Subject: Smooth black non-reflective surface for aluminium?

From: [email-masked] (Asmo Saarikoski)
Date: 20 Jan 1995 17:21:26 +0200

I'm refurbishing a couple of old enlargers and will have new metal masks
made for their negative carriers.  The material will be aluminium.  Most
masks I have seen have been painted black.  Is there any reason why 
anodizing should be avoided?  It would be an inexpensive and durable 
solution.

One thing that comes to my mind is that an anodized surface might not be
quite as smooth as a painted one.  A built-in negative scratcher would
definitely be The Gadget From Hell.  Is the difference significant and 
if it is, does careful polishing of the mask before anodizing help?

Asmo Saarikoski
[email-masked]

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From: [email-masked] (J. Kimberlin)
Date: 23 Jan 1995 18:06:47 -0800

In article <[email-masked]> [email-masked] ([email-masked]) writes:
>Asmo Saarikoski ([email-masked]) wrote:
>
>: I'm refurbishing a couple of old enlargers and will have new metal masks
>: made for their negative carriers.  The material will be aluminium.  Most
>: masks I have seen have been painted black.  Is there any reason why 
>: anodizing should be avoided?  It would be an inexpensive and durable 
>: solution.
>
>: One thing that comes to my mind is that an anodized surface might not be
>: quite as smooth as a painted one.  A built-in negative scratcher would
>: definitely be The Gadget From Hell.   

Back in 1976-77 whenever, I wrote an article for 'Live Steam' about 
chemical conversion coatings for aluminum.  Whatever surface you have on 
the aluminum is what you end up with, but black.  No dimensional 
change and maybe using chemicals you can get easily.

Aluminum oxidizes rather quickly to a self protecting surface.  So for a 
matte finish I recommend bead blasting and then putting the workpiece 
into the following solution as fast as possible, but in any case no 
longer than a two hour wait in air.  Bead blasting and then putting the 
workpiece in water could extend the time a bit depending on the oxygen 
content of the water.

take 750 ml distilled water, add 5 ml concentrated nitric acid.  Then add 
25 grams of copper nitrate and 10 grams of potassium permanganate.  Once 
those are disolved, add water to make 1 litre.  put the aluminum 
workpiece into the solution (which can be multiplied accordingly to fill 
the container you have) and it will become black.  Time and temperature 
play a part, as usual, so the colder the solution the longer the 
immersion time.  It could take a half hour at 75 degrees.

for the chemicals, you may have access to a local college chem dept.  You 
don't need much really.  Also, it would be best not to pour the dregs 
down the sewer but take to your local hazardous disposal site.  Or do it 
at your local college and let them dispose of the solution since you 
won't really have much, presumably.  The usual precautions about skin 
contact, drinking it, etc., pertain.

JerryK

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From: [email-masked] ([email-masked])
Date: 23 Jan 1995 18:15:28 GMT

: One thing that comes to my mind is that an anodized surface might not be
: quite as smooth as a painted one.  A built-in negative scratcher would
: definitely be The Gadget From Hell.   
	--Talk to your local anodizer about this one. It is possible to 
specify the amount of etch, i.e. heavy or light , before the anodizing is 
done. Also,  it is common to take the raw aluminum that is used to 
assemble machines and remove the draw and tooling marks by power-sanding 
the surface with a "jitterbug" or random-orbit sander, prior to sending 
it out for anodizing. Finish depends, of course, on the grit used. 
	Possibly, after this operation, you might try using one of the 
Scotchbrite pads that are now available with a backing to attatch them to 
sanders. Ya gotta experiment, maybe use some old negs and slide them over 
the surfaces. Inspect with a small magnifier.

--
"Steamboat Ed" Haas		:	Co-founder of the 
Voice: 805-968-7534		:	    SBSMEEBBQ
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From: [email-masked] (Bruce Holder)
Date: Mon, 23 Jan 1995 18:02:52 GMT

Asmo Saarikoski ([email-masked]) wrote:
: I'm refurbishing a couple of old enlargers and will have new metal masks
: made for their negative carriers.  The material will be aluminium.  Most
: masks I have seen have been painted black.  Is there any reason why 
: anodizing should be avoided?  It would be an inexpensive and durable 
: solution.
: One thing that comes to my mind is that an anodized surface might not be
: quite as smooth as a painted one.  A built-in negative scratcher would
: definitely be The Gadget From Hell.  Is the difference significant and 
: if it is, does careful polishing of the mask before anodizing help?

  You guessed exactly right.  Anodizing creates a crystalline layer of
aluminum oxide, the same stuff they put on sandpaper.  Polishing the
substrate doesn't help because the crystals build up on the surface.
  It wouldn't surprise me though to find out that the painted ones are
anodized first to give the paint a good surface to adhere to.

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From: [email-masked] (Peter Monta)
Date: 25 Jan 1995 01:15:21 GMT

[email-masked] (Bruce Holder) writes:

> > ... One thing that comes to my mind is that an anodized surface might not
> > be quite as smooth as a painted one.  A built-in negative scratcher would

But polished, anodized aluminum doesn't feel like sandpaper; it feels
very smooth.  The alumina layer, while polycrystalline, is going to
have a surface comparable to the base metal, it seems to me.

I think that matte anodized articles are pre-etched in NaOH to
roughen.  Perhaps those would be more abrasive.

Peter Monta   [email-masked]
Qualcomm, Inc./Globalstar