Coating metal plates with emulsion

Hi everyone,

I am looking for an effective method to coat silver gelatin emulsion on metal plates (iron or aluminium).

I have done some tests with a brush or by moving a thick layer of liquid emulsion as if I was handling a collodion plate. Both of these methods were not satisfactory. In both cases the layer is not even and in the second case there are bubble (besides the fact that too much emulsion is used).

I have read that there are multiple coatings devices. Here some examples:

https://fotospeed.com/11-28cm-coating-rod.html#tab-label-description-title

http://www.edeldruckshop-franalog.eu/PUDDLE-PUSHER-25-CM/en

https://www.gardco.com/Products/Pai...re-wound-Mayer-Film-Applicator-Rods/c/p-56916


Would you recommend investing in that, or do you suggest some other method?

Thanks a lot.

Emilio

Hi @Emilio_, welcome to Photrio! Sounds like you're working on an interesting project; I hope we some day will get to see the results! Out of curiosity - are you doing some form of dry plates for gelatin-basec ambrotypes?

Out of the three items you listed, only the first (the Mayer rod) makes sense for your purpose. The other coating rods are best used for coating watery solutions onto paper; they won't work for coating an actual gelatin emulsion onto a non-absorbing surface.

As an alternative, I would suggest placing the plates to be coated onto a temperature-controlled hot plate set to something like 38C, pouring a suitable amount of emulsion onto it and spreading it with something like a comb, and then allowing the whole thing to cool. You could do this by for instance by carefully lifting the coated plate from the hot plate and placing it onto a cool surface (stone/marble, heavy glass, metal) so that the gelatin sets, and then allow to dry. This approach would be somewhat similar to making pigmented gelatin tissue for carbon printing, although we generally don't use metal plates for this.

One area of attention is adhesion of the gelatin layer to the metal plate. I suspect you may have to sub the plate with a very thin layer of hardened albumen or gelatin prior to coating the actual gelatin emulsion onto it. Otherwise the emulsion will likely lift off the plate at some point.

That’s very helpful, thanks a lot @koraks !

As for your question, I am just printing on metal plates with silver gelatin emulsion overlaying images printed as negatives on transparent plastic sheets (as you would do with cyanotypes - though with an enlarger).

Regarding the methods you suggested, I have a few questions, if I may.

Regarding the coating rod:

• which size would be best in your opinion in terms of thickness (mils/¬µm)?
• do you think that the rod will leave scratches on the metal plate?

Regarding your pouring method:

• what is the reason why you have you suggested to heat up the metal plate to a temperature of 38C - is it to eliminate potential bubbles?
• what device can I use to control the temperature of a metal plate?
• do you have a recipe for hardened albumen and normal gelatin and/or can you refer to a website where I can buy that?

Sorry for all these questions, and again thanks a lot for your support!

Best,

Emilio

Emilio_ said:

Regarding the coating rod:

• which size would be best in your opinion in terms of thickness (mils/¬µm)?
• do you think that the rod will leave scratches on the metal plate?

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I don't know, really. You might search carbon printing guides/forums/threads and see what is used there to obtain a certain layer thickness and if there's any mention of paper being burred by these rods. I never heard about the latter, so I suspect it's not necessarily an issue.

Emilio_ said:

what is the reason why you have you suggested to heat up the metal plate to a temperature of 38C - is it to eliminate potential bubbles?

Click to expand...

To get a layer of an even thickness. If you pour a warm gelatin emulsion onto a cool metal plate, the gelatin will set before it has the chance to spread out evenly.

Emilio_ said:

what device can I use to control the temperature of a metal plate?

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Creativity will come into play. Things that come to mind are soldering hot plates (provided they have a low temperature setting) or even the kind of heating pads used for seedlings and exotic pets/reptiles.

Emilio_ said:

do you have a recipe for hardened albumen and normal gelatin and/or can you refer to a website where I can buy that?

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Albumen you can easily make from an egg white. Whisk it in a bowl (with a mixer) until it's stiff, then let the bowl sit for a few hours. Pour the liquid from beneath the foam and save it; this is your liquid albumen. Dilute it e.g. 1+25 or 1+50 with water and dip the thoroughly cleaned plates into this solution. Set to dry. Then fill a misting bottle with ethanol and mist each plate. This will harden the albumen and the plates are ready for use as soon as they're dry (which doesn't take long with 96% ethanol).

