Coating 'practice' with inert emulsion

rmazzullo · Jul 5, 2024

Hello all,

I am curious what mix of gelatin, other components, etc to use - without adding the silver nitrate - that would (as close as possible) mimic the properties of a finished emulsion ready for coating. The idea is to avoid wasting finished emulsion needlessly when learning how to coat film, paper, etc. I have completed some experiments with gelatin mixed with a bit of food coloring to test setup times for different coating thicknesses, but just gelatin and water doesn't seem to be a reasonable facsimile. I suspect the real deal will coat much differently....especially given the environment where you do the coating.

Additionally, once coated, dried, etc, can the 'test' emulsion be washed off of Duralar, etc, for re-use?

Your thoughts are appreciated

Thanks,

Bob

koraks · Jul 5, 2024

Just use a mixture of gelatin and water at the same concentration as the actual silver emulsion. The rheology will be close enough for practice.

rmazzullo · Jul 5, 2024

Thank you. Do you mean gel concentration?

koraks · Jul 5, 2024

Yes, gel. Sorry, posted from my phone...

The main factors in how a gelatin solution behaves are the type of gelatin (gel strength), temperature and concentration. Note that the temperature can change rapidly as the warm liquid hits a cool surface, spending on the conductance of the surface and it's thermal mass. But for e.g. a glass plate, you usually have a minute or two with the gel remaining liquid - unless you add a lot of hardener just before coating, which will make it stiffen up rapidly.

In what way do you suspect that just gel and water won't be representative of the real deal?

rmazzullo · Jul 5, 2024

I was using gelatin from very old container (+/-15 years)...I suspect that this may have skewed my observations - the tests weren't gelling well (no real cohesion). Especially since now learning that using the same gel concentration (and fresh gelatin) as real silver emulsion can get me closer to what I am aiming for.

Thank you again....

eli griggs · Jul 6, 2024

You could always practice with Jello, the regular, sugared stuff and, with a flat, level board to lay these out in a fridge or freezer, once the jello is applied, you should get a readable results quite quickly and beging to get a feel for a proper application and thickness.

Good luck.

koraks · Jul 6, 2024

rmazzullo said:
the tests weren't gelling well

What concentration did you use?
In principle, unused gelatin remains "like new" pretty much forever. Unless it's affected by bacterial activity, but you would have noticed that as your jar if gelatin would have been a stinky, soggy mess.

rmazzullo · Jul 6, 2024

It worked out to 8.3% concentration in this case, and the 'aroma' coming from the container was amazing; however, the gelatin itself was not soggy. It was basically a test to see what could be learned / observed. I am wondering if the other ingredients will affect coating. The original intention was to explore what would be needed to make a 'dummy' emulsion that could be used to get the coating / setting / drying technique nailed down without wasting a live emulsion batch. My next steps will be with new material.

koraks · Jul 7, 2024

8.3% should set quite well, unless you're using an exceptionally low bloom gelatin (<120 or so). How long did you wait for the gel to set?

rmazzullo said:
I am wondering if the other ingredients will affect coating.

The other stuff won't do much in terms of rheology, except the hardener. With the hardener, it depends how much you use and which hardener is added. Some hardeners will have a more or less instantaneous effect and will make the gel set very firmly within a minute or two (e.g. chrome alum, especially if it's added in excess). The hardener mostly affects how much time you have to work with the gel. Initially there will be no difference between a gel with and without a hardener in terms of how it sets.

rmazzullo · Jul 7, 2024

The test took almost 3 minutes to set. Something wasn't right. I used a 1/2" diameter steel dowel pin with the ends wrapped in layers of tape to create a gap of about .010". The box was labeled 230 bloom (could have been 250 but was partially obscured), but again, was very old and stored improperly and its provenance is long forgotten. Would there be a difference if a surfactant is added to the next experiment? Maybe comparing batches with no additions, hardener only, surfactant only, surfactant and hardener? From your notes, there may be a balancing act using a hardener correctly. New ingredients will reveal more.

I may be overthinking this a bit....

Thank you everyone!

koraks · Jul 7, 2024

rmazzullo said:
The test took almost 3 minutes to set.

That sounds quite normal. As said, it depends on a couple of factors, but 3 minutes is right on the money.

eli griggs · Jul 7, 2024

Has anyone tried rabbit skin glue?

grain elevator · Jul 7, 2024

eli griggs said:
Has anyone tried rabbit skin glue?

