PDA

View Full Version : RA4 baryta papers: difficulty?



Pages : 1 [2]

Photo Engineer
01-08-2007, 11:32 AM
I'm not sure about that explanation. Glass transition temperature of gelatin is only lowered by adding small amount of sorbitol. Plus, I did this experiment myself some time ago and srbitol-treated gelatin is permeable to oxygen and peroxide. If sorbitol treatment indeed prolonged the image longevity I suspect it did so via radical scavenging action on excited dye molecues, or perhaps by scavenging the offending molecule or its intermediate forms.

Glass transition temperature of gelatin is raised by adding sorbitol.

Permeability to oxygen was decreased and scavenging of free radicals was increased.

Results differ on RC and FB papers due to the permeability of the supports to oxygen, and the inherent ability of FB to absorb more of the sorbitol and change penetration from the back, or support side. This effect is also pH dependant.

The sorbitol stabilzers could not be used on RC papers very well due to blooming of the sticky sorbitol on the surface of the print. In fact, just about any stabilzer could cause problems when used with RC based color prints.

This is why the method of glass transition elevation was incorporated into the coating to improve dye stability of later versions of color paper.

PE

Photo Engineer
01-08-2007, 11:42 AM
As I have pointed out before, Kodak uses about 200 fc for dye fade and Fuji uses about 500 fc for dye fade. These two tests mimic in one case the average home or museum and the other mimics average office environments. It is, in a sense, a reciprocity condition related to time vs intensity.

Kodak tested the conditions in thousands of homes across the world to come up with the value of 200 fc, and Fuji has tested many office environments to determine their value.

The ANSI committee is still out on this subject. Wilhelm uses Fuji's figures for light intensity IIRC. On top of this are the environmental factors of sulfur dioxide gas, and other pollutants that cause dyes to deteriorate. Add on humidity and temperature and experiments become very complex as does interpretation.

I have even seen a test in which short term stability of prints was worse than long term. The image was recovering with age for a reason that was quickly determined. Even so, this was not desirable, so the problem was solved and the product went out with the fix.

PE