[pure-silver] Re: b&w print has discolored - selenium toning RC

At 05:25 AM 3/27/2008, you wrote:


On Mar 27, 2008, at 4:48 AM, Tim Rudman wrote:


However, you say you didn't selenium tone the print because it was
RC. Why?
You will still give protection with selenium , (or polysulphide or
gold)
with RC papers. It doesn't have to be used only with FB.
Tim


I didn't know there would be any affect, toning resin coated paper.
Thank you!  And I didn't know it gave any kind of protection.  I
thought it was just for sort of a color change.

Janet
There is really very little difference between RC and fiber paper. The emulsions are for practical purposes the same. The support of RC paper is coated on both sides with a plastic (that's the resin) so that its waterproof. In this way it behaves more like film. Fiber paper is unprotected by any coating. The disadvantage of fiber is that it absorbs the processing solutions which must be subsequently washed out. For the emulsion washing is pretty much by diffusion. The rate of a diffusion process is mostly controlled by the difference in concentration of the ions to be washed out in one side versus the other. So, for instance, the hypo left in the emulsion after fixing leaves pretty rapidly when fresh water is used for washing. The paper support, OTOH, washes out only partly by diffusion. some of the hypo gets trapped in the fibrous structure of the paper and moves out very slowly even when a wash aid is used. For protected support, meaning RC, the support does not absorb anything and washing time is whatever is sufficient for the emulsion. Most RC papers will wash out to "archival" levels in about 4 to 5 minutes in running water. A double-weight fiber print will take about two hours for the same level to be reached. When a wash aid is used the wash time for fiber can be reduced to about 20 minutes but RC washes out so quickly that a wash aid is not recommended with it. Note that the emulsions of printing paper are quite thin because the light must pass through it twice, so, relatively high reflection densities can be obtained with a relatively thin layer of silver. The effect of toning is exactly the same with both kinds of support. Toning helps prevent chemical reaction of the image with air-borne pollutants and those in mounting materials. The toners which have been found effective in image preservation are those that cause conversion if the image silver to silver sulfide, silver selenide, or plate the silver with gold. The sulfiding toners are those usually called Sepia toners although Sepia is actually the name of a color. Gold toners have fallen out of wide use because of the cost of the gold chloride, however, they are very effective preservatives. It is also possible to get some degree of image protection by other means such as the material in Agfa Sistan or Fuji Ag-Guard. These are NOT as effective as proper toning but do not change the appearance of the image. Any print which is to be displayed should be toned. In the past the recommendation was to tone using a 1:19 solution of Kodak Rapid Selenium Toner. This toning was applied to microfilm successfully for many years but about fifteen years ago it was discovered that film treated with it was showing signs of image oxidation. Research by the IPI determined that the toner was no longer working as it had in the past. Selenium tends to tone high densities before low densities so it was leaving some parts of the microfilm unprotected. The IPI determined that the use of a polysulfide toner would tone evenly and could be used for partial toning with complete image protection. Selenium toner WILL provide complete image protection if toning is carried out far enough. The recommendation of Dr. Nishimura, of IPI, is to tone prints in a dilution of no greater than 1:9 for a period of not less than 3 minutes at room temperature. The disadvantage of this toning is that it will affect image color and density. The idea of the high dilution was that it had minimal effect on the image appearance. Kodak Brown Toner is a polysulfide toner which is effective as an image protector when partial toning is used. For microfilm the criterion is a minimum of 60% conversion of the silver as measured by densitometric methods but for printing paper adequate protection is probably had with any visible change in the appearance of the image. In the past RC paper has suffered from short life due to the evolution of peroxides from the material used to produce the highly reflective surface under the emulsion. This is a different material than is used for fiber paper. The effect of the peroxides was to attack the image and the upper layer of plastic causing it to flake. This problem was cured by adding anti-oxidants and oxygen scavengers to the coatings. These are such as to be self-regenerating so they should last the life of the paper. However, they can not protect the image against the effects of residual processing chemicals or the presence of insoluble fixer reaction products. To be reasonably permanent any paper or film must be completely fixed so that all of the unused halides are converted to a soluble form, and must be adequately washed so as to remove these materials and the bulk of the hypo. Because well washed film and prints are susceptible to attack by oxidizing agents in the atmosphere they should be toned, especially if they are to be displayed or kept under uncontrolled conditions. Full toning in any sulfiding toner or in selenium or gold toner provides a very high degree of protection and, hence, permanence. However, it does result in a change in appearance, this seems to be unavoidable. Probably the least noticeable difference will be made by using partial toning in a polysufide toner (KBT OR T-8) or by toning in a gold toner. Gold toners tend to produce a blue shift in image color which may be more acceptable for some prints. Like selenium toner it does not shift the color of neutral or cold tone papers much but can intensify them slightly. As mentioned above the main drawback of gold toner is the cost of the gold. Note that the metallic substitution toners like iron-blue or copper toners result in _lower_ image stability than plain silver so should never be used for image protection. 


--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 
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