Stange accidents on aluminium

[Emilio_]

Hello,

I have been practicing over the last few months with applying emulsion on aluminum plates (after coating them with a polyurethane spray) using brushes. The results were unsatisfactory due to visible brush strokes. I then started pouring the emulsion onto the plates (still pre-coating them with a polyurethane spray) and spreading the emulsion with a comb.

In my first attempt at pouring, I used an increadibly excessive amount of emulsion. Because I had some light issues during the drying of the emulsion, I decided to overxpose the plates to get them completely black. I then fixed and washed them. When I returned to retrieve the dry plates, I found some inexplicable defects (see the photos attached).

Could someone explain the physics/chemistry behind this?

Thanks,

Emilio

P.S. Today, I use the right amount of emulsion (a professional printer specializing in emulsion gave me the correct ratio). I tried aluminium and stainless steel (inox). I have done trials pre-coating the plates in two ways (with a polyurethane spray or by spreading highly diluted chrome alum with a soft tissue). I have some black spots on aluminium, though they are much less significant. However, nothing appears on stainless steel (which is what I finally decided to use for my project)...

 

[Ian Grant]

Aluminium oxidises very easily, some alloy worse than others,

Not a good choice.

Ian

 

[Emilio_]

Aluminium oxidises very easily, some alloy worse than others,

Not a good choice.

Ian

So it has oxidized. What I found interesting is that I did not have this problem when I poured very small quantities of emulsion with a brush. It only happened when I started pouring emulsion using bigger quantities. The bigger the quantity, the more significant the oxidation. There must be some reaction between the emulsion and the aluminium...

 

[koraks]

There must be some reaction between the emulsion and the aluminium...

And/or the processing chemistry. As @Ian Grant said, aluminium oxidizes very readily. It does so in alkalis (e.g. developer), but I imagine silver salts will also corrode it.

Apparently your PUR coating doesn't effectively seal the surface of the plates.

I then started pouring the emulsion onto the plates (still pre-coating them with a polyurethane spray) and spreading the emulsion with a comb.

It's also possible you damaged the PUR coating locally with the comb. If you use a brand new comb, you may find that the teeth are actually quite sharp due to imperfections from the injection molding process. This easily scratches soft surfaces (your PUR coating may not have been hardened out or adhered properly).

by spreading highly diluted chrome alum with a soft tissue

Chrome alum as such doesn't seal a surface. It's a soluble compound and it'll simply dissolve into the silver halide emulsion as soon as you pour that on top of it. Do you mean a gelatin emulsion with some chrome alum, perhaps? I can see how that would effectively seal it.

 

[Emilio_]

And/or the processing chemistry. As @Ian Grant said, aluminium oxidizes very readily. It does so in alkalis (e.g. developer), but I imagine silver salts will also corrode it.

Apparently your PUR coating doesn't effectively seal the surface of the plates.

However when I applied small quantities of emulsion with a brush I did not have any black spots. So maybe bigger quantities of emulsion amplify the oxydiation of the developer...All this seems strange to me becauase I have seen artists using aluminium with silver emulsion...

It's also possible you damaged the PUR coating locally with the comb. If you use a brand new comb, you may find that the teeth are actually quite sharp due to imperfections from the injection molding process. This easily scratches soft surfaces (your PUR coating may not have been hardened out or adhered properly).

To be honest I doubt because I have oxydation traces and spots (though much much less and more brownish than black) even on the back side...

And/or the processing chemistry. As @Ian Grant said, aluminium oxidizes very readily. It does so in alkalis (e.g. developer), but I imagine silver salts will also corrode it.

Apparently your PUR coating doesn't effectively seal the surface of the plates.


It's also possible you damaged the PUR coating locally with the comb. If you use a brand new comb, you may find that the teeth are actually quite sharp due to imperfections from the injection molding process. This easily scratches soft surfaces (your PUR coating may not have been hardened out or adhered properly).



Chrome alum as such doesn't seal a surface. It's a soluble compound and it'll simply dissolve into the silver halide emulsion as soon as you pour that on top of it. Do you mean a gelatin emulsion with some chrome alum, perhaps? I can see how that would effectively seal it.

I do not use pure gelatin to sub the plate. I first, I rub the plate with a cloth soaked in diluted chrome alum. I let it dry. Then, I pour the emulsion at 40 C (13g for a 16x24cm plate). The plate is on a heating pad at 35 degrees. I spread everything with a smooth wooden comb with wide teeth. Then, I move everything to a cold plate. When it solidifies a bit I put the plate on a closed armor to finish drying (24h at least). This way I have never had problems with the emulsion peeling off. I have also used a polyurathane spray to sub the plate before coating with emulsion, whic is good, but it takes longer, it is more expensive and it alters the colour of the plate.

 

[koraks]

I have oxydation traces and spots (though much much less and more brownish than black) even on the back side...

Then it's likely your processing chemistry, in particular the developer has corroded the aluminum. You'd have to seal the entire plate somehow to prevent this from happening.

I first, I rub the plate with a cloth soaked in diluted chrome alum.

Ah, I see. That would allow a thin film of the emulsion to harden as it hits the chrome alum, I think. Sounds like a nice technique!