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rdg
01-28-2014, 12:04 AM
I have never played with coating emulsions, or even started to make my own emulsion for that matter, but know that I am likely going to be doing it later. In doing a great deal of reading I have more questions than answers.

I know that this is substantially above creating and coating my first emulsion but is it possible with hand coating to do multiple layers? And if it is possible would it be preferable to do them sequentially as part of the same coating session or would it be better to wait and do it after the previous layer(s) have hardened/stiffened?
As I see it, and with my experience I more than likely can be very wrong, having the ability to have multiple layers with hand coating will allow for such wonderful things as an anti-halation layer or even a top protection layer for the emulsion. I also realize that this would be imperative if anyone wants to ever coat colour emulsions by hand.

Thanks.

Richard

Photo Engineer
01-28-2014, 12:45 PM
Richard, I have coated many layers on Baryta paper, RC paper, and film. It is almost impossible to do a good job on Baryta due to curl. Also, on any support, defect rate goes up with the # of layers, but it is not excessive. We usually got on the order of 18 good sheets out of 24 coatings.

Each layer must dry before the next one is applied or you scratch or scrape the wet previous layer. In fact, you have to use the right coating blade for each layer to account for the "height" increase due to the previous layer(s). So, you might coat at 5 mil, then 7 mil then 9 mil and etc to clear the previous layer and yet apply the right amount of emulsion.

PE

rdg
01-30-2014, 11:12 PM
Thank you. I was thinking about the same thing, although when to add the new coating was not something I was certain about.

I am not yet ready to try it, there is a lot of things I have to learn and do first, but at least I know that the potential exists.

At what point does the emulsion start to become too thick? Considering the number of layers that existed for colour films, although I am not saying that the layers would be the same thickness as with hand coating, there is probably a practical limit, beyond where additional layers are prone to failure just from the coating techniques.

Again thanks.

Richard

Photo Engineer
01-30-2014, 11:57 PM
Richard, the first layer is thicker than zero and thus has an effect.

PE

dwross
01-31-2014, 09:26 AM
The thickness with handcoating adds up really fast. The thinner you try to coat, the more difficult it is to get a smooth, uninterrupted coating, especially after the first one. It's far easier to get nice multiple coats on glass plates with a passive pour technique. But, you still have the problem of thickness. It's not a very thick total coating that becomes too thick to fix evenly, or at all. This is especially true if you add hardeners to any of the layers.

Beyond the mechanical problem of hand coating very thin, there is capability of the emulsion chemistry itself to deliver the goods when it is coated very thin. Gawd knows I love the old emulsions, but they are not the most efficient dmax delivery systems. (To be clear, that is the reason they are so beautiful and different from what you can go out and buy.)

If you want multiple coats, you'll be happier with a more 'modern' emulsion -- unless the goal is something totally artsy. Near-modern emulsions can be made in a home darkroom, but it takes a lot of practice and commitment to technique. Standing on my favorite soapbox, I can only recommend that anyone interested start simple.

Photo Engineer
01-31-2014, 06:03 PM
I have coated multilayer materials using our lab equivalent of AJ-12. We coated 6 layers. It is a matter of practice and attention to gelatin and other chemical "load".

PE

cliveh
01-31-2014, 06:09 PM
I have coated multilayer materials using our lab equivalent of AJ-12. We coated 6 layers. It is a matter of practice and attention to gelatin and other chemical "load".

PE

PE, you obviously advocate this, so can you say why multiple layers are beneficial?

wildbillbugman
01-31-2014, 07:24 PM
How about putting a nonreactive clear coat between each layer? To prevent damaging each coating with the next coating. I think that glass plates will be more conducive.to multiple coats than clear plastic. Although I have never succeeded making in-camera negatives with multiple coats, I have been making multiple final coats on glass for more years than I like to admit. Think multiple decades.
Bill

Photo Engineer
01-31-2014, 08:10 PM
Clive, I can give several reasons!

One is to put on a protective overcoat for your film or paper. You can also add matting agents to change surface characteristics. Examples are starch in gelatin. Great matting agent.

Another is to put on two different speed emulsions to extend the latitude. Or, if paper, to create a VC paper with emulsions of 2 different contrasts but the same speed. I did some work on this with my Azo type emulsion AAMOF and described the potential here on another thread.

And, of course there is the application to color!

PE