Reilly, James M. The Albumen & Salted Paper Book: The history and practice of photographic printing, 1840-1895. Light Impressions Corporation. Rochester, 1980.
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The change of sensitised paper in the dark is found a serious evil by practical photographers. I know some persons, who, from this cause, waste as much paper as they use.
--C.A. Seely, 18601
The sensitization step,. in which the previously "salted" papers are rendered light sensitive by treatment with a silver nitrate solution, is vitally important to successful printing with albumen and salted papers. This chapter is devoted to the solutions and procedures employed for this purpose. The composition of the sensitizing solution may include other substances such as citric acid, but it always has one basic ingredient: silver nitrate. Silver nitrate is formed by the action of nitric acid on silver metal. It is sold in the form of colorless crystals, normally in quantities of 1 ounce, 4 ounces, and 1 pound. As with nearly all chemicals, a considerable saving is realized by buying in large amounts, and the one-pound package is economical compared to the one-ounce size. For albumen and salted paper printing a "technical" or "ACS" grade is sufficiently pure, and there is no need to purchase the highly purified grades that are required in photographic emulsion making.
The primary light-sensitive substance in all the papers described in this book is silver chloride. Silver chloride is always formed in situ by a "double replacement" reaction between whatever chloride is used to "salt" the paper--sodium chloride, for example--and silver nitrate. In a "double replacement" reaction the ions of two compounds change partners, forming two new substances. The reaction between sodium chloride and silver nitrate yields sodium nitrate and silver chloride. Since silver chloride is nearly insoluble in water it is immediately precipitated as a cloudy white mass. A chemist might symbolically write the reaction this way:
NaC1 + AgNO3 → NaNO3 + AgCl4&darr
Sodium chloride and Silver nitrate produce sodium nitrate and silver chloride.
The symbol ↓ means that the silver chloride is precipitated; the sodium nitrate which is formed is a soluble substance and remains in solution. Sodium nitrate has no effect on the printing process and is washed away during processing.
For reasons explained in Chapter 1 there must be sufficient silver nitrate in the sensitizing solution not only to react with all the chloride present on the paper, but also to leave a considerable excess of silver nitrate present in the light-sensitive layer. In practice the silver nitrate solution must be approximately four times as strong as the original salting solution. If a 3% salting solution was used, then the strength of the sensitizing solution must be approximately 12%, depending in part on the type of binder material used on the paper (see below). The sensitizing solution itself must be made with distilled or at least de-ionized water, because if made with tap water, the silver nitrate will react with chlorine and "hard water ions" such as carbonates, etc., that exist in many water supplies. The result is a cloudy white precipitate of silver chloride, silver carbonate, etc., which robs the bath of strength and makes sensitizing difficult. During the initial wash step of processing albumen and salted papers, the clouding of the water which occurs is this very reaction between the excess silver nitrate left in the paper and various ions present in tap water.
CAUTION: Always wear eye protection when compounding or handling silver nitrate solutions!
The simplest sensitizing solution may be made as follows:
|Silver nitrate||120 g|
|Distilled water||to make 1 liter|
The strength of the silver solution for any given paper is governed by several factors; the first is the amount of chloride that has been used to "salt" the paper. In general less "salting" requires a less strong silver bath and vice versa. For most salted papers it is best to keep the chloride content at about 2-2.5% and the silver bath at 10-12%, because lowering the chloride content tends to produce prints that lack brilliance and density. Albumen paper is a somewhat different case, in that the presence of the glossy albumen surface tends to compensate for a lack of a heavy deposit of image silver. By lowering the chloride content of albumen paper to the range of 1-1.5% thinner negatives may be more successfully printed, and a weaker silver bath--in the area of 8-9% .-- is called for. Such a low chloride content and weak silver bath were commonly used in the period 1880-1900 to try to accommodate the negatives with relatively lower density ranges produced by gelatin dry plates. On the whole, however, modern practice with albumen paper is best conducted with a chloride content of 1.5% or above and a silver bath strength of 10% or above. Of course, this means that some thin negatives will not be printable. This is a small loss, because better looking and more permanent prints will result from the increased silver image deposited by stronger salting and silvering (see Chapter 11).
