 8:1 mixture of Blacken-It solution mixed with bottled water, waited until the parts turned the appropriate colour, then rinsed in bottled water to “neutralize” the reaction. I tried this several times on different parts and each time it resulted in a flaky, blotchy appearance that could not be made even despite buffing with a soft cloth.

 

 

Here is an image of my results. Note the blotchy and flaky texture.

 

Inspired by the fine results of other modellers on the boards, I resolved to master the “mysteries of the blue Liquid”.  I began with research; modelers, gunsmiths, jewellers, instrument makers, and mechanics all use various products and processes to chemically blacken metals and a great deal of information is available from forums, blogs, websites, magazines, and books.

 

With this knowledge at hand, I decided to conduct a series of trials to determine the best process for blackening metal using Blacken-it. I chose Blacken-it as it seems to be the most commonly used product on Model Ship World, and, perhaps more importantly, I had a supply available. However, the techniques I use here should be applicable to other metal blackening products.

 

Before I outline my tests I should begin with a note on safety:

 

The chemicals used in the blackening process are dangerous. Rubber gloves, safety goggles, and a well-ventilated room (or fume hood) must be used EVERY time you handle the chemicals.

 

The Process:

 

From my research, I learned that producing consistently good results requires seven steps, in this order:

1)      The surface of the metal should be mechanically prepared. This roughens the surface and removes synthetic coatings that are often used to give stock metal a shiny appearance.

2)      The surface of the metal should be treated with an acid pickle to remove any scale or corrosion.

3)      The metal should be cleaned with a solvent to remove organic contaminants such as oils, fingerprints, and other dirt.

4)      The metal is chemically coloured using a diluted blackening agent.

5)      The reaction should be “fixed” or halted, using a neutralizing solution.

6)      The surface of the metal should be buffed to remove excess blackening products and to polish the new surface.

7)      The metal should be coated in a protective agent to prevent corrosion, soiling, and damage (optional).

 

The Equipment:

 

My research indicates that the following chemicals most often produce consistent results:

1)      Muriatic acid (31.4%). This is commonly used to remove scale and corrosion on the surface of the metal.  Most hobbyists and professionals use 1:1 concentration of water and acid. Remember, you should always pour the acid into water, as it can be dangerous to pour water directly into acid. You can purchase muriatic acid in most hardware or pool supply stores.

 

2)      Deionized water. This tip was given to me by Druxy on these forums. I’m convinced that the blotchy, scaly results on my first use of Blacken-it were the result of my use of mineral-laiden bottled water. Dionized water is treated to remove mineral ions which could react with Blacken-it. Use the deionised water for all stages of the blackening process, including rinsing between baths, diluting chemicals, and for neutralization.

 

3)      Acetone (100%). This is a widely used degreasing agent employed to remove finger prints, oils, or other organic coatings which might contaminate the metal. It can be purchased at any hardware store.

 

4)      Baking soda. The final stage of the blackening process should include proper neutralization. A common solution is two table-spoons of baking soda in a cup (250 ml) of warm deionised water. Often, hobbyists will use running tap water to neutralize the reaction with good results; baking soda seems to be preferred by jewelers and instrument makers.

 

Here is a photo of the equipment I used in my tests.

 

The Tests:

 

My trials involved testing two variables: 1) the concentration of Blacken-It (undiluted, 1:1, 5:1, or 10:1), and 2) water neutralization versus baking soda neutralization.

 

The test parts prior to preparation.

 

Step 1: I thoroughly sanded the surface of my brass test parts with 400 grit sandpaper. This is similar to the preparation of any metal part even if it isn’t going to be painted or chemically coloured.

 

Step 2: I buffed the metal with ultrafine steel wool. Be certain to carefully remove any steel wool filings that remain as they can react with the chemicals used in the next stages.

 

The parts after mechanical preparation. 

 

Step 3: Immerse the part in muriatic acid (diluted 1:1 with deionised) water for 30 minutes. You can immerse the parts for longer, but the acid will eventually etch the surface and soften sharp edges and other details if you leave them in too long. You may notice that the pickle will change the colour of the brass or that some corrosion may appear – this is normal and is caused by impurities or inconsistencies in the metal.

 

 

Step 4: Rinse each part by agitating vigorously in a bath of deionized water for at least 10 seconds. Allow to dry thoroughly on a clean paper towel. Change the water in the bath for the next step.