With gelatin, one possible way is to make a 0.25% or 0.5% gelatin solution and add a little chrome alum to it. Then dip the plates in this and set to dry. The gelatin should be hardened and the plates ready to use after a day or so. Alternatively, just dip the plates in a 0.25-0.5% gelatin solution without any additives and set to dry. After drying, dip them in a cool tray of water with a little formalin added to it. Dry again, and allow to harden for a few days at least (preferably a few weeks).

There are dozens (probably hundreds) of variations to this theme and it's usually a matter of experimentation to find out what works best for your purpose.

That's great, thanks a lot @koraks !

Welcome, @Emilio_! Let us know how it works out and feel free to ask any follow up questions!

sorry to get back again @koraks : when you say 1+25 (or 50) of your albumen solution, I guess you mean 1 unit of pure processed egg white (without foam) + 25 (or 50) of water, is that correct ? I guess I will go with this method to pre-coat the plates, looks easier and more rapid.

And yes, defintely, I will let you know how it goes

Emilio_ said:

1 unit of pure processed egg white (without foam) + 25 (or 50) of water, is that correct ?

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Yes, that is correct! The ratio isn't very critical.

Albumen is indeed generally quicker and easier than gelatin. Gelatin does give better adhesion, however. In commercial applications (i.e. film and paper), gelatin is usually used.

Sorry @koraks (I swear I will stop after this), are these products the gelatin you are talking about (but already made)? I see one type is "coating", the other is "precipitation" and the other is "ripening"... not sure what the difference is....

No need to stop
I'm not familiar with the Adox gelatin offering, other than that I'm aware that they exist and that they should be very suitable for the production of photographic emulsions. But which gelatin you'd use for which application, exactly, I don't know. I'm more of a carbon printer myself and the gelatin requirements are different for that process.

One thing I do know about photographic gelatins is that the 'photographic' part as to do with the impurities that are (not) present in the gelatin, in particular sulfur. This will affect the properties of a silver halide emulsion coated with such a gelatin. In commercial film and paper, trace elements such as sulfur, gold, and certain rare earth metals are deliberately added to the emulsion to for purposes such as sensitization (especially sulfur and gold) and reciprocity characteristics (e.g. osmium if memory serves, but don't quote me on that).

Many of the amateur emulsions you find recipes for online, including on Photrio, often assume the use of a reasonable pure regular gelatin, i.e. not a photo gelatin. The reason for this is that these normal gelatins do contain trace amounts of particularly sulfur, and this works to sensitize the emulsion coated with the gelatin. That makes it a bit of a non-exact science, because too much sulfur will cause fogging, and in a regular gelatin, the exact sulfur content is usually unknown or at least not tightly controlled. However, the short of it is if you're coating a typical home-made emulsion, it's important to note what kind of gelatin is prescribed for the emulsion, in particular if a photographic gelatin is required or not.

Reading this page: https://www.adox.de/Photo/chemistry/toners-helping-aids/colloida-gelatin/ it seems that the Adox Colloida gelatins differ in the degree of purifications; while they're all labeled 'photo grade' (or something to this effect), there are apparently subtle differences. See e.g. this quote about the type R/Ripening gelatin:

It is technically inert but has a very mild chemical activity to increase the film´s speed.

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As with all things photographic, and especially anything in the 'alternative' corner, experimentation is usually the only way to determine with (almost) definitive certainty whether a certain material works in a certain application.

Of course, it doesn't hurt to reach out to the manufacturer and ask their advice. In the case of Adox this is something that would certainly make sense. They're generally quite responsive.

You might want to try the C version which is meant for subbing glass metal etc. .
I haven't tried it myself as it is only available on their website. Good luck

For liquid photo emulsions on metal, polyurethane spray is a good sub coat - scuff the surface lightly with a fine sanding sponge; or very fine sandpaper if you don't want a matte look. It adheres very well to clean metal. Try two coats, and give the poly coat a very light scuffing (like fine steel wool) to give it a little more "tooth". Use a tack cloth before coating to get any dust or steel-wool bits off.