Thanks guys.
Mark, can you describe the "micro cracks" you are seeing... we may be talking about the same thing... the word crack makes me think of an irregular or jagged discontinuity in the coating..
I'll check on the office humidity.
::Michael
Well, it's hard to diagnose without actually seeing a sample, but I would add that mechanical scratches tend to show jagged/abrasive components because the physically displaced coating material has to pile up somewhere, whereas microcracks which can occur in brittle coatings appear as very fine lines that propagate, sometimes in straight lines, but often with gentle curves (e.g. think of glass in a car windshield that starts with a small stone chip and then a crack eventually propagates from that point of impact over time). In the article for which I gave the URL in my earlier post, it was flexing of the substrate by the technicians handling the prints as they came off the printer that induced the cracks I observed in the prints. The cracks were more severe than the fine cracks I've seen in EEF but the principle is the same. Also, in the EEF paper, the cracks I observed were there when I opened the package, suggesting they were induced during slitting and packaging operations rather caused by dry environmental conditions in my studio.
In very dry indoor humidity (e.g., wintertime conditions indoors in my home in the northeast US, I can literally both hear and see cracks forming if i pick up a sheet of say, HN photo rag Baryta, and flex it so that it is curled to about a 2 inch radius, in other words bending it much like the wrapping it requires to go around the 3 inch diameter roll core. Fortunately, the manufacturer usually does these initial roll winding operations in moderate humidity controlled environments. The cracking sounds I can induce under very dry conditions are extremely faint but think of a sound like "Rice Krispies" cereal makes when you put milk on it
. Lastly, if you take a conventional silver halide color RC photo paper you can usually crease it so tightly as to cause a permanent crease throughout the substrate, but if you look at the crease, the top coating still hasn't cracked. Try that with an older "peel apart" Polaroid print, and you will definitely induce cracking as well. Ditto for many of today's "photo type" inkjet papers. The old Polaroid prints had a Polyvinyl acetate (PVA) type of binder layer rather than the gelatine binders used in conventional silver gelatin prints. PVA and PVB are often cited in patent literature for use as a binding agent for the microporous particles used in inkjet compatible coatings. Thus, while we do have some historical precedents like Polaroid prints and Kodak Dye Transfer prints that demonstrate how some hygroscopic binder layers become very brittle in dry environmental conditions, the microporous particles also embedded in modern inkjet coatings make the brittleness tendencies of the coatings even more precarious.
