Acid-Free Processing Sequence
for Fiber-Base Black and White Prints

Lloyd Erlick.
December 28, 1999

Why "acid-free"?

There are several acids used in the black and white darkroom. By far the most common is acetic acid. Others, such as boric acid and sulfuric acid, are relatively rarely used. Citric acid is rather rarely used also, but is valuable for its lack of smell.

The most prominent use of acid in the darkroom is as stop bath. Generally acetic acid in a one to two per cent solution is used following the developer. It 'stops' the development reaction because most developing agents will work only in an alkaline environment. Many workers find the odor of acetic acid unpleasant; citric acid can be substituted (fifteen to twenty grams of citric acid anhydrous per liter of working stop bath solution) for an absolutely odorless bath.

The next most prominent use of acetic acid is in fixers. In a hardening fixer the acid is demanded by the hardener. It also makes carried-over developer unable to develop any exposed photosensitive material still in the print when it reaches the fixer. If development were able to proceed in the fixer, stains would result.

Boric acid is used in some developers (we're not proposing to eliminate acid from our developers in this article, but it is a fairly rare print developer that contains acid!) and it may also be found in some formulas for fixer. Boric acid may be valuable to some workers for specialized purposes, but for ordinary use such as the processing of fiber-base black and white prints it is unnecessary. I have never had boric acid in my darkroom.

Sulfuric acid appears in some toners, but on the whole it is a special-purpose material. I have never had sulfuric in my darkroom.

For our purposes, the acid we wish to eliminate appears in stop bath and fixer. It will most likely be acetic acid, and next most likely, citric acid.

Now we can address the question: "Why acid-free?" Why do we want to eliminate acid from the processing sequence?

There are two reasons:

First, to eliminate irritating odor in the darkroom. When sodium sulfite (a common darkroom chemical, found in developers, fixers, and other common materials) mixes with acid, the result is sulfur dioxide. This is a noxious, irritating gas.

Second, to prevent staining in the selenium toner. This is a major factor. Selenium toner is essential to the overall process, and it is easy to get yellow stains on the print if it is in an acidic condition when it enters the toner.

Most darkroom workers process their black and white prints to the point in the process just before the selenium toner step. They store the prints in a holding bath of plain water, and wait for the end of the session before they tone the prints. They tone all their prints at once as a time-saving measure. An advantage of this method is that the soak in the storage-bath tends to bring the prints to a neutral condition (i.e. not acid and not alkaline). This is a good way to guard against stains in the toner bath.

I dislike this method. I find that the final result of the toner is extremely dependant on the exposure and development of the print. I find that small adjustments in the final look of the print after toning can be made by varying exposure under the enlarger. The option to vary the toning time is essential too, and impossible with batch-toned prints.

My preference is to start a print and see it through to completion, including toning and hypo clearing agent, before putting it in the wash.

With acid in the process, my preference is difficult to achieve. The acid in the stop bath must be completely removed, and if there is acid in the fixer it must be rinsed away or neutralized too, before the print goes to the toner.

My alternative is to remove the acid from my process entirely. The price I pay is to rinse thoroughly whenever I would otherwise have to use acid. Thus, my 'stop-bath' is a quadruple rinse with plain tap water. In other words, I fill my tray (please see my article on the single-tray method of print processing elsewhere in this 'Technical' section of the website) and dump it four times. I pay close attention to making sure the water gets underneath the sheet as well.

My fixer is the mildly-alkaline 'plain fixer' described by Ansel Adams in "The Print." (Please see my article on 'plain fix' elsewhere in this Technical section of the website.) It contains no acid, so prints can go straight to the toner from the fixer with no intermediate step or rinse.

I have found that the life of the toner is quite different if there is no acid in the process. Even if prints are soaked or otherwise neutralized (workers often treat the print with sodium sulfite or Kodalk solution before treating it with toner) the toner inevitably takes on a dark, murky quality. Long before it is exhausted, it will be nearly opaque with dark brown or black precipitate. With no acid, the toner remains quite clear over its entire life. If it is filtered through a plain paper coffee filter before each use, it will remain nearly water-clear over its entire life.

