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andrewkirkby
07-09-2010, 01:31 AM
Hi there,

I am working on making a warm tone chlorobromide & cadmium emulsion for coating to paper.

I am wondering if i can use a magnetic stirrer hotplate to make the emulsion? i could easily use a 3000ML beaker and a long egg shape stirrer as i already have tese.

Should i be looking at overhead stirring and using the hotplate for providing the heat to ripen the emulsion or will the mag stirrer work?

I already have the mag stirrer hotplate but i am concerned that the emulsion will not mix properly due to the random nature of the stirring.

Andrew

dwross
07-09-2010, 08:48 AM
Hi Andrew,

Whether or not to use a mag plate depends on the size of the emulsion batch and/or just how good a vortex you can keep going as you ramp up the size. Different models seem to have different capacities. I feel that a 400 ml beaker 2/3 full is pretty much the top end of mine. Any bigger than that and I'd need an additional overhead stirring tool. My darkroom is too small to coat much more than about 250 ml of emulsion at a time anyway. Paper emulsion goes a long way. And, once you get a little practice, it goes fast. I coated twelve 9"x11" sheets last night in one hour from dry chemicals to coated paper hanging to dry. I just had to remember to bloom my gelatin ahead of time. Since practice is an important part of the game, I strongly suggest you make frequent, small batches rather than honkin' big ones. (Saves on silver costs, too, when the inevitable Big Fail happens :))

If you really want to make huge batches, you'll need to test your stirrer's capacity. Make a plain gelatin solution that is about the thickness of very heavy cream when it's at 45-50C. At any amount you go with, you'll need to see a good, strong vortex that is visibly moving the liquid at the beaker interface. In other words, you don't want any dead zones. This is critical, but not complicated. Any tool or mixing contraption that keeps the salted gelatin moving as you add the silver will do the trick.

Good luck and fun.
d

Kirk Keyes
07-09-2010, 10:16 AM
I prefer tall, skinny beakers to maximize the coverage of the stir bar vs the bottom of the beaker. Look for "berzelius" beakers, also known as "tall form" beakers.

wildbillbugman
07-09-2010, 05:27 PM
I have abandoned magnetic stirrers entirely. I hate it when the stir bar starts running wildly all over the container while I am in the middle of a precipitation ! I have two overhead mixers. one is for viscous mixes. High torque,low speed. The other will go to 1500rpm, but will not handle viscous mixes. So I usually switch over, just before precipitation. My mixing blades are Jiffy Stirrers. These have 2 horizontal blades connected by 2 vertical blades. These come in various radii. Mine are 2 and 3 inches.
My containers are 1-2 inches wider than the mixing blade.
At the risk of being ridiculed, I will confess a practice that I have adopted to minimize air while mixing. Early in the process, just after I have added the halide salts, I add a drop of Triton X-200. This releases much of the air introduced by mixing, reducing volume and turbulence.
Bill

traveller
07-11-2010, 02:25 PM
I don't know anything about emulsion making but I'm selling laboratory equipment.

Magnetic stirrers a great with low viscosity liquids but have limits with higher viscosity liquids.

In my opininion, from the specs I have read here, a overhead stirrer with a paddle stirrer would be the better tool for the job. You don't have to care about viscosity, get good mixing results at low speeds and 3000 ml can be managed without problems.

John

Photo Engineer
07-11-2010, 03:43 PM
I use a magnetic stirrer at volumes up to 400 - 500 ml max and at up to 300 RPMs in 10% gelatin. I use glass beakers, not SS due to the heat transfer problems with SS beakers.

Above these values, an overhead prop mixer is usable, but has one big drawback. If used alone, it does not mix properly at the bottom and the emulsion and gelatin can "burn" on the bottom. So, I use a combination of prop and magnetic bars above 500 ml to prevent "burning" on the bottom.

PE

andrewkirkby
07-11-2010, 09:49 PM
I am going to look for an overhead stirrer. I think this is something that needs to be done properly, for reasons mentioned above. I don't want to waste chemicals due to poor equipment choice.

Thanks everyone for your responses. I will keep you updated on my emulsion making endeavours.

hrst
07-12-2010, 07:54 AM
But you must consider the fact that emulsions of 400 - 500 ml and bigger are quite large and you probably don't need to do them unless you are going to "mass produce" some film&paper. It's better to do small batches, then you waste less chemicals when you search for the good procedures. 200 ml of emulsion coats a huge number of sheets, and this amount is not a problem for a magnetic stirrer. Magnetic stirrer at least ensures proper mixing at the bottom. We have really had no mixing problems with a magnetic stirrer. Our batch size is 150...200 ml and we use glass beaker.

During the critical precipitation the gelatin level is not so high, so the viscosity doesn't seem to be a problem either.