[pure-silver] Re: Hardening fixer for film
----- Original Message ----- From: "Laurence Cuffe" <cuffe@xxxxxxx> 
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, April 19, 2007 5:08 AM
Subject: [pure-silver] Re: Hardening fixer for film
On Wednesday, April 18, 2007, at 03:28AM, "Richard Knoppow" <dickburk@xxxxxxxxxxxxx> wrote:
----- Original Message ----- I am not snipping because I want to respond to some 
things in both posts.
First of all, I am NOT a photochemist or a chemist of any sort. I know what I have picked up from those who are. Ryuji 
Suzuki is one and there have been others. I am, however,
something of a skeptic due to early training in the sciences 
and, perhaps, because my dad was an attorney and taught me
to be careful of what I accepted as fact.
  I was sure that IPI never formally tested Sistan, this
turns out to be correct. They evidently tried some very
early testing which did not turn out well. In any case Agfa 
decided against testing. Sistan may very well work as
advertised but there has never been any independent research published. I am somewhat skeptical of Ctein's work although 
he may well have done proper accelerated aging testing.


Richard, Good points.
For my sins, I am a chemist, though not a photo chemist, and I also find the explanations plausible.
Ctein describes a fairly methodical set of tests aimed at resolving a particular problem of silvering out where prints were displaying a surface tarnishing and bronzing which he was encountering with Agfa and to a lesser extent Kodak RC prints which were displayed in framed cases. He does so in sufficient detail to allow an objective observer to reproduce his results. This was around 1995, and the work was showing problems after periods of between 8 months and two years. 


  Lots of snipping...
The problem Ctein discoveredd (?) became well known, namely degradation of both the image and the surface of RC paper. This turned out to be due to the Titanium Dioxide suspended in the upper layer of plastic and used as the reflective material. The TiO slowly emmited a peroxide that attacked both the image and the plastic material. The image showed the usual effects of oxidation including the silvering effect and the plastic became brittle and flaked off. The effect was much exagerated when the surfaces of the print were sealed so that the gas emmited by the TiO could not escape. The solution arrived at by Agfa and Kodak was to include an oxygen scavenger in the surfaces. This material is supposed to be self-regenerative so it does not wear out or get used up. All currently made RC papers have this substance in them and do not suffer from the problem. Toning in highly diluted Selenium toner (KRST at about 1:19) was a standard method for protecting film and prints against the effects of oxidizing gasses in the atmosphere for about three decades. Its use came from experiments at Kodak Research Labs and the patent for Kodak wash aid suggests a formula including Selenium. In practice, it was possible to dilute the Selenium toner with the wash aid. For reasons still not fully determined this combination stopped working properly about 20 years ago. This was discovered when the Image Permanence Institute of RIT was asked by the National Endowment for the Humanities to investigate the degradation of microfilm in the NEH archives. IPI determined that the Selenium toning was ineffective in the low density areas of the images allowing oxidation in the form of "red spots" to occur. The microfilm had been properly processed and treated so it became clear that the protective treatment was not working. IPI determined that the current version of KRST simply no longer worked at the high dilutions that were recommended and had become routinely used for image protection. While pictorial emulsions, both film and paper, are less vulnerable than microfilm they are still subject to the effects of oxidative gasses in the atmosphere and it was clear than the Selenium toning method was failing to provide the protection. The reason for this is not known. Kodak claimed that the formula and method of production for KRST had not changed, IPI speculated that the earlier successful protection may have been due to an impurity in one of the ingredients that produced some indidental sulfiding in addition to the conversion of silver to Silver selenide. IPI decided that better protection was provided by a Sulfiding toner of the Liver of Sulfur type such as Kodak Brown Toner but decided to formulate its own toner so that users could mix it themselves and be assured of a consistent product. This was marketed by IPI as Silver-Lock. It also operates at a lower pH than KBT and is essentially odorless. Such toners tone all sizes of silver grains equally so there is no loss of protection in low density areas. There is no doubt that stabilizers like Sistan and Fuji Ag-Guard provide some degree of protection. I will point out that none of the toning methods or stabilizing methods will have much effect on the oxidative damage to the plastic surfaces of RC paper. They works mainly by either converting the image silver to a more stable compound or, if Ctein's idea of how Sistan works is correct, by reconverting Silver Oxide to metallic silver before it has a chance to migrate throught the emulsion. Fuji did publish a research paper describing accellerated aging effects with Ag-Guard. They showed that there was significant image protection but less than for Selenium toner. Other than Ctein's work there appears to be no research data for Sistan. The usual method of testing using accelerated aging is to bake prints at high temperature and high humidity and to subject them to hydrogen peroxide gas. The peroxide test is quite effective in showing up the effects of oxidation of the image. In the distant past the main culprits causing image degradation were excessive hypo in the emulsion and the effects of incomplete fixing. Both produce uncontrolled sulfiding of the image with attendant fading and staining. Over the last about fifty years the effects of oxidative gasses in the atmosphere causing oxidation of the image have come to be the most important factors in image degradation. It is ironic (see my note about T.H.James work) that films or prints processed according to the best archival practices of perhaps fifty years ago are _more_ vulnerable than those given routine processing, assuming neither are toned. This is due to the protective effect of _very small_ amouns of Thiosulfate retained in the emulsion, which has a stabilizing effect on the silver, probably due to a slight amount of Sulfiding of the silver. 

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