U of S | Mailing List Archive | alt-photo-process-l | Re: Dark reaction

Re: Dark reaction

Mark wrote:

In copper plate photgravure, they often senstize the gelatin tissue
with dichromate then store it in a refrigerator until they use it. I
am not sure how long it keeps.
I dry my sensitized carbon tissue in a dehumidified refrigerator to minimize the dark reaction, and can expose it up to 24 hours after sensitizing without noticeable degradation. I've observed that a number of contemporary carbon printers using room-temperature drying seem to get noticeable fogging even if the tissue is exposed the moment it won't contaminate the negative. I think this is, in part, the modern trend toward thinner, more deeply pigmented tissue and stronger dichromate sensitizing solutions. I've also observed that much of the dichromate available trough photo channels seems less than pure, and that printers these days seem to think that dichromated media don't need "real" safelights -- instead, using yellow "bug lights" or even low-wattage incandescents. I did not have problems with room-temperature sensitizing in the past, and judging by the 19th century carbon images I've seen and the ones in my collection, neither did our forbears. But then, I use thick, less heavily pigmented tissue, relatively weak dichromate sensitizer by today's standards that is mixed from fresh, laboratory-grade dichromate in distilled water, and a red lithographic safelight.

I suspect photogravure is somewhat more forgiving in this respect than carbon, but I do not practice photogravure and therefore have no direct experience.

best regards,