Quote Originally Posted by Ryuji
Are you sure about this?

I've tested a LOT of surfactants and biocides before settling on the ones I use, but the difference in the biocides is minimal, as long as they are added to the finished emulsions. Both phenol and thymol are very old generation biocides and there are better ones that are safer to humans and environments, and inexpensively available because they are used in many industrial applications. I've tried all sorts of stuff but the choice of biocide is rather irrelevant to photographic property (except for certain quarternary ammonium salts), and the choice can be made by the biocide functionality alone.

Surfactant is also a very important factor in making beautiful prints, but I see little need to change the surfactants added to the sizing, subbing, emulsion or overcoat layer to accomodate the differences among paper stocks. The surfactant choice is more strongly influenced by the coating speed, coating method, emulsion viscosity and the coating temperature.

I don't know how fast you coat, but unless you are coating at the industrial speed, there are many surfactants that work very well.

There is a larger issue that is not mentioned in your post. The binder system in modern emulsions incorporate nongelatin polymers in a small proportion (5 to 40%). The formulation of the binder system has a large impact on the quality of coated layers, such as swelling property, wet strength, brittleness, etc. If you are having problems in obtaining good coating, you should be looking at these issues together with the coating speed and temperature issues, because they are mutually dependent factors.

Good luck
Ryuji;

I have an AgBr and an AgCl emulsion that take two different surfactant levels on one type of paper and two different surfactant levels on another type of paper for a total of 4 surfactant levels at what amounts to the same coating formulation of silver, gelatin, hardener and excess salt (Br or Cl being the difference as well as a slight difference in density and ionic strength due to the different halides).

In addition, I see differences in response to the same paper from different batches. One example is COT320 and another is plain baryta support. All papers vary in some way or another. One batch of Strathmore I got repelled heavily and on inspection had small red dots in the center of each repellancy spot. These dots were embedded in the paper itself. Only about 1/2 of the sheets in the package had the spots and they were only visible by inspection with a loupe.

Regarding the biocide or the surfactant in the baryta, these are the differences that I know about. I'm not saying that these are what cause the problem, but if the baryta is sized with an anionic surfactant then this would alter the coating properties compared to a paper with no surfactant. That is just an OTOMH example. The phenol level is so high that the odor of it in my darkroom is quite heavy while coating on baryta support. Just open a new pack of Ilford MGIV paper and take a sniff. Phenol!

I do know that the baryta is very strongly repellant to anything unless I use much higher levels of surfactant than needed on good COT320. I have demonstrated this time and again in my own lab.

Also, there appears to be a mild reducing agent in some of the baryta supports. It comes and goes, but introduces black streaks in the emulsion. It is also a batch to batch variation that I have observed and test for.

In any event, I don't know the formulation of the baryta, but I do know that what I have behaves differently than any baryta I worked with at EK. I don't know the exact reason(s), but I have derived methods to work with these differences.

But then, all of this is just a small snippet of what I will cover in the workshop, with examples and specific formulas that were duplicated 2 - 4x in the lab to be sure that what I see is correct and what I pass on to the student will work in their labs.

PE