Acid-Free Processing Sequence for Black and White Fiber Base Prints:

By means of a single tray (please see the article about the single-tray method of print processing elsewhere on this website, in the 'Technical' section) placed on a low stand (about 6 to 8 inches) and a group of wide-mouth containers in my processing sink, I use the following sequence for acid-free FB print making:

Under safelight:

1. Move the dry sheet from its box to the enlarger. Handle with clean, dry fingers, at edges only.

2. Move the dry, exposed sheet to the empty (dry or wet) tray. Handle with clean, dry fingers.

3. Pour developer into the tray. Agitate as usual by rocking the tray. Pour developer back into its container at end of development time. My favorite print developer is the Ansco 120 formula, diluted 1+2 with distilled water, for a 3 minute development time. This developer yields a soft print with a long tonal scale. It is suitable for very rich results in selenium toner at 1+5 dilution.

4. Quadruple-rinse the sheet by pouring plain water into the tray. Agitate and be sure to cause water to go *under* the sheet as well. Pour water off and repeat for a total of four rinses. Describing how to be sure the water goes under the sheet is like talking about how to tie shoelaces. A few minutes spent rocking the tray and observing the 'wave' and how it ripples back and forth will teach anyone how to make sure the rinse or particular solution (like hypo clear) gets under the sheet. The quadruple-rinse should remove all or nearly all developer from both surfaces of the sheet, and also from within its fibers. This is an important step (and is a replacement for the stop-bath step) that demands care, but the payoff is a print that is unlikely to stain.

5. Pour Fix1 (first fix) into the tray and agitate 3 to 5 minutes. Both fixers are the (cheap, easily prepared and absolutely odorless) 'plain fixer' described in Ansel Adams, "The Print." Also please see the article on "Plain Fixer" elsewhere on this website. White light may be turned on after 90 seconds to two minutes in the first fix. (Some workers say one minute is sufficient.) Return Fix1 to its container.

NOTE regarding Fix use and lifespan: Using any fix to its exhaustion point, or even close to it, is false economy. Fixer byproducts become closer and closer to insoluble the closer we approach fixer exhaustion. Washing the print is easiest when the fixer is fresh. The acid-free method of processing under discussion here adds another reason to avoid over-using fixer: we would like to avoid problems from developer carry-over to the fixer. Even though we are quadruple-rinsing the print after the developer, no method is one hundred per cent effective. Overusing the fix will eventually bring us to grief from developer staining as well as impossible washing. I usually mix three liters of Adams' 'Plain fix', which is rated at a capacity of 75 8x10 equivalents (i.e., twenty-five per liter.) I use mine only to about 50 8x10s per three liter batch. And I throw away both Fix1 and Fix2; I don't promote Fix2 to first place when Fix1 is through. So I'm hyper-conservative about my fix, but it's cheap and easy to mix. Please see my article on "Darkroom Effluent", on this website, regarding fixer use.

Another important point is that this type of fixer is very sensitive to acid. It is protected to a degree by the presence of sodium sulfite in the fixer formula, but nonetheless acid should be kept out of plain fix. If enough acid gets into it, for long enough, sulfur will appear as a yellowish color in the fix. At this point it is dead.

Under white light:

6. Pour Fix2 (same type of fix as Fix1) into the tray and agitate 3 to 5 minutes. No rinse between fixes. Return Fix2 to its container.

7. Pour selenium toner into the tray. NOTE: no rinse is necessary between Fix2 and toner, although it would be harmless. Agitate for requisite time. Selenium toner can be applied in many dilutions from very dilute to quite concentrated, for many different lengths of time, from about one minute to fifteen minutes. I often use it at 30 to 34 degrees Celsius. Return toner to its container.

Although many darkroom workers pile their prints up in a holding tray and tone them all together, I prefer to do each one separately. I find 'shuffling' a stack of large prints (frequently 16x20 or 20x24 in my case) very unpleasant. Also, I touch the face of the print with absolutely nothing, including the gloves people often use when shuffling prints.

8. Rinse the print two or three times, pouring off rinse water each time. Agitate print in each rinse being sure water goes under the sheet. Pour off last rinse water.

9. Pour first hypo clear (2% sodium sulfite solution, used for one session only) into the tray. Agitate 3 minutes or more being sure solution goes under the sheet. Return first hypo clear to its container.