 

 

Allow the parts to dry thoroughly.

 

Step 5: Immerse the parts in an acetone bath for 30 minutes.

 

 

Step 6: Rinse each part by agitating vigorously in a bath of deionized water for at least 10 seconds. Allow to dry thoroughly on a clean paper towel.

 

 

Allow the parts to dry thoroughly.

 

Step 7: Immerse the parts in the Blacken-it solution. Maximize the surface area of the part exposed to the chemical by placing it on end if you can. Gently, without scratching the surface, turn the part every few minutes to ensure all surfaces are exposed equally to the solution. Carefully monitor colour changes, and remove the part when the desired colour is achieved.

 

Different concentrations of Blacken-it. 

 

The parts after 30 seconds. 

 

After five minutes.

 

After 60 minutes (other parts removed when desired colour achieved). 

 

Step 8: Instantly dunk the part in the warm baking soda bath. Agitate vigorously for ten seconds. You will notice that the part will begin to corrode and a blotchy green or red film will cover the surface. Do not worry.

 

 

After a bath in the baking soda solution, the part will appear green (or sometimes red). 

 

Neutralizing with water leaves a cleaner surface (but caution is warranted, see below). 

 

Step 8: Carefully buff the part with a clean soft cloth (an old t-shirt works perfectly). Do not touch the part with your fingers. You will notice that the corrosion products resulting from the neutralizing bath will scrub away.  Buff until all portions of the part have an even colour; continue to buff if you want a shinier surface.

 

Buffing the parts fixed in the baking soda solution removes the green/red coating. 

 

After buffing, all the parts appear roughly similar in colour and finish from a distance (see below for differences). 

 

Step 9: Wait 24 hours to ensure the reaction was effectively neutralized. If “sweating” or pitting is noticed, the reaction was not properly neutralized, and a further rinse may be required. Usually the part can be salvaged by buffing with a soft cloth. Sometimes, it may need to be blackened again.

 

Step 10 (Optional): Spray the parts with a thin acrylic matte coating to protect the surface.

 

I use Krylon Matte Coat.

 

The Trial Results:

 

Undiluted Blacken-It solution:

 

The undiluted solution produced a very dark, but somewhat uneven black surface in about five minutes. Fixing the reaction with baking soda caused a significant amount of corrosion, but it was mostly removed by buffing. 

 

Undiluted immediately after buffing.

 

However, after 24 hours both parts began to sweat, indicating that the chemical reaction had not been neutralized even with a baking soda bath.  This is not unexpected, as the product guidelines indicate that the product is meant to be diluted.

 

Undiluted after 24 hours. 

 

Recommendation: Do not use undiluted solution.

 

1:1 Blacken-it Solution

 

This is the concentration recommended by the manufacturer. After ca. 10 minutes the part reached a deep black, but after neutralization with baking soda solution the surface appeared to be quite blotchy. After 24 hours the edges of the part began to sweat and corrode and the surface appeared pitted.

 

1:1 immediately after buffing. 

 

1:1 after 24 hours. 

 

The water neutralized part had a slightly more even surface, but unfortunately began to sweat after only 24 hours.

 

1:1 unfixed (water neutralization) after 24 hours. 

 

Recommendation: Do not use 1:1 solution.

 

5:1 Blacken-it Solution

 

The 5:1 solution required approximately 25 minutes to reach a deep black.  Immersion in the baking soda solution initially produced a green corrosion but buffing resulted in an even black surface. The part remained stable after 24 hours (and is still stable a week later).

 

5:1 immediately after buffing.

 

 

5:1 after 24 hours. 

 

The unfixed, water-neutralized part began to corrode at the edges after 24 hours.

 

5:1 unfixed immediately after buffing. 

 

5:1 unfixed after 24 hours. 

 

Recommendation: Works very well in conjunction with a baking soda rinse.

 

10:1 Blacken-it Solution

The 10:1 solution required approximately 60 minutes to reach a dark even black. Immersion in a baking soda rinse produced a slight corrosion, but buffing resulted in a very even and deep black surface (in my opinion better than the 5:1 concentration). The part has remained stable after a week.

 

10:1 immediately after buffing. 

 

10:1 after 24 hours. 

 

Similar results were achieved with the water-only neutralization, and the part remained stable after 24 hours. However, after ca. four days corrosion began to appear at the edges of the part.

