Ag Guard
From Silvergrain Research
Fujifilm Ag Guard is a final rinse solution containing an image stabilizing agent. The product is currently available only in Japan, but it is worth in-depth discussion.
For best image protection, consider toning with polysulfide or gold, whenever possible. These are the gold standard of making silver image permanent.
However, Ag Guard treatment has a few advantages:
- odorless
- simple operation
- does not add a processing step
- does not change image hue, density or contrast
- can safely overtreat with no ill effect (other than prolonged wet time)
- can be as permanent as gold toned material
Contents |
Fujifilm Ag Guard
How it is used
Ag Guard may be applied by immersion. The supplied solution is simply diluted with water. Standard dilution is 1+4. Processed films and prints are squeegeed or dripped to remove water, and soaked in Ag Guard working solution for 1 minute, just like you would in PhotoFlo or other wetting agents. The treated material is hang to dry (no need to remove the excess solution). (For nonabsorbent substrate, such as RC prints, Ag Guard is used undiluted or 1+1 dilution. For absorbent fiber based prints, the product is diluted 1+3 or 1+4.)
Ag Guard may be applied to prints by sponges, cloth or brushes. This technique may be preferred for larger prints and mounted prints. It is most effective to use the undiluted solution rather than 1+4 diluted solution.
Processing capacity of this product is 60 times 80 square inches of film, or 50 8x10 prints.
The product label indicates that this product is harmful, and it instructs to use gloves and goggles. The text also instructs to wash hands thoroughly after use.
How it works
According to MSDS, the main ingredients of Ag Guard is 2-(amidinothio)ethanesulfonic acid (CAS 25985-57-3) in concentration range of 0.5 to 1.5% (concentrate solution) at pH of about 5.7. (It is probably more like 0.5 to 0.7% range.) Ag Guard MSDS (in Japanese)
2-(amidinothio)ethanesulfonic acid
US Patent 4500632 assigned to Fuji Photo Film Co., Ltd. discloses a process for stabilizing silver images. 2-(amidinothio)ethanesulfonic acid is not mentioned by this name in this patent, but it is the general structure in formula (I) in columns 1 and 8, wherein R1 is hydrogen, A sulfonate, and n = 2. It is also described in unexamined published Japanese patent application 58-43452 (1983) and Japanese Patent JP63-45577 (1987).
2-(amidinothio)ethanesulfonic acid is an organic thioether compound that strongly adsorbs metallic silver surface and "shield" the image from oxidative attacks. The virtue of this compound is that the adsorption of the molecule is so strong that the treatment does not come off the silver image surface even if the image is rinsed in water. It is probably fair to say that this is a compound similar in affinity to polysulfide or sulfide, so that the treatment does not "wear off" by washing, but the compound only adsorbs on the surface and shileds the image from oxidants, rather than converting the silver image to silver sulfide.
Fuji published test result using Ag Guard, and its treatment efficacy is comparable to gold toning.
Iwano, H., Kishimoto., S., Koshimizu, T., and Ohmura K. 1994. Method of measuring the stability of silver images to aerial oxidation and the image stabilization by a thiol, J. Imaging Sci. Tech., 38, 140-144.
Availability
It is a real pity that Fujifilm does not export this product. As far as I know, several people in the U.S. and the U.K. tried to produce an Ag Guard knockoff by mixing 2-(amidinothio)ethanesulfonic acid with a wetting agent. There are manufacturers in Germany and Japan (guess what, Fuji has technology to synthesize this family of compounds cheaply, and they used to make it themselves---discontinued March 2006). The remaining Japanese and German sources for this compound turned out to be prohibitiviely expensive. Consequently, there is no commercial source of Ag Guard or its knockoff in North America, Europe and the rest of the world.
As we see below, Ag Guard is very well designed. Making a product that is as well designed and as well tested as Fujifilm Ag Guard is pretty difficult.
Alternative approaches
Thiocyanate (AGFA Sistan)
AGFA sold Sistan, which is a Triton X-100 based wetting agent containing potassium thiocyanate.
Within a limited range of small concentrations, thiocyanate makes insoluble silver complex, which prevents migration (diffusion) of oxidized silver ion in the gelatin layer. When oxidative attack occurs, free Ag+ may be released but this is scavenged to a relatively inert, light-insensitive, insoluble silver-thiocyanate complex in the gelatin layer.
Sistan has a few shortcomings:
- Overdosing will accelerate image degradation.
- The window of effective concentration range is rather narrow.
- Thiocyanate is hygroscopic, that is, attracts moisture by itself.
- The image protecting effect is abolished once the image is rinsed.
- The image may not be adequately protected after a number of humidity cycles.
