Engine blackening or how to turn your engine black without paint. - RC Groups
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Mar 08, 2014, 11:18 AM
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kenh3497's Avatar

Engine blackening or how to turn your engine black without paint.

I started this in my thread on bead blasting https://www.rcgroups.com/forums/show....php?t=2113984 and thought the blackening process warranted a thread of it's own.

A short recap.... I glass bead blasted the parts I wanted to blacken per the Beechwood instructions. I've never done that before and it was worth the trouble. I came up with a nicer finish than I had on a couple of previous attempts MANY years ago. With that said, it's not perfect and I think some of that is the chemicals I'm using are quite OLD, like 25 years +. Still I'm satisfied with the outcome. This WILL NOT be a show engine. It will be a work horse and the blacking plus the bead blasting should make it cool a bit better.

OK, so I said the bead blasting should help the engine cool better. I know your are asking, "How the heck can that make a difference". Well "theoretically" The bead blasting should make for more surface area. Also the rougher surface should make for a bit more turbulence to help the air grab the heat off the engine better. "Black" is supposed to give up heat the best of any color. This process unlike paint does not add to the surface so should not have ANY insulating properties like paint. OK, my flame suit is on so have at it

After the bead blasting was done I soaked the parts in Lacquer thinner to remove any residual oil. This part is VERY important to the process. I did the blasting at work so after degreasing I put the parts in a plastic bag to take home.

Once at home I set up a PLASTIC tub for the blackening solution. You will see a photo in the next post to show why. The instructions state to swab on the solution. So I opted to dunk them If swabbing is good, dunking is better... right In any case that was the plan. The parts were then degreased for a second time and great care given not to touch them with bare skin. I warmed the parts in the oven set as low as it would go. I didn't want them cold but I don't know that it made any difference. Into the soup they go and it boils and bubbles and turns kind of murky. I didn't take any photos as I was concerned it would take to much time and the process goes pretty fast. I left the part in the solution for about 15 or 20 seconds, agitating slightly by shaking the container, removed it to see if it was "done yet" and then dunked it again for another 15 seconds or so. The second dunking seemed to make the solution work even better. Don't quite know why. I removed the part and continued on untill all four parts were done. The instructions recommend a cold water rinse and for good measure a rinse in a baking soda solution to make sure the blackening is neutralized and another water rinse. After the rinsing is done the blackened parts are fairly fragile as it takes a while for the black to "set". They look very flat and not very attractive at this stage, just black. I doused the parts in a WD40 lookalike to displace the water and popped them in the oven at 200 degrees to cook and set.

Photos of the finished product to follow shortly after they are done baking. I hope they don't rise to much and spill over the side of the pan

Last edited by kenh3497; Mar 08, 2014 at 11:44 AM.
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Mar 08, 2014, 01:01 PM
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kenh3497's Avatar
The parts are fresh out of the oven. You can see the stainless steel forceps was also turned black by the solution. I rubbed it with my finger and the black is pretty tough. I think it will take a scotch bright part to shine them up again. This is the reason I recommend a plastic container. I guess you could also use glass but plastic is a bit safer.

I have some scratches in the nose of the crank case. This is where I was trying to turn the crankcase to get it completely submerged in the blackening solution. I didn't have any more of the solution so this was my only option. I may try to touch it up or may just leave it alone

Over all I'm pretty happy with the process. It is not hard, just a bit fussy with having things clean and oil free.

I was just reading and the instructions state that the bead blasting is, at least I take it as so, a necessary thing to remove the micro layer of corrosion that ALL aluminum has. This layer can have a negative effect on the blackening process. Bead blasting is not the only way to remove this layer. any mechanical or chemical means will work IMO.
Mar 08, 2014, 01:36 PM
Registered User
I can understand that shot peening, or in this case sand blasting or bead blasting will improved the surface hardness of the material by work hardening the surface. That is commonly done on some machine parts to achieve surface hardness. I do not understand why black is cooler than say while colors. Black is absence of color and thus absorbs heat, ergo gets hotter by retaing heat rather than radiating heat. But this is not my area of expertise at all so I just do not know.

Otherwise the engine looks neat. Good luck with it.
Mar 08, 2014, 02:22 PM
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surfer_kris's Avatar
Originally Posted by Chip01
I do not understand why black is cooler than say while colors. Black is absence of color and thus absorbs heat, ergo gets hotter by retaing heat rather than radiating heat. But this is not my area of expertise at all so I just do not know.
It is a symmetric process, i.e. a black surface is good at absorbing light but this also means that it is equally good at emitting it. So a warm black surface will emit more heat/thermal radiation than a white surface would...

Look at beduin's clothing for example, a black robe makes sense as long as you stay in the shadows...
Mar 08, 2014, 02:39 PM
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kenh3497's Avatar
I couldn't help myself. I had to do a mock assembly of the engine to get an idea of what it looked like. I'm waiting for new crankshaft bearings to arrive so not 100% able to do a proper job

I degreased the back plate to install the vent with Loctite. You can see it is much flatter that the oiled surface. One could spray a little clear on to seal the black but IMO that would defeat the heat rejecting properties I'm looking for. I don't think an oil free surface is achievable in our model engines so it won't be an issue

I still need to paint the muffler. I intend to use some VHT brand ceramic header paint. That has to be baked on to achieve it's potential. It, the paint, "should" help hold some of the heat inside the muffler further reducing the in cowl temps.....maybe

Mar 08, 2014, 05:51 PM
SrTelemaster's Avatar
The prop hub should conceal the scratches on the nose.
Mar 08, 2014, 06:44 PM
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kenh3497's Avatar
After looking over the blacking job I could see a spot here and there where there was no color. All the spots were tiny and in a corner. I think the bubbling and boiling trapped some "air" in those corners and prevented the solution from reacting in those spots. None are obtrusive in any manner and you have to be paying attention to see them.

