Digital BW, The Print

Thank you all for your thoughts on this topic it is just the kind of information I was looking for. It is such a wonderful resource to have members of such expertise available and willing to give their time and knowledge to the community. Bravo!

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Mark" <mark@...> wrote:
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> RC Papers commonly use Titanium dioxide as the whitening agent embedded in the Resin layer (typically polyethylene) just below the image receiver layer. When low levels of light strike the TiO2, free radicals are released that eventually embrittle the resin coating. The prints become much more susceptible to cracking when the plasticity of the coatings is reduced. This embrittlement process is very hard to reproduce in accelerated testing because it takes relatively low levels of light over sustained periods of time to cause it rather than high light doses, plus as Paul suggested, there is a temperature and relative humidity cycling component that physically induces the cracking. 
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> The phenomenon was first brought to the attention of the photo industry through notable field failures occurring within a fewf years after the introduction of RC papers (they were developed for rapid photo processing in the late 1960s). Since that time, the RC paper technology has been greatly improved by adding anti-oxidants into the resin, and current opinion in the photo conservation field is that modern RC papers should hold up for 50-100 years under "normal" display conditions before cracking. Note that some RC inkjet papers additionally suffer from poor adhesion of the ink receptor coating to the RC base layer. I suspect CD Tobie has witnessed one or two of those horrid events as have I. 
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> Also, as Paul suggested, prints that have coatings are a composite system whereby the combination of layers can be stressed by environmental cycles, generally RH cycles more than temperature, but both will induce internal stresses within the layers that can ultimately lead to microcracks which then ultimately lead to bigger more visible cracks.
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> I wrote a paper discussing the envrionemental cycling issue among other factors concerning the care of photographs in 1996.  Although it addressed traditional photos with gelatin layers, the findings are still widely applcable to modern inkjet media (indeed swellable polymer inkjet papers are comprised of highly soluble gelatin). The mechanical tests needed to calculate internal layer stresses to my knowledge have not been done on modern microporous coatings. My guess is the results would be fairly consistent with other coated paper systems. My technical paper entitled "The Allowable Temperature and Relative Humidity Range for the Safe Use and Storage of Photographic Materials" can be downloaded for free here:
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> http://www.aardenburg-imaging.com//cgi-bin/mrk/_4820ZGxkLzBeMjAwMDAwMDAwMTIzNDU2Nzg5LyoyMQ==
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> regards,
> Mark
> http://www.aardenburg-imaging.com
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