If too much thiocyanate is present in the gelatin layer, they form more soluble higher order complex with silver, and this will accelerate image degradation. Some reports say that doubling of recommended dose can accelerate image fading with some material.
Thiocyanate is not very strongly adsorbed on metallic silver surface, and therefore the image protection effect is abolished once the agent is washed away. Similar problem may occur in humidity cycles, where a part of the paper may retain higher concentration of thiocyanate and others lower concentration.
I am not aware of a formal study demonstrating Sistan is effective, although a few casual studies (such as Ctein's article on Photo Techniques) indicate that it may provide some image protection compared to untoned silver image.
Halides
Due to their ability to form only very sparingly soluble salts, halides, particularly iodide and bromide are effective in retarding oxidative attacks. However, these agents also suffer the shortcommings of thiocyanate. In addition, these salts may enhance the moisture retention of the gelatin and the paper bese, which is a definite disadvantage.
Another disadvantage of halide treatment is that the salts formed with halides are light-sensitive.
Halide treatment is not recommended.
Aminated s-triazines and hydantoins, and methylolureas
US Patent 2629660 Harsh (1953) and US Patent 2629661 Mackey (1953) disclosed three families of compounds that are claimed to make silver image more resistant to oxidative fading.
Harsh disclosed water soluble addition products obtained by condensing a s-triazine or a hydantoin with formaldehyde. Mackey disclosed a water soluble addition product of formaldehyde and urea, such as N,N'-dimethylolurea. All of these compounds were used as the final rinse bath. Many of the suggested compounds are inexpensive but they are also toxic and carcinogenic.
Organic thiols and thioethers
There are many organic compounds that have an active sulfur element with some affinity to silver. Quite a few of these compounds were used for stabilization processing.
Thiourea is one of them, and its image protecting effect is known. However, thiourea shares all of the problems listed for thiocyanate. Plus, thiourea decomposes in presence of oxygen and light to produce corrosive acid, which can destroy silver image. That is, thiourea is not stable enough to use for image stabilizing applications. Indeed, thiourea was once used to remove cloudiness on Daguerreotype plates in 1950s, but images on those thiourea treated plates were destroyed within 10-15 years. (Kurth, D. G., 1999. The reaction of thiourea to dicyandiamidine sulfate on silver surfaces investigated by reflection-absorption infrared spectroscopy, Fresenius J Anal Chem, 365, 465-6.)
In selecting suitable agent, it is extremely important to consider possible decomposition products and also test for these long term effects before applying the treatment to production material.
There are many other thiols and thioethers that have strong affinity to metallic silver, and form insoluble silver compounds. Many of them are used as antifoggants and emulsion stabilizers. A good example is 1-phenyl-5-mercaptotetrazole. This agent, even if added to the developer, is known to protect silver image from mild oxidative attacks. However, this type of agents are very expensive and not very soluble in water. Many organic thiols are also toxic, have offensive smells, or more likely both.
Therefore, what we need is a compound with a surfactant structure, where one end has high affinity to metallic silver surface, and the other end provides good water solubility.
For the argentophilic end, perhaps most useful compounds have a structure: N=C-N with a thiol attached to the central carbon. Amidinothiols, 2-mercaptopyrimidines, 2-mercaptoimidazoles, 5-mercaptotetrazoles, and some other compounds have this structure, and emulsion chemists have very good understanding of their behavior in emulsion as well as processing chemicals. Indeed, some of these compounds have been used in photographic emulsions as well as developers to prevent dichroic staining, physical development staining, and other undesirable staining phenomena appearing most frequently in roller transport processing.
The hydrophilic end can be adapted from any standard solubilizing groups, such as amino, carboxyl, sulfonate, sulfate, ethoxylate, etc. as long as they are in soluble form at the usual pH range of emulsion storage condition (pH of 6 to 8). The best may be amino, hydroxy or sulfonic. In order to minimize the interference with the argentophilic end, it is probably best to "insulate" these groups.
Along this line of thinking, the molecular design of the active ingredient of Ag Guard makes a good amount of sense. On the argentophilic end, it has amidinothio group with two nitrogens attached to the carbon, followed by the thioether. Then the solubilizing group is ethanesulfonic acid. The ethane part keeps the sulfonate two carbons away from the argentophilic end.
Similarly usable compounds may be obtained by substituting mercaptotetrazoles, 2-mercaptoimidazole or 2-mercaptopyrimidine with a solubilizing group at 4, 5 or 6 position, or at two of these positions.
However, before applying these compounds to important images, it is essential to confirm that:
- the treatment does not sacrifice images permanence in ideal storage condition
- the treatment is indeed effective in severe storage conditions
