Backstage Library Works

Protect Your Microfilm Against Red-Spot Oxidation

Silverlock Polysulfide Treatment FAQs

This document was originally created by the Image Permanence Institute (IPI), modified by Backstage preservation services, and approved by IPI.

1. Question: What is SilverLock? Will it prevent red spots?

SilverLock is a protective chemical bath that makes silver film resistant to the effects of atmospheric pollutants. It does this by converting most of the metallic silver that makes up the image to silver sulfide. Metallic silver is very susceptible to oxidation — a fact you can appreciate if you have a silver table service that must periodically by cleaned of tarnish. The silver in a film emulsion can react in somewhat the same way, except that the deterioration typically appears in the form of spots (sometimes referred to as red spots). SilverLock completely prevents red spots.

2. Question: Who invented it?

The formula for SilverLock was developed by the Image Permanence Institute (IPI) as a result of five years of federally funded research. Work was supported by grants from Office of Preservation, National Endowment for the Humanities (NEH); the National Historical Publications and Records Commission (NHPRC); and the New York State Library Preservation Grant Program as part of a general effort to preserve the informational content of the nation’s brittle books. The specific purpose behind the design of IPI SilverLock was to ensure the stability of preservation microfilm.

3. Question: Where can I get the treatment?

In 1997, IPI discontinued the production of SilverLock. Some organizations increased inventory of IPI’s SilverLock, however, at some point these supplies will be exhausted. Although the formula is freely available, controlled testing and specialized equipment are required to formulate the raw materials of the polysulfide solution to the IPI formula.

Currently, Backstage Library Works is the only organization producing SilverLock that is tested and certified by IPI. Backstage will apply polysulfide treatment to film produced in our studios or to film produced by other organizations.

4. Question: Why should I treat microfilms at all? Aren’t they archival already?

Microfilm, properly processed and stored under ideal conditions, is indeed considered to be archival and will retain information indefinitely. But, presupposing an ideal world can lead to trouble in the real one. The typical photographic storage environment can present stresses and atmospheric pollutants that may damage or destroy silver images. High humidity, high temperature, and oxidizing and reducing gases all work together to take their toll on silver gelatin emulsion imaging media.

Most micrographics people are aware of the deterioration problems of microfilm in long-term storage, and some go to extraordinary lengths to prevent them. Archives have installed expensive environmental monitoring and maintenance systems in their vaults to protect their stored film from atmospheric pollutants that degrade silver. SilverLock creates a pollution barrier, too — right on the film itself. It makes film chemically resistant to damaging atmospheric agents, without altering important film characteristics. While there is really no substitute for a healthy storage environment, SilverLock can be regarded as a cheap insurance policy against the many hard-to-control environmental contaminants that can damage microfilm over the long haul of archival storage.

5. Question: If we provide a vault that has proper controls, use archivally safe boxes, and watch our air quality, isn’t that good enough? Why do this treatment then?

Because there are so many potential sources of contaminants that cannot be foreseen and that may be very close to the film. For example, many building materials — paints, plastics, adhesives, concrete sealants — can give off peroxides. In most actual occurrences of red spots or fading, the specific sources of contaminants cannot be pinpointed. A simple analogy: it is better to vaccinate against disease than rely on avoiding exposure to germs. In addition, HVAC equipment can fail, or not be maintained for lack of funds. The longer we expect the film to last, the more time there is for slow processes or corrosion to occur.

6. Question: Does treating SilverLock mean we no longer need to have environmental controls?

No, SilverLock-treated microfilm still should be stored in the recommended temperature and relative humidity (RH) conditions, because the gelatin emulsion and film base are subject to deterioration.

7. Question: We were always told that proper processing was the main issue in archival permanence. Now you are saying that corrosion of silver is the key. What scientific evidence exists, and what do Kodak and Fuji think about all this?

Proper processing is still important, everyone agrees on that. But that is only the beginning; most modern processing machinery does a more than adequate job of hypo removal. That the corrosion idea is correct is supported by scientific publications from all the major manufacturers and by independent researchers who analyze actual examples of deterioration in film collections. Kodak and Fuji are aware of IPI’s research but do not specifically endorse SilverLock treatment of microfilm. However, the idea that gold, sulfide, or selenide treatment increases permanence is officially recognized by all manufacturers. Kodak recommends the use of its proprietary polysulfide solution (Kodak Brown Toner) for increasing the oxidation resistance of black and white films (Kodak Pub. G-23, 1989; Kodak Pub. A-1671, 1998).

8. Question: Speaking of Kodak Brown Toner, can this be used to treat microfilm? How is SilverLock any different?

Yes, Kodak Brown Toner can successfully be used. IPI did a parallel series of tests that showed it to be effective. However, the SilverLock formula is optimized for the purpose of treating microfilm; it has the advantages of being less likely to decompose in a tank or tray and giving better control over the process of converting the silver image to silver sulfide. Specifically, with SilverLock, the silver sulfide image contrast is close to that of the original silver image.

