Well, Denise, this thread was an adjunct to the dispersion making thread that I posted the same day, at least in part. The other thread demonstrates the use of homogenization or "blending".
No method should ever be used with emulsion crystal making that breaks up or changes the grain size by force, as this will fog the emulsion. This is destructive and uses a lot of energy which is expended on the emulsion in the form of shear and heat both. The method used must be mixing action that does not shear the emulsion and that does not allow gelatin or silver halide to accumulate on the bottom of the container. It must not whip air into the emulsion.
Basically then, to summarize, this thread was to point out that:
1. Up to about 500 ml, stirring at about 300 - 700 rpm will work well for most emulsion making.
2. Over 500 ml, you need a mixer running at that same speed or higher, but not high enough to cause air to be whipped into the emulsion.
3. Over 500 ml, you may need a mixer and a stirrer both to prevent gelatin from "burning" or just collecting at the bottom of your container, especially if you use a hot plate.
4. Over about 3L and with most modern emulsions at any scale, you need some sort of mixer running at up to 5000 rpm.
5. To make dispersions, you need a homogenizer.
6. Modern emulsions need a special sort of mixer that can act like a homogenizer but without cracking or shearing grains.
Originally Posted by Photo Engineer
(Apologies if you've answered this somewhere else. It is hard to keep track unless you read every word posted -- and who has time for that, especially with all the extraneous stuff that creeps in :blink:)
I'm working on that post. It will be here in this thread Denise.
A special sort of mixer!
The ideal mixer will not heat the emulsion, cause shear forces nor will it beat air into the make. It will be a perfect mixer for any and all emulsions.
One way to do this is in a totally enclosed chamber with no air gaps and into which you add your starting mix and the making ingredients. A high speed impeller or prop type mixer would virtually fill this chamber and the conditions would be met.
But wait! How do we add ingredients? The answer is to have an on-line UF unit to allow for constant volume throughout the make. However, this still limits you to a given fixed size, that of the chamber. This is usually a cylinder, with closed top and bottom and a full sized mixing impeller inside the chamber.
This does work, but is rather limited in scope.
A broader scope is allowed by taking the cylindrical side off the chamber, drilling holes in the top and bottom of the "lids" on the above, and then submerging it in the large kettle. Then you can make any size emulsion you wish.
This device is called a shrouded turbine. It sucks liquid in from the top and bottom (both kettle ingredients and newly added ingredients) and the reactor is within the top and bottom lid and just before expulsion due to the rotational force of the turbine or impeller.
Coming out the edges of the shroud (the top and bottom of the original cylinder), the reactants are spread evenly by the high mixing energy and form rising and falling walls of mixture at the edges of the kettle. Baffles break up any eddy currents and provide full mixing.
Since there are no shear forces, there is no excessive heat generated, crystals are not broken, but good mixing is achieved. This type of unit is a unique mixer, particularly adapted to emulsion making. Further refinement can adjust this device to be fully scalable and to also have constant throughput regardless of make size or input rate.
Unfortunately, I have found that emulsions designed to take advantage of this type of mixer are difficult to make with other types of mixer and are difficult to scale with other types of mixer.
So, there you have a special sort of mixer explained. Earlier, we were in the midst of a huge thunderstorm with heavy rain and hail. I deferred posting this until we had an "all clear".
Hello to All,
I am curious about the minimum amount of stirring required for what is usually the last addition to any emulsion. the hardener. What would be the minimum mixing RPM and time for,say 250 ml of emulsion. Can it be "over mixed" I usually give it 5 minutes of rapid mixing on a magnetic stirrer.
Overmixing just before coating is represented by the generation of bubbles in the coating or the whipping of air into the emulsion. Correct mixing does not generate bubbles by cavitation or by just whipping the emulsion but does mix all ingredients thoroughly. I generally figure that the hardener is mixed in by a good hand stirring with a spatula or glass rod at 40C for about one minute. That should do it.
Well, about 2 weeks or so ago, I took delivery of a shrouded turbine which I have posted a picture of elsewhere. If you want another post, just scream a bit.
Today, my high speed mixer arrived. It is capable of up to 6000 RPMs. I've already cranked it up with the shrouded turbine in a 1 L beaker and really mixed things up right and proper. At 300 RPM, it mixes a drop of dye into 1 L in less time than I can measure, giving a uniform solution. It causes little if any vortex, and a small metal baffle removes the tiny vortex that doe form giving a smooth, nearly ripple free surface to the liquid.
At present, the turbine I am using must be rebalanced properly since this was the first use, and I am also getting one 1/2 the size as this one is designed for 1 L and larger and I need one for 1 L and smaller as well. So, I had a fun day in the lab. What about all you guys?
Pictures of the setup to follow as I can get time.
That sounds like a real MIXER - I've come to the conclusion that a standard magnetic "stir bar" is good for making things go in circles, but not for the quick and intimate mixing of emulsion.
I trust most of the test mixture stayed in the beaker:)
The test did indeed stay in the beaker.
And, the OP and my following posts describe that stirring will work for emulsions smaller than about 500 ml or even maybe 1L and that stirring is only reasonable at up to about 300 RPMs in gelatin.
Originally Posted by Photo Engineer