 

10:1 unfixed after four days. 

 

Recommendation: The 10:1 solution performed very well in conjunction with a baking soda rinse, and in my opinion produced the best colour and surface.

 

Final thoughts:

 

1)      Fixing the parts by agitation in a warm baking soda bath appears to be a critical step in blackening brass, at least with Blacken-it. Even at lowest concentrations, and with a water-neutralizing rinse, the acidic reaction appeared to continue for some time, especially around edges and in nooks and crannies.  

 

2)      5:1 and 10:1 solutions appear to produce relatively similar results, even though they both require proper neutralization. The 10:1 solution appears to produce a slightly more even and deeper colour. Using Blacken-it at its recommended concentration is a waste of product and results in corrosion even after proper neutralization.

 

3)      Buffing is a critical step in achieving the proper surface appearance.

 

4)      I was able to rejuvenate “sweating” parts by dunking them in a baking soda solution and then buffing. Regardless, faint hints of the corrosion remained.

Edited June 6, 2014 by E&T

[druxey]

Thank you for these exhaustive test results, E&T. They are very informative.

[dvm27]

Brilliant, this one's a keeper.

[popeye2sea]

I use blacken-it for steel and the recommended concentration is 1 part blacken-it to 9 parts water.  I prepare the steel by immersion in muriatic acid diluted in water 1:10, then rinse in de-ionized water.  I have had good results with it.

[jud]

Good work, now you have a repeatable method of blacking brass. I would expect that the muriatic acid bath followed by a good soak in a neutralizing agent would result in a surface to blacken without the need for an acetone bath. I have used that acid as a cleaner when re-soldering caterpillar radiators that had shaken loose after years of use.

jud

[mtaylor]

Thanks E&T.  I think there's now a definitive answer for when someone asks about it. Excellent research and well-presented.

[Chuck Seiler]

I had the same thought as jud, is the acetone bath really necessary after the muriatic acid bath?

[Keith_W]

Fantastic work, thank you!! The mods should consider making this a PDF and moving it into the articles section. 

[Mirabell61]

Many thanks E&T,

 

for sharing your intensive experimental tests on blackening, great work..

 

Nils

[themadchemist]

thanks for sharing, very useful info.

[wq3296]

Greetings E&T,

 

With all due respect to your exhaustive experimentation, you lost me. As your final step, you sprayed the parts with a flat finish. I think you could have achieved the same result by using flat black spray paint directly on the brass and avoided all the extra work and expense. One of the posts mentioned using steel instead of brass which, in my opinion, makes more sense for pintles and gudgeons and other parts. These items were originally made of wrought iron which would have oxidized through any coating applied to them. Accordingly, steel would mimic iron in that it would start out grey and age to a light rust color, possibly eliminating the need for blackening.

 

Don't misunderstand - your work was instructive, but I don't think the result of your method justifies the effort involved: particularly when you suggest coating it with a finish anyway. wq3296

[Erebus and Terror]

Thank you everyone for the great comments; they are really appreciated. 

 

Jud and Chuck, your point about the acetone is well taken.

 

All of this sequential preparation is probably overkill, but from what I've learned it is useful for achieving consistent results, especially when coated brass stock is used.

 

Apparently the acetone is often employed as a backup-step to remove any hand oils or dirt that might have crept in after the acid bath. However, it will also remove any lacquers or finishes that muriatic acid will not. This is especially important if you don't adequately sand all portions of the metal prior to the chemical treatments.   

[Erebus and Terror]

Hi Wq. 

 

Thanks for the comment. I certainly take your point.

 

It's a matter of preference, certainly. From my perspective the blackening results in a more realistic look in comparison to paint.

 

In short, it looks like black metal, rather than metal painted black.

 

A thin matte coating doesn't distract from this (at least in my eyes) and is barely noticeable after drying. I'd protect painted metal this way as well. 

 

To each his own, however. I've seen some superb work using painted metal, and I'm certainly not disparaging it. 

Edited June 6, 2014 by E&T

[jml1083]

E&T,

 

Great job, very methodical! I'm doing a Tech Session at the next monthly meeting of The Ship Model Society of NJ and your analysis will be very helpful.

 

Thanks for taking the time to document this process so well. I'm sure this topic will get many hits in the years to come as this is something many people struggle with.