9. Question: How does SilverLock work? What’s the idea behind it?

It works by converting approximately 70% of the silver image to silver sulfide, a compound that is many orders of magnitude more stable than silver itself. Pollutants won’t harm silver sulfide. If 100% of the silver were converted, the image would be brown; leaving a little silver keeps the image black. The key point is that the information content of the microfilm is now in the form of silver sulfide. We could bleach away the 30% of silver remaining and still have enough density and contrast left in the residual silver sulfide image to make a perfectly good print or duplicate. It’s the silver sulfide that we rely on as the “insurance image” to guarantee the survival of the information content of the film.

10. Question: How do we know silver sulfide images are stable? Has this ever been done before?

Literally hundreds of millions of photographic prints and silent movie films have been toned to silver sulfide and have proven themselves practically immune from fading. This was common practice from 1910 to 1950; it was done to “tone” the images brown, but the side effect was unmatched permanence. Several chemical techniques — including polysulfide — were used to produce silver sulfide images during this period, with no ill effects. So there is at least 80 years of experience with polysulfide toning to give us confidence that the approach is safe and effective. The new twist on this idea is the discovery that film need not be “toned” all the way to a brown color in order to protect the information content of the image — it can remain apparently unchanged and still be resistant to oxidation.

11. Question: What are the special advantages of SilverLock? Why not use gold (GP-2), as recommended by Kodak in the 1960s? What about selenium?

Gold (GP-2) is very effective; the one drawback, however, is the extremely high cost. The cost of stabilizing a very large film collection using the gold treatment is prohibitive.

Selenium treatment, on the other hand, is something we do not recommend. Although it was suggested as a film treatment several years ago, standard tests of oxidation resistance performed by IPI showed it to be partially ineffective in preventing oxidation damage to microfilm.1,2 The low-density areas of selenium-treated microfilm are not protected.

12. Question: How do you know that SilverLock (or any other treatment) protects against corrosion of silver? Are there ANSI Standard Tests?

There are two test procedures: the hydrogen peroxide fuming test, developed jointly by IPI and Kodak, and the dichromate bleach test. These procedures exist as the ANSI IT9.15 Methods for the Evaluation of the Effectiveness of Chemical Conversion of Silver Images Against Oxidation. Treated film samples from multiple batches of Backstage SilverLock have been tested by IPI. The SilverLock produced by Backstage passes both the dichromate bleach test and the hydrogen peroxide test specified in the ANSI standard.

13. Question: Will polysulfide treatment eventually be required by ANSI Standards for “archival” microfilm?

No. ANSI specifications no longer certify any material as “archival.” As of 1991, materials have had an “LE” (Life Expectancy) Designation. The LE rating for silver film on polyester base is 500 years. This assumes processing and storage in accordance with ANSI Standards — in other words, it assumes the film will never be exposed to any pollutants. Since post-processing treatments like SilverLock are intended to protect against possible exposure to contaminants, they are considered to be optional and not part of the primary ANSI specification for film. It is up to the user (or to secondary standards organizations such as AIIM) to decide if they want to specify the use of a treatment to increase oxidation resistance. However, the ANSI Standard IT9.15 specifies test procedures to make sure that the treatment chosen is effective.

14. Question: How do we know that SilverLock is safe to use — that no unexpected side effects will show up later?

IPI devoted five years of research to silver image treatments, including an entire two-year project (funded by NHPRC and NEH) just to look for any possible drawbacks and thoroughly investigate its effectiveness. Accelerated-aging tests concerned with the image, the gelatin, and the support were done with exhaustive thoroughness, using different film types and brands.

15. Question: When should film be treated — during processing or afterwards?

Film can be treated in-line as part of original processing, or later on, using a dedicated processor.

16. Question: Can older film be safely treated?

IPI fully recommends SilverLock for use on films up to ten years old. Further research has been done on the gelatin qualities and high image densities of older microfilms. This research has revealed adjustments that should be made to the dilution of SilverLock during application to ensure adequate drying with thicker emulsions.

According to IPI: Retrospective polysulfide treatment of microfilms which already contain redox blemishes is a judgment call. If the redox blemishes are not in image areas, polysulfide treatment is encouraged since the film and/or storage environment are conducive to oxidative attack. However, if they are in image areas, the consumer must weigh the disadvantage of possible additional degradation of the original or copies against the advantage of arresting further degradation.3


1 P.Z. Adelstein. James M. Reilly, D. W. Nishimura. and K. M. Cupriks. “Hydrogen Peroxide Test to Evaluate Redox Blemish Formation on Processed Microfilm.” Journal of Imaging Technology, Vol. 17. No. 3 (June/July, 1991), pp. 91-98.
2 James M. Reilly. D. W. Nishimura, K. M. Cupriks. and P.Z. Adelstein, “Polysulfide Treatment for Microfilm,” Journal of Imaging Technology, Vol. 17. No. 3 (June/July, 1991), pp. 99-107.
3 James M. Reilly and Kaspar M. Cupriks. “Final Report on NEH Grant PS-20565-92 July 1992 to June 1995: Polysulfide Treatment of Existing Microfilm Collections.” Image Permanence Institute. (September 30, 1995), pp. 25-26.

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