A second consideration in the strength of the silver solution is the nature of the binder material used. For papers in which starch is the binder material strong silver baths and short times of floating on the sensitizing solution are required. The reason for this is that unlike albumen, starch is not coagulated or rendered insoluble by the silver solution. Too long floatation will cause a loss in image brilliance because the starch layer is very permeable to water and absorption of fine silver chloride particles down into the paper fibers is more likely if floatation is prolonged. Also, the strength of the sensitizing solution regulates the size of the grains of precipitated silver chloride; stronger solutions produce larger grains that are less likely to be absorbed into the paper fibers. Hence for arrowroot and matte albumen papers strong silver baths and short floatation times are required 2
The duration of the floating of albumen paper on the sensitizing solution depends partly on the strength of that solution. If the silver bath is very strong, the coagulating effect of silver nitrate on the albumen layer is correspondingly strong, and the solution will not permeate through the albumen very quickly. This means that a longer time of floatation is required for albumen paper on strong sensitizing solutions. On the other hand if the silver solution is relatively weak, then prolonged floating may be detrimental because the albumen--which is water soluble unless coagulated--may begin to dissolve off the sheet before it is completely coagulated. This phenomenon is especially noticeable with freshly made-up sensitizing baths, because the sodium or ammonium nitrate which accumulates in older baths as a result of the "double replacement" reaction involved is absent when a bath is fresh. These salts also have a coagulating effect on albumen, and some old manuals advise actually adding some ammonium or sodium nitrate to sensitizing baths to insure proper coagulation of the albumen. However, this advice pertained to relatively silver-poor baths of 5 to 8%, and is not applicable when a bath of 10% strength is used.
CAUTION: Always wear approved eye protection and tightly fitting surgeon's gloves during the operations of sensitizing paper.
Silver nitrate can cause permanent, irreversible eye damage. Although infrequent contact with silver nitrate and the resulting staining of the skin does not appear to be dangerous to health, prolonged exposure can lead to permanent staining of the skin. Ultimately silver absorbed through the skin will be deposited in various locations around the body causing irreversible staining of the conjunctivae, blood vessel walls, gums, mucous membranes, etc. This condition is known as argyria. Consequences more serious than staining of the tissues have also been reported, and all necessary precautions should be taken to avoid repeated absorption of silver nitrate through the skin. Always wash hands immediately after contact with the silver nitrate solution. The most important precaution, however, is to wear eye protection; sheets dripping silver solution pose a special hazard to eyes.
The operation of sensitizing and drying must always be conducted under proper safelight illumination. See Chapter 2 for details on appropriate workroom lighting. An average time of floatation of albumen paper on the sensitizing solution is 2½ to 3 minutes. This insures adequate sensitization of even doubly albumenized paper. Albumen paper should not be excessively dry at the time of sensitization. Conditioning the paper by storing it overnight in a damp basement or placing it for a few hours in a closed box with a dish of water will confer two benefits in the sensitizing procedure: first, it will allow the albumen layer to evenly take up the sensitizing solution and second, the sheets will not be so stiff and hard to manipulate. The most common approach to applying the sensitizing solution to the sheet is floating. The actual technique of floating is no different from the floatation methods described in Chapter 3 for applying the sizing-salting solutions. Care in necessary to prevent the silver solution from reaching the back of the sheets, since patches of uneven density may result. A three-minute egg timer is a handy accessory when sensitizing paper. Always time the duration of floating from the moment all bubbles have been broken and the sheet lies flat on the surface of the solution. The bubbles that appear usually originate from air trapped under the sheet or are formed by the droplets draining from a previous sheet. Blowing gently on the bubbles before floating the sheet usually bursts them, or at least moves them to the side of the tray.
Fig. 28. Table for sensitizing paper. A is a sheet of paper hung to dry after sensitizing, together with a small dish underneath to catch the drippings. B is the tray containing the sensitizer. C is a sheet waiting to be sensitized, and D is a cushion holding pins used to hang the sensitized sheets.
