Isowashing Emulsions - Patent vs. Practice
In the "Patents - No Words" thread, PE asked, "How would you equate the patent with what we really did? I think this is germaine to this thread."
He's asking in reference to what we did in his class on emulsion making vs. what's described in US Patent 2,614,929 - found here:
What we did - we made an emulsion, then added dilute sulfuric acid to it. The emulsion reacted by coagulating - enough that it made a nice little rubbery ball in the bottom of the beaker. We then added water to the beaker - I don't remember if we acidified the water or not. We swirled the ball of coagulum around in the water and dumped it out. I think that we then washed it again with another portion of water. We may have even wrung out the ball at the end of the wash. To redisperse the emulsion, we then added dilute sodium hydroxide solution and while being careful not to add to much at a time, we adjusted the pH back up to about 6.5 or so. We checked the pH of the solution with a pH meter to verify where we were. It's interesting to watch as the emulsion readily coagulated and stuck to itself with we added the acid, and then it was as if it was dissolving into the little bit of sodium hydroxide solution that we were adding and it really took very little to bring it back into a liquid form from the coagulated mass.
In the patent, in example 1, it looks like they added sufficient acid needed to take the emulsion through the isoelectric point of the gelatin, they went from say pH 7 to pH 3 with the isoelectric point being at pH 4. They let the coagulated emulsion settle out of suspension and they then decanted the supernatant liquid from the vessel. They then added water to resuspend the coagulum and a bit of ammonium hydroxide to raise the pH to 4, which then reprecipitated the emulsion. They allowed it to settle again, and then decanted the supernatant. Following that they added additional gelatin and water to bring it to a desired concentration, and chilled it for storage it looks like.
So the big difference I see from what we did as we manipulated the emulsion to form a ball for washing - and didn't waste/spend time with letting it settle.
I've got to do some non-photo things like fix dinner, but I'll be back to see where this goes. I bet there's something in the other examples I haven't noticed yet...
You missed the temperature cycle. It works without the temp cycle but it must settle slowly before decanting the water. The temperature cycle was used to accelerate the formation of the coagulum and the redispersal of the emulsion. We used proportionately more PA gelatin to insure that the putty like coagulum formed as well.
So, the patent works, but that exact method was not used in actual practice. It is one example of many where the patent does what it says, but is not 'practical' when done in production due to the need for speed. You use more PA gelatin and cycle the temperature to get it to go faster.
It also does not state that the PA gelatin has side effects on the emulsion, on coating and a whole host of other changes that take place in the coating formula. That is not needed in a patent. Just a demonstration that it works.
Well, this is not a subject to generate much interest it seems, but I have added some other information from a previous post. The incorporated picture, also from another post will illustrate.
When you ISO wash, you can get a series of forms of coagulum (this is what it is called in technical terms) that is created by acidifying an emulsion made in PA gelatin. You can get a loose flocculant material, a curd like material, or a putty like material. This depends on temperature and amount of PA gelatin used in proportion to regular gelatin. It also depends on the percentage of phthalate groups grafted onto the gelatin.
The first type of coagulum settles slowly, the second more rapidly and the third is formed almost instantaneously. Now, which do you think is desired in a manufacturing situation? Well, the quick one is. However, you run the risk of not being able to re-disperse the coagulum upon adding base. So, you have "the permanent coagulum". (also the name of a band formed by a group of emulsion makers at Kodak who played instruments and who suffered this fate at one time or another in the lab)
The slow loose type is most difficult to separate from water as well as being the slowest to separate from the water and salts. Most of the time it is useful to stay in the middle of the range. I use the higher level of phthalated gelatin in my coatings for purposes of teaching as it makes for a rapid wash and is almost foolproof for the beginner.
In any event, when the latter, putty like, type of coagulum forms, it forces out about 90% of the salt and water. At this time, more water can be added and it can be washed again. You can also knead it like bread dough to force out more water and salt. The attached picture shows the coagulum, adhering to a stirring rod and most of the the water and salt remains in the beaker.
Now as far as using PA gelatin and then coating, hardness does suffer slightly for two reasons. First is that regular make up gelatin is added for coating, and secondly, PA gelatin is NEVER phthalated to such a degree as to completely eliminate its capability to harden. Therefore, under normal circumstances, only a percentage of hardening sites would be occupied by phthalate groups.
The evidence is the fact that many of you, as my students, have either done this or watched me do it in the darkroom, and we have coated, exposed and processed the resultant films and plates. Picture examples were posted here.
One problem I might mention is that due to the time constraints in the workshop, the film coatings only dried and cured for about 12 hours or so, but when done at home I have had more time to cure (dry and harden) the coatings.
I hope this clarifies some additional facts about PA washing that are entirely unreported in the patent literature and that are poorly understood and often misreported elsewhere.
Lack of comment, at least from me, does not indicate a lack of interest. I'm just not equipped to add much except the occasional question.
Originally Posted by Photo Engineer
"I only wanted Uncle Vern standing by his new car (a Hudson) on a clear day. I got him and the car. I also got a bit of Aunt Mary's laundry, and Beau Jack, the dog, peeing on a fence, and a row of potted tuberous begonias on the porch and 78 trees and a million pebbles in the driveway and more. It's a generous medium, photography." -- Lee Friedlander
I would second Ed's comment. I have some PA gelatin and will be using it in June after the journey to the east (Rochester). Ron, your added comments are quite helpful.
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I read all of this stuff, but there's not often anything useful I can add.
Lens caps and cable releases can become invisible at will. :D
plenty of interest
Thank you for a very clear explanation and example to look at. I have no doubt that "others" lurking from all parts of the globe are taking notes.
Will portions of your DVD / book cover the this in any greater detail?
That photo above is a still from the DVD section on ISO washing.
One thing I've forgotten, or perhaps missed in the class note/formula, is that you want a mix of the phthalated and unphthalated gelatin. Definitely make sure there is some discussion about this in your book.