Would that not be more expensive than gelatin?
I recall reading, probably on this here forum, someone using starch thickened water for practice. But maybe that wasn't for gelatin but to emulate some other medium.

eli griggs · Jul 7, 2024

Rabbit skin is, in effect, gelatine, powdered animal skin and though it would most likely need a hot application, I think that will simply result in a well practiced hand in applying gelatine, quickly, and in good layers.

A wine maker's hydrometer should help get to the specific ratio for best results and you can find reference to emulsion's "Specific Gravity", is that the correct phrase, online and books on the topic?

Are you planning to use a glass spreading rod and does anyone here have thoughts on make, technical use, vendors, etc?

koraks · Jul 8, 2024

Another very suitable material to practice coating gelatin with is...gelatin. Since it's an affordable material (although prices have come up over the past few years), there's no real need to practice with something else like rabbit skin glue, bone glue or isinglass. I'd in particular be hesitant to try these alternatives (apart from the smell) due to the likely different rheology/flow characteristics than gelatin.

eli griggs said:
glass spreading rod

Haven't tried it for gelatin, mostly because gelatin coating isn't much of a "puddle pushing" application like the coating operations a glass rod is normally used for. It could still work.

Most people AFAIK use either a Mayer rod / wire wound rod, or pour and spread with something like a comb or fingers. A glass rod is conceivable if it's used in conjunction with spacers to set the desired thickness.

If a rod (of any kind) is used, it needs to be heated to above the gel melting temperature and cleaned and dried between uses. I never bothered for the gelatin applications I do, and simply pour & comb.

Some kind of rod is useful if you want to more accurately control layer thickness and you don't want to go to the lengths of building some kind of coating head.

If you Google a bit, you'll find a source in the US for the threaded Mayer rods.

Peter Schrager · Jul 8, 2024

Rod information
RD SPECIALTIES
#lab-35t
3/8 inch #35 formed
I put the rod in a tray of hot water and wipe off in-between with paper towel.
Coating at 105 degrees F.

koraks · Jul 8, 2024

@Peter Schrager thanks, that's the one!!

Peter Schrager said:
I put the rod in a tray of hot water and wipe off in-between with paper towel.

There's a YouTube by a carbon printer - might be John Lockhart - who shows how he keeps his rod inside an upright section of PVC tube, sealed at the bottom and filled with hot water. I found that a neat trick.

Kino · Jul 8, 2024

The Light Farm website by Denise Ross has a lot of information on how to coat emulsions.

The Light Farm

rmazzullo · Aug 16, 2024

I just realized embalming fluid has been used as an emulsion hardener (Formalin). Yes, I know formalin has been discussed here before, but somehow, it seems a wee bit more ghoulish now. Learning of the proclivities of formalin / formaldehyde to cause irreversible (fatal?) physiological damage to the careless user, I am looking more closely at chrome alum or glyoxal, of which much has been written here. Interestingly, looking at a new embalming supply catalog for appropriate PPE, (https://thechampioncompany.com/content/ChampionProductCatalog.pdf), they are moving away from aldehyde-based compounds to more plant-based products. I will research this more thoroughly to see if those items have any application here...before I freak out a little.

- Bob

koraks · Aug 16, 2024

You can use chrome alum, glyoxal, glutaraldehyde etc. for hardening gelatin. IDK to what extent any of these may cause problems with a silver halide emulsion. I think glutaraldehyde or glyoxal is used in modern, commercial photographic silver gelatin emulsions.

rmazzullo · Aug 16, 2024

Thanks! I will look more closely at the alternatives. The idea of 'embalming fluid' gives me the willies.

koraks · Aug 16, 2024

Formalin is not a benign substance. Having said that, the exposure you'd subject yourself to when adding a tiny bit to a small amount of silver gelatin emulsion would be orders of magnitude less than what people have been subject to in biology or pathology labs in the past. It's good to be careful and if you want to steer clear of formalin, that's definitely a commendable choice. However, I wouldn't freak out at using one or two drops once in a while when brewing up a batch of emulsion.

grain elevator · Aug 16, 2024

I find it weirder that people are ok with putting tons of formaldehyde into the soil and ground water inside dead bodies. Small amounts in the lab seem less absurd, although I also don't dare use the lith developers that contain it, especially considering they are used warm. Small amounts of formaldehyde are everywhere, it outgasses from wood (especially from glues in bonded wood products, but some even from timber itself). I don't really understand whether the magnitude is similar in careful darkroom use, but it would the relatively easy to research.

Peter Schrager · Aug 16, 2024

I use a 4% glyoxol up to 20 ml per 650 ml emulsion

Coating 'practice' with inert emulsion

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