Ideally, the tray, and all containers used for the silver solution, should be made of glass. A stainless steel tray is not recommended. If a plastic tray has to be used, it should be new, because plastic tends to hold on to chemicals; unwanted staining could be the result of a tray contaminated with developer or fixer from ordinary photographic work. PyrexTM baking dishes are not precisely the right size for the usual negative formats, but are serviceable. A glass rod is also a handy accessory when sensitizing paper. It is useful for bursting stubborn bubbles and for lifting the corners of the sheets when it is time to remove them from the silver solution. The lifting of the sheets should be performed very slowly in order to allow the silver solution to drain evenly off the sheet. Lift one corner of the sheet and gently and slowly peel it off the surface. When the sheet has been properly lifted, hardly a drop will leave the sheet while it is held in the air over the tray to drain. The sheets should be hung from clothespins by two corners of the long edge so that the runoff has the shortest distance to travel. Inclining the line at an angle of 5-7 degrees allows the runoff to collect at one corner where it can be conveniently blotted. Blotting of the drops that form on the lower corner is essential to speedy drying and even sensitization. Do not over dry the sensitized paper; albumen paper will become brittle if excessively dry, and most salted papers--especially matte albumen paper--will not produce satisfactory prints if they are too dry.
With matte salted papers--but not glossy albumen paper--an alternative method of sensitization is to brush on the sensitizing solution. This has the advantages of requiring less made-up silver nitrate solution and of doing away with the danger of getting sensitizer on the back of the sheet. However, brush sensitizing has the disadvantage of the constant possibility of streaks resulting from uneven application of the sensitizer. Brush coating should be performed with a wide, flat Japanese brush, one in which the bristles are bound with thread rather than a metal ferrule. The reason for this is that the silver nitrate will react with the metal of the ferrule and cause stains in the print.
Slightly stronger solutions are employed in brush coating than in sensitization by floating. The brush should be well washed with distilled water after each use and reserved exclusively for sensitization. The brush will soon be discolored but this will not affect its performance, if it is kept clean. Another way to apply the sensitizing solution is with a "Blanchard's brush," which consists of a 4-inch wide, ¼-inch thick piece of wood or PlexiglasTM, over which are stretched several folds of clean flannel. Sanding smooth corners on the working edge of the wood or PlexiglasTM strip is helpful. Dip the Blanchard s brush into the silver solution and apply it to the paper with smooth even strokes.
Fig. 29. A drying room for sensitized paper. A small stove can be seen at right.
Two applications may be needed if sufficient density cannot be obtained in shadow areas of the print, especially with papers coated with a relatively heavy layer of binder material. The appearance of blotches or light spots in dense areas of the print is a phenomenon described in old manuals as the "measles," and is caused by insufficient sensitization. This trouble may appear in either floated or brush-coated papers. The cause may either be a too-weak silver solution or insufficient residual sensitizing solution in the paper. Nothing can be done for prints that already have been exposed, but unexposed sheets suspected of having the same difficulty may be re-floated or rebrushed with a stronger sensitizing solution. The appearance of paper-white round spots with hard, definite edges is the result of air bubbles trapped under the sheet during floating on the sensitizing solution; these areas have therefore not been sensitized at all. Similar white spots also arise when air bubbles prevent the sizing-salting solution from covering an area of the print, but instead of being paper-white they typically have a light brown stain from the action of the silver nitrate.
After some sheets have been sensitized by the floating method--which in most cases is the best and most satisfactory approach--the remaining silver solution will be reduced in both strength and volume. The strength of the silver solution will be lowered by the reaction of the silver nitrate with the chlorides in the paper, and the volume will be reduced by the simple mechanical absorption of silver solution by the sheets of paper as they are floated. Although the amount of silver nitrate removed from the bath by both causes remains fairly constant and predictable, in practice a relatively weak bath needs more constant attention and replenishment than a relatively strong one does, since the weaker bath is brought to a dysfunctional point more rapidly. It is bothersome to be constantly monitoring and replenishing a silver solution, so it is better to start with a 12% bath and attempt to maintain it in a range of 10-12%, where there need be little concern over insufficient sensitization or the "measles." Of course, 10% baths are less expensive than 12% ones, but the lost time and expense of inadvertently spoiled sheets and frequent determinations of the strength of the solution make the relatively weaker baths a false economy. The 12% solution will remain functional and satisfactory for perhaps two or three batches of prints--say a total of 30 11 x 14 prints--before a determination of strength need be made, whereas the 10% bath will need replenishing much sooner.
