Ammonium Dichromate replacement for Carbon printing

[Struan Gray]

I too would like to make it clear that I in no way think that other people should not be using dichromates in their private photographic work, or that I think it is unsafe to do so.

One thing that has changed in the time since I started working in labs is the definition of 'adequate ventilation'. A fume hood or well-designed darkroom ventilation would make a lot of sense, rather than trusting to luck. Similarly, I would wear safety goggles to guard against accidental splashes or dust entering my eyes.

Sandy: do you use a reducer to convert the chromium VI before disposal? I have seen suggestions to use sodium thiosulphate - do you have any other recommendations?

[mdm]

DAS is a satisfactory way of making carbon prints, if you are uncomfortable with dichromates use an alternative and get on with it, dont look back. Look at Todd Ganglers wonderful colour work, there is nothing to fear, all it takes is some new work habits. Hard if you have an established workflow but if you are just starting its a good way to go.

[Erik Larsen]

[QUOTE

Sandy: do you use a reducer to convert the chromium VI before disposal? I have seen suggestions to use sodium thiosulphate - do you have any other recommendations?[/QUOTE]

Sodium sulfite works as well.


One nice thing about ammonium or pottasium dichromate (if there can be a nice thing) is that the stuff I have purchased in pound quantities is granular, not powdery fluff so there is no dust created when weighing out and mixing. That being said please use common sense, wear gloves etc when handling. The amounts we use is minuscule per print and it's hazards are sometimes blown out of proportion.

[sanking]

"I would like to make it clear that I was somewhat apprehensive about starting this thread, should it in any way cast the tremendous work and effort by Sandy and others in a bad light."

No problem on my part with the discussion. And the issue is one of interest no only to carbon printers, but also to all of the other photographic processes that are based wholly or in part on dichromate, including gum bichromate and most of the alternative printing processes.

At this time the only alternative I see to dichromate is DAS and there are pros and cons with the work flow of each. If the question is what will be available in the future, who knows? Right now, in my place and time, DAS is both more expensive and less readily available than dichromate.

Sandy

[sanking]

"Sandy: do you use a reducer to convert the chromium VI before disposal? I have seen suggestions to use sodium thiosulphate - do you have any other recommendations?"

In my work flow most of the unreduced hexavalent chromium is removed from the sensitized and exposed tissue in the mating bath, where the tissue is combined with the final support. This bath is reused, not discarded. The sandwich is squeegeed over a paper support that picks up excess solution. The paper support is discarded as solid waste. The only thing that goes down the drain, to a septic tank, is the developing water, which would contain a very small amount of chromium VI. My understanding is that in the pH of the septic tank the Cr(VI) will eventually be reduced to Cr(III) by organic matter.


Sandy

[Struan Gray]

Thanks Sandy, good info.

[sanking]

The object is to ensure that carbon printing and prints will exist for another 100 years.

What other compounds could possibly be used to photochemically harden gelatine?

Can we step back and look at the basic process needed and find solutions?

Maybe these questions belong on APUG.

/gth

I started a thread on the yahoo carbon forum about DAS to have a conversation about non-dichromate alternatives for carbon printing.

I am not optimistic about DAS itself. Although this is a chemical known to work, with certain work-flow issues, it is not easy to purchase, and is quite expensive even in bulk orders. Try $500 a kilo in bulk orders ot 10 kilos or more from China. That would make it difficult for even suppliers like Bostick & Sullivan and Photographer's Formulary to buy and sell the stuff at a reasonable price to hobbyists. It seems that the main industrial use of DAS no longer exists, thus the reason for the very high price since it has to be produced for a specific purpose without broad general application.

On the other hand there are some other potential sensitizing alternatives from the world of silk screening that may have properties that work for carbon printing. If any of the skilled carbo printers on the Yahoo forum get interested in the subject at least we will have somebody to test the chemicals who actually knows how to make a good carbon print.

Sandy

[gth]

The TCIi price for DAS is about $90 per 25 g. There is a some mention of 2mg of DAS per sq in of tissue on APUG. That translates to $0.60 per 8x10 print.
The 25 gram price is advertised on the TCI site, but I have no idea if the emulsion concentration is valid. Assume it's twice that of the APUG data and that you have 20 % loss in handling and your are at $1.45 per 8x10. If one out of ten prints makes it through the workflow it is $15 per good print and might still be justified considering other costs to produce that print.

Assuring long term availability might be a nastier question.







What about the

[Ed Bray]

If anyone needs Ammonium Dichromate in the UK, there is some available on ebay at the moment. I bought 5kgs yesterday.

[gth]

Further required DAS concentration and cost.

David Chalmers at

http://carbonprinting.wordpress.com/category/process/

Says he uses 0.5% DAS sensitizer in the emulsion.

If I use an emulsion of 1/16 " x 8" x10 " I use up 5 cubic inch of carbon emulsion. Counting only weight of gelatine that is 104 gram (gelatine relative density of 1.3).

Does that sound reasonable?

At 0.5% DAS we get 0.5 gram of DAS per 8x10 print.

THat is $1.8 in DAS cost for a 8x10 print using the commercial price of $90 per 25 gram or three times my previous calculation. Which would start to hurt if one considers losses to get to a final print.

Chalmers' concentration was high so the cost might be in the $1 per 8x10 which is comparable to other costs in the process although much higher than the cost of dichromate.

That may be a necessary cost of business for a more "safe" process.