Potassium Dichromate vs. Sodium Dichromate: Direct Positive Photobooth Process
Okay, I also posted this in a thread about 3 years old, so thought it better to start a new one. I'm new here, so first off, thanks for a great forum!
Here's the deal:
Out of necessity, I'm trying to formulate my own bleach for my vintage photobooth. The bleach I usually buy premixed is often out of stock, or hard to get. The formula on the safety spec sheet is Sodium Dichromate 15-25% and Sulfuric Acid 10-15%. The paper is a direct positive roll paper. The exposed strip travels quickly through a series of tanks/baths. So, the strip is only in each tank / checmical for about 10 seconds, or so. (Since it's an automated process.)
So, my question is, what is the difference, if any, between sodium & potassium dichromate? Will they yield the same results? Would I need a different quantitiy with potassium? Also, I can't seem to find a place to purchase Sodium Dichromate... However, Photographer's Formulary does sell Potassium Dichromate.
FYI: Process for the complete process is as follows:
1 Tank/Dip Water
2 Tanks/Dips in developer @ 110-115 degrees. (Potassium Sulfite 25-35% & Hydroquinone 1-5%)
2 Tanks/Dips in water.
1 Tank/Dip in bleach: (Sodium Dichromate 15-25% and Sulfuric Acid 10-15%)
2 Tanks/Dips water.
2 Tanks Clearning Powder: Sodium Sulfite (100%)
1 Tank/Dip Water
1 Tank/Dip Toner: (Sodium Hydroxide 10-15% & Thiourea 3-5%)
2 Tanks/Dips Water.
Entire process is about 2.5-3 minutes, and generally yields nice strips...
Any help would be much appreciated! Thanks!
There isn't much difference. The potassium salt is probably more soluble and may be a bit more active. The sodium salt has a somewhat lower molecular weight, but for practical purposes you can probably use the same amounts.
They are inter-changeable. As the bleach is to finality the very slight difference in molecular weight is irrelevant.
Why are you reversal processing a Direct Positive paper, it's designed to produce a Positive with normal developing & fixing ?
Maybe it's not a standard paper and/or process? I was just trying to re-create the checmicals I usually use in this process, which does work. Think about an old photobooth... that's what I have. The chemical baths that work for that paper are as I noted above. The paper is exposed by a camera in the booth, with a prism in front of the lens. So, there's no negative, just the paper. So, developer, bleach, clearing powder, then toner. I don't claim to be a chemist, but do know what works... unfortunately, getting the materials is often difficult. Luckliy, I think I may have found a new source, so that's a plus! If you have anymore insight, shoot away. -Ant
I took a look at your website, great idea.
The process is very standard "Reversal processing" but at a higher than usual temperature, which isn't a problem. It would be very easy to give you substitute formulae for all the process. Only the developer is critical and would need some experimenting to get optimal processing.
The crunch would be tweaking the process for a different paper.
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Thanks again. The temperature is higher becuase the paper can only be in the developer for a short period of time. There are 2 tanks of developer... As for tweaking to accomodate different papers, I think there may only be one manufaturer. So, not much tweaking? I've searched everywhere for other suppliers of the photobooth paper, but no luck. And, the distributor/s here in the US and Canada are very tight-lipped about who makes the film. When I get it, it has already been re-packaged. However, I think it might be made by Slavich, but I have yet to confirm this?
Here's a few pictures of the inside of a typical vintage photobooth. There are courtesy of photoboof.com. Nevertheless, you can see the tanks, the spider which cycles the paper through each chemical bath, + the camera and roll-paper magazine.
The original Kodak Super Speed Direct Positive paper used in Photobooths required reversal processing, as described.
Originally Posted by Ian Grant
So strictly speaking "Kodak Super Speed Direct Positive paper" isn't like direct positive film or paper as they only require normal processing.
Originally Posted by nworth
Use of potassium dichromate may slow down fixation. It is known that some silver salts dissolve less readily in the presence of potassium. I have not seen it in this process, so I think you are safe.
Interesting indeed.... so, the quuestion now is, where can I find a similar roll paper? I currently purchase it from 2 different companies. 1 in the US and the other in Canada. I'm sure they both distributers purchase the paper from the same compnay in Russia, or somewhere in Eastern Europe...? As I may have noted in previous comments, both distributors are tight-lipped about where they get the paper. And, by the time I get it, it's repackaged with their markings. Initially, I thought it was made by Slavich, but FreeStyle photo checked it our for me... they couldn't find it. I'd like to identify another source for this paper, since I buy a lot of it. And if I could buy it cheaper, great too! A few people have suggested I make some contacts in China to have them "reverse engineer" and manufactuer it? ANy thoughts? Anyone know where I can track this paper down outside of where I get it now? I think I'm going to start a new thread on this... since it's a different topic all together! Thanks!