I'm still happy, but then I'm prejustice

Mar 08, 2014, 09:47 PM
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mosquitowings's Avatar
Ok Ken that's just cool! Gives me a lot of ideas. But you haven't told us what chemicals you use to do this?
Mar 09, 2014, 08:34 AM
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kenh3497's Avatar
Originally Posted by mosquitowings
Ok Ken that's just cool! Gives me a lot of ideas. But you haven't told us what chemicals you use to do this?
It's a secret

Birchwood product called Alumablack A15.

https://www.birchwoodtechnologies.co...antiquing.html scroll down a bit

I would really like to try this product as it looks like the results would be more predictable and tougher. Of course it comes with a price


This might work as well. Very well might be the same as the A15

Last edited by kenh3497; Mar 09, 2014 at 09:43 AM. Reason: wrong product
Mar 09, 2014, 09:00 AM
SrTelemaster's Avatar
Your 1st link is for copper based alloys.
Mar 09, 2014, 09:36 AM
supreme being of leisure
ZAGNUT's Avatar
i saved this DIY method years ago but have never tried it. i assume it is a similar process to the stuff you can buy....probably cheaper if you have access to the ingredients.

From: kimberln@crl.com (J. Kimberlin)
Newsgroups: rec.crafts.metalworking
Subject: Blackening Aluminum
Date: 12 Aug 1995 20:11:13 -0700

There have been several postings about blackening aluminum recently. I
wrote an article for *Live Steam* magazine back about 1977 on the subject
and have modified it a little since then. I do use it myself and thought
that it would be of interest to the newsgroup. So:


There are several ways to color aluminum black and among them
are black anodizing and paint. You could rub dirt into the aluminum
surface, I suppose, but of all the methods, I think chemical coloring
is the superior method. It is certainly cheaper, faster, and home use
allows the model engineer greater flexibility in the timing of his
decoration of models in progress.

Surface preparation of parts to be colored black is all
important as any irregularities are not covered by this finish. Paint
does build up and fill in scratches and other voids. Castings,
however, should look like castings if the prototype used castings, so
surface finish is always adjustable to the builders idea. The point
here is to emphasize that this blackening technique will not cover up

You will need three chemicals. These are: Nitric Acid, Copper
Nitrate, and Potassium Permanganate. You will also need some good
quality water - either distilled or deionized. I will give the
dimensions of the mixture in both metric and English units so that
both types of measures are accommodated:

Take: water 3 quarts 750ml
Add Acid 1/2 oz 5ml
Add Copper 3 oz 25gm
Add Permanganate 1 oz 10gm
Add Water to make 1 gal 1 liter

Obviously you will have to make up more or less solution to
fill the container you will use to color aluminum parts and the parts
to be colored should be completely covered by the solution. You
should use a glass or plastic container. A metal container will
poison the solution prematurely.

At 75 degrees F (24 C) temperature, the blackening process will
take about 15 minutes using a fresh solution. If it takes longer it
means the solution is deficient in one of the components. Usually,
copper nitrate and nitric acid need be added.

Aluminum is a strange metal to most of us. While we cannot see
it, the surface of a newly machined or cleaned piece of aluminum
combines with oxygen in the air to form a self protecting coating of
aluminum oxide. This happens within minutes. If this surface
continues to grow (get thicker) the blackening solution described here
will not work satisfactorily. Thus, the piece to be colored should be
cleaned just before immersing into the coloring solution. In my
experience, glass bead blasting is a superior way to clean the
aluminum surface and the choice of bead size determines surface
finish. Once the bead blasting has been accomplished, the beads can
be washed off with hot water and the aluminum piece immersed in the
blackening solution. I recommend that the time between blasting
(cleaning) and immersion in the blackening solution be less than two
hours. I once waited five hours and was disappointed in the results.
Once the blackening process has been completed, wash off the workpiece
with tap water, drain and spray with WD-40 or other water displacing

There are a number of ways to clean aluminum satisfactorily.
It is possible to simply sand the surface clean, or scrub it clean
with an abrasive. One can also chem clean aluminum by degreasing the
workpiece then dipping it into lye (Draino, for instance) for a few
minutes or seconds as required, then rinsing. The shape of the
workpiece and the model engineer's facilities often dictate what
method of surface preparation will be used.

Model engineers wishing to use this solution to blacken
aluminum castings or other parts should be aware that the chemical
components may be hazardous. While the solution itself is not
particularly dangerous it can make your hands purple, so use rubber or
plastic gloves. Potassium Permanganate is classified as an oxidizer
even though dilute solutions of it are used throughout the world to
sterilize vegetables used in salads, etc. Concen trated nitric acid
is just plain bad. The technique for using it is to pour out a little
in a glass container and then use an eye dropper to transfer the
liquid to a measuring container when the volume wanted is small, such
as that described here. Nitric acid also turns your hands yellow,
hurts, and removes fingerprints. A good way to avoid eye damage is to
wear a face shield such as the one you should be wearing when working
in front of your grinder.
Mar 09, 2014, 09:44 AM
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kenh3497's Avatar
Originally Posted by SrTelemaster
Your 1st link is for copper based alloys.
I edited the post. You need to scroll down a bit to the A15 product.
Mar 10, 2014, 05:47 AM
Registered User
Martin5465's Avatar
In the model railroading world there is a blackening solution that you apply with a brush. Intended for small parts so I don't know how it would work on an aluminum block. Also not sure if it even works on aluminum. Been a long time since I used it. .

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