Obviously, it is necessary somehow to determine the strength of the solution and replace its lost strength and volume long before such drastic consequences as insensitive, flat prints or the "measles" make their appearance. The most accurate approach is to periodically chemically analyze the silver content of the solution by means of a titration analysis (see below), but this is not always possible. In the absence of a reliable means of testing the silver content, a rule of thumb for replenishment may be used instead, and fairly reliable results obtained. This rule of thumb may be summarized as follows:
Replenish the lost volume with a silver nitrate solution twice as strong as the original bath. For example, if the original strength and volume were 1 liter of 12% silver solution and after a printing session only 850 ml remained, replenish with 150 ml of 24% silver solution. This should restore the silver bath to the original strength and volume. If citric acid is used in the sensitizing solution, it is replenished strictly on the basis of volume lost from the solution. For example, if the liter of sensitizing solution had originally contained 5% citric acid (i.e., 50 g) and 850 ml was left after a printing session, then the missing 15% of the original volume of 1 liter represents a loss of 15% (i.e., 7.5 g) of citric acid. Thus the 150 ml of replenisher would contain 7.5 g citric acid in addition to the required amount of silver nitrate.
The rule of thumb method of replenishment is not suitable for continuous use, or when a large amount of printing is to be done. The errors which are inherent in the method may accumulate with unpleasant and expensive results. For more critical and serious work the best method is to analyze the silver content of the bath using a titration procedure. The following adaptation of a classical procedure in quantitative analysis requires a modest outlay for equipment but affords accurate, speedy determinations. After the initial calibrations have been performed, a determination of silver-bath strength can be made in only a few minutes. In practice, a determination is required only about every third printing session, unless a great many sheets are sensitized at one time. The details of these procedures were worked out by Irving Pobboravsky, a research technologist at the Graphic Arts Research Center at Rochester Institute of Technology, and have been successfully used by the author for several years. They appear here through the permission of Mr. Pobboravsky.
The method consists in slowly adding a reagent solution (sodium thiocyanate) to a measured amount of silver solution mixed with an "indicator" (ferric ammonium sulfate) until a color change takes place that cannot be made to disappear by stirring or agitation. To make sure that only the desired reactions take place between the silver nitrate, the sodium thiocyanate and the "indicator," the silver solution is acidified with a small quantity of nitric acid. At the point of the irreversible color change, the amount of the thiocyanate consumed indirectly tells the concentration of the silver nitrate solution. The method is known as Volhard's method (after the 19th-century chemist who discovered it), and it was in use for the purpose of keeping track of albumen paper sensitizing baths as early as 1875. The complete working details of the method may be found in Appendix B.
It will become apparent after the first printing session with albumen or salted papers that although the silver solution begins as a perfectly clear and colorless solution, it soon turns brown and eventually almost black with use. The cause of this is the fact that some organic matter from the sizing of the paper or whatever organic binder is used--albumen, gelatin, etc.--always partiallydissolves into the silver solution. The reaction of this organic matter with some of the silver of the sensitizing solution eventually causes a spontaneous reduction of the reacted silver to the metallic state. The metallic silver is responsible for the coloration of the solution. In much the same way a sensitized but unexposed sheet of albumen paper will spontaneously "print-out" over time and become dark brown.
Fig. 30. Siphon apparatus for use in decolorizing silver solutions. A siphon was used to extract the decolorized silver solution without disturbing the layer of powdered kaolin on the bottom.
The silver solution may be used in a slightly discolored state, but not when it is nearly black. Some way must be found to remove this organic material or the silver bath quickly becomes useless. The best way is to use a finely powdered white clay known as kaolin, which if shaken up with the bath, will absorb the organic matter and settle to the bottom of the bottle, thus clearing the solution. About 15 grams of kaolin is sufficient to repeatedly clear one liter of silver solution. The kaolin should be left in the container in which the silver bath is stored, and after each use, the bath and kaolin should be shaken up together. The kaolin will settle out overnight and the bath will be clear and ready to use the next day.
To avoid the difficulty of trying to decant the clarified solution, a siphon arrangement like the one pictured can be easily constructed. All that is required is a two-hole rubber stopper and some glass tubing. To cut glass tubing, first score it with a file and snap at the score line, then round the sharp edges on a gas flame. To use the siphon, blow in the short tube to start the flow, and the bottle will empty itself down to the level of the bottom of the drain tube without disturbing the kaolin in the bottom of the bottle. If for some reason there is not time to allow the kaolin to settle out of the silver bath, the kaolin may be removed by filtering the solution through medium filter paper. If a silver bath has acquired a surface scum which shows up either in the tray or as a metallic marbled sheen on exposed prints, filtering the solution is necessary. Always make sure that the surface of the silver solution is free from scum before floating a sheet of paper.