10. Pour second hypo clear (2% sodium sulfite solution) into the tray. Agitate 3 minutes or more being sure solution goes under the sheet. Return second hypo clear to its container.

Note re Hypo Clear: If a quantity of sodium sulfite is purchased in bulk (usually sold in 25 kilogram bags) the price per gram is very low. Commercial hypo clearing products are mainly sodium sulfite. I prefer to dissolve one tablespoon (a little over 20 grams with my tablespoon) of sodium sulfite per liter of hypo clearing bath. It dissolves easily and quickly, so I simply do it for each printing session. I use it only for one session, so my hypo clearing baths are always fresh. I use a double hypo clear for the same reasons as the double fixing bath. The first does most of the work, and the second is extremely effective in completing the job. I find the anhydrous form is the best to buy in terms of price per gram of sulfite itself and in terms of ease of use in the darkroom. (It dissolves easily at room temperature.)

Sometimes, depending on water supply, a white precipitate appears in the hypo clear solution. This is usually an insoluble calcium compound due to hard water (i.e., water containing calcium salts. Magnesium salts will cause the same problem.) To prevent this, add about one gram EDTA per liter of solution. Depending on the hardness of the water, less or more EDTA may be needed. An alternative to EDTA is Calgon (the famous water softening compound). Kodak Hypo Clearing Agent powder can be used, too, at about one teaspoon per three liters of sodium sulfite working solution. KHCA contains EDTA, so that is why it works to keep the solution from going cloudy. One pack of KHCA powder goes a long way used like this.

It is possible for the cloudy precipitate formed from hard water to stick to the face of the print. Some people say it can be swabbed off with cotton batten or a sponge, but I prefer to prevent it forming in the first place. Once again, the desire is to eliminate anything that might touch the face of the wet print. In this case, we can eliminate both the precipitate and the swabbing.

If the hypo clear solution is mixed with distilled water, none of these measures will be necessary.

11. Rinse the print two or three times, pouring off rinse water each time. Agitate the print in each rinse being sure water goes under the sheet. Pour off last rinse water.

12. Lift print carefully with clean fingers and place in print washer.

Clean the tray before starting the next print. Run plain rinse water over all the top edges of the tray and down the inside walls, and dump. Then tilt the tray and pour the rinse water down the sloping tray interior. Dump and repeat until you are satisfied. Drain excess water. There is no need to dry the tray. The next print can be placed into the wet tray with no ill effect. The length of time between placing the fresh sheet into the wet tray and pouring on the developer need not be more than a few seconds.

Dry the top edges of the tray (where your hands touch the tray!) with a bit of paper towel (or, for real cheapskates like me, a square of toilet paper.)

Make as many prints as desired, or enough to fill print washer. Prints may be washed immediately or wait overnight, or up to 24 hours. A long soak, such as overnight, should always be in cold water. Warm water soaking can dissolve the gelatin coating of the print and cause white spots. Be careful in summer weather because cold water can warm up enough to harm the print.

Please note that my prints are never hardened (there is no hardener or associated chemicals, such as acid, in my fixer). Some people feel prints made without hardener and washed in warm water are prone to damage because the emulsion may soften. I have never experienced any ill effects of going without hardener. It is extremely easy to be meticulous and gentle with prints.

13. Turn on print washer and wash prints 90 minutes. Water temperature must be no lower than 20C (68F), temperature in 75F to 80F range may be best. Inspect prints from time to time (ten to fifteen minute intervals). Lift from washer and replace if print surfaces are covered with bubbles. Be absolutely certain your hands are clean. If you use different types of paper, observe the differences in how the bubbles disappear when the sheet is lifted from the washer. On some types of paper, the bubbles disappear almost immediately. On others, the bubbles persist strongly, and might need two or three dips in and out of the water to remove them.

14. Lift the prints from washer with plastic clips that have copper or plastic loops to go over a line. Hang prints by their clips from a stiff line (ordinary string or rope line sags too much). Hang large prints from both top corners, 11x14s and smaller prints from one corner.

15. Using a squirt bottle (the kind of bottle used to package soft drinks, with a 'nipple' closure fitting), run distilled water down both sides of the print. If the floor must be protected use a bucket on top of newspaper or rags.