[Ulises Victoria]

Hey... can you make a trial for wood glues?        (just kidding)

 

Very informative. I use Blacken-It extensively as I hate gold and shinny parts on my ships, except where it's meant to be.

[druxey]

Recently I've experimented with abrading the metal surface before blackening in order to avoid nasty chemicals. I've been using Scotch-Brite^TM radial bristle disks that are 400-grit. These disks, in a rotary tool, make short work of getting to a clean metal surface. The flexible bristles get into nooks and crannies nicely. I use the same technique prior to silver soldering.

[Erebus and Terror]

Hello Druxey, 

 

This is very good advice. In fact, very thorough mechanical preparation of the surface would likely remove the need to use muriatic acid. I admit my steps are in some respects redundant; each stage cleans up anything missed from the previous step.

 

Are these the discs you are using?

 

http://www.woodworkscraftsupplies.co.uk/index.php?cPath=94_101_147

 

I also wonder if these little discs might work for the smallest pieces?

 

http://www.thepolishingshop.co.uk/acatalog/Dremel_Wheels.html

 

Even after using these, I would probably still use an acetone bath to remove any oils or grease. 

Edited June 10, 2014 by E&T

[druxey]

Yes, E&T, the Scotch-Brite are the ones I'm using right now.

[cristikc]

Thanks for sharing, very useful info.

I will tray soon to see how it works to me

Cristi

[Erebus and Terror]

Thank you everyone for the comments....

 

Bart, your story about the gun blue process is very interesting. I have never heard of heating the metal, though "cold blue" is a term often used, so perhaps heating is part of a different process. It's an interesting idea, and perhaps something to add to the topic as a test. 

 

E&T

[jud]

Brownells is a good place to get ideas, options and the means to complete the method chosen. I have used the cold blue method, never the hot blue which used to require tanks and chemicals to get it done. Have two S & W K-22 Masterpiece revolvers, they were users, "actually acquired them to replace one that I let get away", much of the bluing was gone and after I had them rebuilt they needed protection. The gunsmith I was using sent them to someone in Idaho who put a kind of  parkerised finish, black in color that has proven to be tough. Today there are many different methods and options to choose from to color metal, Brownells is a good place to start finding out what some of those options are. Looking there may encourage the use of aluminum in modeling because of the different treatments that can be used for different effects. One of the earlier finishes was just plain rusting in a controlled manner, they called it browning,  was used on many muskets.

jud

Edited June 10, 2014 by jud

[mtaylor]

I've done the same method as E&T except I Scotch-Brited the brass first (if it wasn't milled or turned by me) and then instead of muriatic acid I used vinegar.  It saves some aggravation as the acid to me is a hazard.

[Erebus and Terror]

Jud and Bart, thanks again for your comments. The process sounds very interesting and I'll have to peruse the Brownells website. From my admittedly limited experience gun blue seems to be more durable than the brass blackening we apply - but I wonder if that is a function of the different metals and their quality. Very interesting comments!

[Erebus and Terror]

Hmmm. I think I will test the scotch-brite and vinegar. Muriatic acid is nasty stuff. How long do you leave it in the vinegar?

[shoule]

 Between the test, the results and the post posts, I have a better understanding.

 It takes a lot of bricks to make wall. Good one E&T.

[mtaylor]

E&T,

I've been using about 1/2 hour.  For a bit there, I was cleaning with acetone after buffing with the Scotch-Brite... but cleaning with acetone after the vinegar seems to work better for me.  I'm not sure why....

[dgbot]

Mark, I use vinegar as well. The time is about 1/2 hour. The vinegar leaves a slight residue from the reaction with the brass. Acetone cleans it up pretty good.

[HIPEXEC]

Thanks for the detailed tutorial.

[Keith_W]

Can anybody with a chemistry background please explain how metal blackening works? I don't understand how it works across different metals. I have an undergraduate level understanding of chemistry, so I don't mind looking at chemical equations. I know there is at least one person here with a chemistry background

[Richard Griffith]

E&T, thank you for the excellent trial and publishing your results.  I have used Blacken-It with decent results but clearly need to modify my methods to align with yours.

 

IN the meantime, I tried the etchants offer\ed by Blue Jacket.  Their chemicals are fast and easy to use.  I am a happy customer, not a salesman. 

 

Duff