Under normal circumstances albumen and salted papers will remain usable only for one or two days after sensitization. In extremely humid and warm conditions they may yellow in a matter of 8 to 12 hours, or even less. The yellow color is actually spontaneously reduced metallic silver in a very finely divided state. As the process of spontaneous reduction of silver continues, the paper turns a deep reddish brown; ultimately the paper turns black with a shiny greenish "bronzed" surface. It may require several months to reach the "bronzed" stage in dark storage. The spontaneous reduction of silver can be slowed by storing the sensitized paper in a tightly closed container in a cool and dry place, since air and moisture accelerate the process of decomposition. Papers that are only slightly yellowed may still be usable, because the fixer will remove some of the finely divided silver from the highlight areas.
Naturally the rapid decomposition of sensitized papers is a real inconvenience in printing operations. For best results it is necessary to sensitize, print and process all on the same day. There does exist an alternative procedure, in which organic acids are either added to the silver nitrate solution or applied to the paper in a separate step (this can be done before or after sensitization). The most effective organic acid for this purpose is citric acid, which is also used in many foodstuffs as a preservative. Citric acid has a marked effect on those papers such as arrowroot or resin-arrowroot, which do not contain an "active" organic binder substance. In these papers it produces a redder, more brilliant print than would otherwise be obtained, and it incidentally preserves them a little longer after sensitization.
The addition of 5% citric acid to the silver bath produces the maximum preservative effect, although as little as 1% will extend usable life to a noticeable extent. In the case of glossy albumen paper the addition of citric acid to either the albumen or the sensitizing solution has an effect on the color of the print, causing it to become reddish brown, even with prolonged gold toning. When the citric acid is present in the silver nitrate solution, the print color seems to be more brownish than when the citric acid is present in the albumen. In both cases, however, the pre-processing stability of albumen paper is improved, and greater sensitivity and maximum density are achieved. The preservative effects of citric acid were known as early as 18603, but apparently it was not realized that by increasing the amount of citric acid in the silver solution, greatly improved stability could be obtained.
The idea of using a large amount of citric acid in the sensitizing solution to preserve albumen paper was first published in 1869 by a Viennese photographer named Adolf Ost.4 In 1872 the first "ready sensitized" albumen paper, a product of the Sensitized Paper Co. of Portsmouth, Ohio, was offered to the public.' Hailed as a great breakthrough, this invention promised to revolutionize the practice of albumen printing, and at least as far as amateur photographers were concerned, it did.
Professionals, however, continued to sensitize their own paper for two reasons: purple tones were difficult to produce in the presence of the citric acid, and it was considerably cheaper to sensitize the paper "at home." Although the use of organic acids--citric, tartaric and oxalic--was the basis of most "ready sensitized" papers, there were a number of ways in which the preservative effects could be obtained (see below). The exact methods used by manufacturers of "ready sensitized" paper were jealously guarded secrets. With the advent of the gelatin dry plate in the 1880's there was a great upsurge of amateur photography and "ready sensitized" albumen paper gained enormously in popularity. Very few amateurs in the 1880's and 1890's bothered to sensitize their own paper.
Fig. 31. An 1887 advertisement for an albumen paper sensitizing service.
As mentioned above, a serviceable technique that requires the least effort is simply to add 5% citric acid to the sensitizing bath. The formulae of many salted papers already stipulate this addition, and where it is not called for it may be used as an option for increased keeping qualities after sensitization. Alternative approaches are to brush a 2% citric acid solution on the paper either before applying the salting-sizing solution, or after sensitization. Papers to be "preserved" must be scrupulously kept out of white light, because the slightest exposure to actinic light forms some metallic silver that acts as a catalyst for further silver reduction.
