Nadeau does talk about carbro in History and Practice of Carbon Processes, as does Friedman in History of Color Photography (probably my favorite book...). A whole chapter in each book is devoted to it.
Here's what Nadeau says:
The Chemical reactions which take place between the sensitized pigment and the bromide print are rather complicated but may be summarized as follows:
1. The ferricyanide oxidizes the silver in the bromide print and so loses oxygen and is reduced to ferrocyanide.
2. The chromic acid oxidizes the ferrocyanide and in doing so forms chromium salts which are capable of tanning the gelatine.
3. The chromium salts, which must be soluble, diffuse back into the gelatine pigment layer and tan the gelatine. Pure chromic acid would only tan the gelatine emulsion layer of the bromide print but would not diffuse back into the pigment.
I'm sure Friedman has a lot more on it as well.
In other news, Kees, is the price from China competitive? Bob, I understand you might be coming to Rochester in January; let's talk about carbro sometime and see what we can see.
This description of the carbro chemical reactions is not completely accurate as its fails to account for the function of dichromate - (and whether DAS #3 could be used in place of dichromate was questioned).
The Basic Carbro Chemical actions:
1. The ferricyanide and bromide react on the silver image, resulting in the formation of silver bromide and potassium ferrocyanide.
2. The water soluble potassium ferrocyanide passes into the carbon tissue and reduces the dichromate in the carbon tissue - it is this reduction of dichromate in the presence of gelatin that results in the insolubilization of the gelatin.
See Charles Lighton's discussion in the August, 1927 Photographic Journal for a more complete description.
Thanks Charles, that is interesting. With that same logic, is it possible for us to say that blank chemical will reduce DAS and tan the gelatin?
By the way, this DAS acronym is a life saver...
Yes, thats the point: Can DAS be chemically "activated" (I don't know if "reduced" is the correct word) to tan gelatin? We know that this happens when it is exposed to actinic light.
BTW: i have recently learned that DAS (thanks to Kees for the acronym) works well in place of dichromate in the albumen adhesion formulation.
Originally Posted by CMB
I recall that question being posed a while back; a welcome answer for anyone interested in DAS... ;)
Yes it is, about $0,37 per gram plus shipping, minimum order 1 kg. This is about double the price of the Secant offer you had, for the group buy, but I was interested in finding a seller that actually produces DAS and can garanty (some) continuety at a reasonable price. The DAS I got came from a 20kg batch produced end august. I have tested it and is is working as it should.
Originally Posted by holmburgers
This is excellent news. I did not have the time to try it myself yet. At what concentration it was used?
Originally Posted by CMB
By the way: the real credit for the acronym is for my chinese contact! ;)
The amount of DAS used in the albumen coating was the same as is used for dichromate: (10g/600mlH20+egg). Its possible that a lower concentration would work as well. FYI, here's the old UltraStable process instructions for the Albumen Substratum:
"1. Take white of one egg, beat to froth and add 450 ML cold tap water. Dissolve 10 grams pot. bichromate (now DAS) in 150 ML warm water and add to egg and water. Mix well. Powdered egg albumen may be used instead of white of egg. Use 15 grams to 450 ML cold water. This solution stored in dark bottle can be used repeatedly. Refrigerate if possible.
2. PET Sheet is immersed in albumen solution approximately 1 minute, then hung to drain. It is then immersed in tray of cold water, shaken for a couple of seconds to clear off excess albumen solution, then hung to dry. Drying may be accelerated with heat. Prior to use it is exposed to strong UV source (exposure unit) for 30-60 seconds to make the thin, invisible layer of albumen insoluble. It is now ready for use and is virtually frill-proof."
Dumb question from the newbie:
Do you have to soak or wet albumen coated film before you mate it with tissue?
just a short 10 second soak is enough
Originally Posted by VesaL
Time to get the word out! The group buy is going forward and orders need to be in by January 27th.
Cheers everybody; hope everyone's well.