16. Allow the prints to dry undisturbed. Room humidity must be 'not too dry, not too wet'. (Human comfort range is ideal.) Turn off sources of air movement such as fans and heaters. If protecting the wet prints from dust (or keeping a bit of humidity close to them while they dry) is necessary, it is simple to create a 'tent' of plastic. Cut up several green plastic garbage bags and tape them to the ceiling above the hanging prints, and let them hang down surrounding the prints. A second row can be taped to the bottom edges of the first to make a deeper tent. A more permanent version of the plastic tent can be made from four spring-loaded roller blinds. Cover them with plastic (use the 4 mil polyethylene available at hardware stores) and attach them to the ceiling in a rectangle surrounding the print drying area. Pulling them down encloses the prints completely.

For very dry conditions, a humidifier could be placed under the tent.


  • The process I use might seem unduly time consuming (four rinses between developer and fix ...) but in reality it saves time, because I go all the way through to toning (and my method makes it easy to see subtle differences in final toned print appearance from small exposure and development changes) and washaid; once it's in the wash it's ready to dry and flatten. I never spend time taking wet prints out of holding tanks or trays and putting them through a second process (Workers who do this are inevitably tired by the time they do it at the end of a session, so there has to be a price to pay in terms of mistakes and damaged prints). I especially never spend time drying prints only to wet them again later for toning.

  • This sequence, used in conjunction with the single tray method, involves almost no touching the print.

  • Touching the wet print occurs only going into and out of the print washer. Tongs and gloves are eliminated.

  • No squeegee or print-drying screen touches the print.

  • Except for corners and edges, the face of the print is never touched at all.

  • The tray never has time for developer stains to build up (this does occur in the developer container, which never touches the print).

  • The tray (not paper!) may have selenium toner deposits, but these are slow to build up and easily cleaned with laundry detergent and laundry (chlorine) bleach.

  • There is only one tray to keep clean.

  • By simply adding wide-mouth containers, additional developers can be used, or added at any time during a working session.

  • By simply replacing the single tray, sheet size can be changed at any time during a working session.

  • Different concentrations of selenium toner can be kept on hand during a working session.

  • Temperature of any solution can be maintained or altered by setting any container or containers in individual water baths.

  • Human skin receives no chemical exposure whatsoever.

Additional Remarks:

Adams, in "The Print", emphasizes that prints can go directly from the second fix (he refers to it as 'plain fix') to selenium toner with no rinse. This is a major time saver.

An acid fix demands a thorough rinse in advance of selenium toner. Usually people store prints in a water holding bath and tone them all at once after the prints are all produced.

I dislike this method because it prevents me from seeing how the finished print will look, after toning. I frequently adjust enlarger exposure according to how the print looks after toning. Saving them all up in a holding bath precludes this fine adjustment.

Also, saving the prints to tone at the end of the session often means toning them in a batch all at the same time. I am prejudiced against this method. I like to observe my print closely as it tones, but especially I hate the tendency for the print surfaces to pick up faint scratches from handling this way. I think the corners of some prints scratch the faces of other prints. Some people use gloves to shuffle the stack, and I think these produce faint scratches too.

I'm pretty biased against anything (fingers, gloves, tongs, squeegee, drying screens) that touches the face of a wet print, which is why I use the single-tray method.

The presence of acid in the system poses problems that demand careful rinsing or washing before toning. This can take quite some time. If acid is eliminated, careful rinsing of the print before the fix (i.e. after the developer step) to remove developer is a necessity, but not onerous. The payoff is carefree selenium toning, and the elimination of the 'holding-tray rinse-wash'. In fact, the holding tray itself can disappear, as it has from my sink.

Also, I have noticed that a no-acid system leaves my selenium toner almost water-clear. The dark precipitate I was so used to appears to be very much reduced or eliminated this way. My tray collects essentially no stain compared to the stain that was deposited when there was acid in my line. Filtering the toner before each session seems to help keep it clear.

My toner seems to last a lot longer, too, although that's difficult to prove. Certainly it looks better longer and does not give the feeling that using it is going to crud up my prints. I usually mix selenium at 1+5 in distilled water, which means each batch costs C$10.00 (I mix up three liters of working solution.) So if my toner will last three months instead of two, I'm ahead.

Copyright Lloyd Erlick. All rights reserved.