Another technique for preserving paper in the 19th century was to wash out a considerable part of the residual silver nitrate after sensitization, then restore sensitivity immediately before printing by fuming with ammonia. For a modern worker to duplicate the 3- or 4-month useful life for albumen paper that commercial "ready sensitized" paper achieved in the 19th century would mean a long process of experimentation with silver bath strength, citric acid content, etc. The end result in the 19th century (as evidenced by many complaints in the journals) was often paper that was so silver-poor that it required a revitalizing treatment with ammonia fumes before even a barely satisfactory print could be obtained. As a general rule, albumen and salted papers deliver the best results when used as soon after sensitization as possible.
In 1840 it was first noticed that adding ammonia to a silver bath until the brown precipitate which forms is finally re-dissolved causes a more brilliant, vigorous image in salted papers.6 "Ammonia-nitrate" was particularly useful for printing from weak negatives. The "ammonia-nitrate" process was very widely used for salted papers in the period 1840-1860, but it had some disadvantages. The prints sensitized on an ammoniacal silver solution yellowed very quickly, and the solution itself tended to discolor very rapidly. Brush sensitizing lessened the latter difficulty, but the first one remained.
When albumen paper began to be used in the early 1850's, the disadvantages of the ammonia nitrate bath multiplied, since it tended to dissolve the albumen off the paper. Photographers tried to correct this by neutralizing the ammoniated bath with nitric acid, and this approach solved the problem. But in neutralizing the ammonia the real effect was to form ammonium nitrate, which builds up in use anyway, so the utility of adding ammonia then acidifying with nitric acid is doubtful, though it does appear to make a slight contribution to the sensitivity of the paper. Fuming seems to be a better way to produce the advantages of ammoniacal silver.
Many old manuals do recommend the use of ammonia--and a number of other substances--in the silver bath. Most of these are superfluous, if not potentially harmful. One clear danger in the use of ammonia in silver baths is the potential for the formation of fulminating silver, a highly explosive substance, if an ammoniacal silver bath is "boiled down," as some old manuals recommend.7 "Boiling down" is supposed to be a way to rejuvenate a bath that has become overloaded with sodium or ammonium nitrates and organic matter, but the author has had no occasion to employ this procedure, and it is not recommended for anyone to undertake this dangerous operation.
The additives recommended by various 19th-century sources had three purposes: increased sensitivity, coagulation of the albumen layer, and preservation of the sensitized paper. .For increasing sensitivity ammonia was the main substance recommended, but citric acid also has an effect on sensitivity, and this recommendation will be found in some manuals. The second class of additives--albumen coagulants--includes the largest and most wide-ranging list ofsubstances. At various times sugar, camphor, alum, sodium, and ammonium nitrate and alcohol were all suggested. The third group of additives, intended to help preserve sensitized paper, has been dealt with above. For modern practice, only preservative additions are recommended.
Only about 6-8% of the silver initially present in a sheet of sensitized paper is utilized to form the image; the rest is washed or fixed away in processing. Much of the silver can be recovered by careful saving and treatment of the processing solutions. Most of the recoverable silver is found in the first and second changes of wash water. Where the volume of printing operations warrant, these should be saved and the silver precipitated out of them by adding either sodium chloride or sodium carbonate. The silver will then be in the form of insoluble silver chloride or silver carbonate, and after settling the excess water can be decanted off. A great deal of silver is present in the fixing solutions, and these should be placed in a large container and allowed to evaporate to a thick sludge, or to dryness when ready to ship to a refiner for processing. Print trimmings and spoiled prints are also a source of recoverable silver. They can be burned and silver (and gold) recovered from their ashes. All of these economy measures were in common use in the 19th century, especially by larger volume operations. In a typical gallery about 60% of the silver consumed was recovered, though in exceptional cases 75% or above was achieved.
All of the residues of wash water, fixer and print trimmings must be sent to a precious metal refiner for conversion back to pure silver. Sometimes volume users in the 19th century did their own smelting and refining, but most often the residues were sent out to be processed. Many of the larger photographic supply houses offered this service, in addition to converting the recovered silver back into silver nitrate. For modern printers who use only a small amount of silver and gold, silver recovery measures may not be economically feasible because of the high costs of refining. Many refiners have a minimum charge or specify minimum amounts of residues. Shipping charges must be borne by the consumer, adding to the cost of silver recovery. Smelting and re-nitrating silver are not operations for the home laboratory. Thus, unless a large amount of printing is done, silver recovery is usually not worth the trouble.
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