Carey lea silver
The subject of Carey lea silver comes up from time to time. It seems to be a part of the history, lore, and technology of emulsions, so at Kirk's suggestion that we start a thread devoted to the subject, I lead off with this very interesting article.
Just to put things in perspective, Carey Lea Silver (CLS) is used as the yellow filter layer in all color photographic products except negative color papers. I have formulas for this and for a black AH layer that use Lea's methods. I hope to reduce them to usable form soon for posting if there is interest.
Thanks for starting this, Denise. That was a good page you linked to.
Here's a couple I found.
A nice, vintage report on colloidal silver and it's preparation by Carey Lea himself:
And another one from the same era with more details:
Except for use in image transfer, colloidal silver has little other use except in color photography as AH and yellow filter layers. It is not used in B&W photography outside of Polaroid type B&W transfers.
The chemistry is a bit exotic and the equipment required and steps required are complex. For example, to isolate this very fine silver "mud" as it was called, a centrifuge was used to wash the colloid. Filtration was impossible due to the fact that it clogged the filters. So, there are a whole host of things that are not disclosed in the early patents and writeups.
Kirks earlier references did not give specific formulas, and the references above do not work.
I'm looking forward to reading those links. I can't imagine why they don't work. I'm about to try one myself. Fingers crossed...
Also, if any of us move beyond the point of pure historical interest and find a good use for colloidal silver, there seems to be commercial sources. I don't know if they are identical to a homebrew CLS, but it might be a good (although expensive) starting place.
I used to make colloidal gold for research purposes and it was definitely a tricky business. I don't know about silver, but there were a lot of poor methods out there for gold. The biggest issue with poor preparations was a wide dispersion of particle sizes--which usually caused problems with the downstream application. A good colloid is the size you want it to be with a fairly narrow spread in particle size. A centrifuge is necessary for concentrating and washing the colloid and a scanning spectrophotometer is used to assess the particle size and spread.
I've always wanted to own a centrifuge.:) After seeing Kirk's collection of totally cool used lab equipment, it's almost tempting, and doesn't seem that unreasonable.
Well, to connect this all to the Polaroid thread we started from, here is a simplified view of how Polaroid B&W works.
One sheet is a negative film the other is a sheet of paper with baryta or titanox overcoated with a layer of neutral colored colloidal silver that is so finely divided and so low in coating weight that the dmin is not turned gray or discolored.
You take a picture and spread a goo between these two sheets. The goo must be bubble free and is composed of a fast acting monobath. The goo must be spread evenly by means of rails and rollers to get an even coating.
As the negative image develops, the fixing agent dissolves the positive image which is silver halide and it diffuses to the reciever sheet with the colloidal silver. There, the colloidal silver acts like a uniform latent image waiting to happen and the dissolved silver halide begins to form a positive silver image. BINGO! Positive print!
Now, the image that forms is colloidal, has a silver halide solvent present, and is therefore subjet to bronzing and fading so you have to coat it with something to "tone" it. This is the little applicator that Polaroid supplies.
Now, where does the speed come from? Assuming a 50 ISO negative, if it were coated on paper, it would be ISO 100 due to back reflection and due to the chemistry involved, you mainly get the toe silver from the negative which is about ISO 100, you can get a print of about ISO 200 from an ISO 50 negative emulsion.
That is a short description of the process.
Colored colloidal silver can give colored (toned) images to start with. Non-uniform colloids can give uneven or blotchy images. So, the silver must be neutral and very low in coating weight per unit area and should be the finest of all possible colloids.
Denise - I think we had that model centrifuge in high school, 25+ years ago. But there are a lot more at less expense you could get.
I used to use an old hand cranked version in HS.