Save the planet - understand film washing kinematics

Gist: You are basically wasting water if you wash your film for greater than 7mins after fix. I wash them for 10 mins because I am paranoid. I never followed ILFord's recommendations and I think I was wrong. Read on if you like nerdy stuff and some theoretical tips with real world implications on what to do when you wash your film.

The Beginning

I recently met my friend, who is an avid film photographer. He mentioned developing some of his ULF negs and I was naturally interested and decided to meet him and watch the negs come out of the final wash. We talked while we waited and somehow the conversation veered towards the water we waste when we wash film and that conversation got me thinking. And that is the reason i am writing this note.

Honestly, I have not really paid much attention to the final stage of my black and white development process apart from the fact that I usually wash my negatives for around 10 mins in running water after the fix and no more. And I do realize that at the rate of about 6 liters/1.5 gallons per minute, I am using approximately 60 liters or 15 gallons of water to wash my film.

Now I am not a tree hugger by any stretch of imagination but I was now curious to figure out why am I really washing for 10mins?  In my case that was what I was told and I had read so I was doing it and it seemed to be working fine so why bother. Right? Well, I knew people who washed their film a longer than that so I guess it was right.

Most of the information below is essentially a collection of my research online. There is a lot more out there if you are interested. After all, this is a hundred plus year old craft. Feel free to explore and double click areas that interest you. If you need any pointers, just ask in comments.

Note that all of what I say below is primarily applicable to the Black and White film development process which is a little simpler than say the chromogenic or color process (negative or positive) that contains one or many layers of silver halide emulsion, along with dye couplers that, in combination with processing chemistry, form visible dyes. We will not discuss that process in this post.

The Question

So the question I wanted to answer was -

How much minimum water is really needed to completely wash a roll of film negative after fixing it?

And to answer this question, I needed to go and answer an even more fundamental question -

What does washing film really mean? Or Describe the science of washing film negatives?

To answer, and to scientifically describe the process, it probably is best to start with the chemistry of creating an exposure on any photo sensitive emulsion and I promise this will help us understand the chemistry of film washing. Indulge me for a bit here.

If all this seems beginner mumbo jumbo, jump right ahead to the Wash section below to read about the topic at hand.

Background

The chemical reaction of the silver salt with light results in the photo-reduction of silver ion to metallic silver

Ag⁺(aq) → Ag(s)

It is the formation of metallic silver that is responsible for the image that appears on exposure of the silver salt to light. The quantity of silver ion that is photo-reduced to silver metal is proportional to the intensity of light.

The Process

When we say cheese, this is more or less what happens under the hood:

1. We use a piece of photographic emulsion that holds silver halides like AgBr salt in a gelatin. 
2. During photography, the film emulsion is exposed to a pattern of photons aka light. 
3. This light ionizes the bromide ion and the freed electron ultimately reduces a nearby silver ion. 
4. Some parts of the emulsion are exposed to more intense light than others. 
5. The amount of silver reduced in a given region will be proportional to the intensity of light. 
6. That variation in intensity creates contrast on the exposed film, and hence creating a latent image.

Develop and Stop Bath:

In photography the action of the silver bromide (AgBr) is intensified by the use of a developer. Stock developing solutions consist of a developing agent such as pyro or hydroquinone, an alkali such as sodium carbonate or borax, a little potassium bromide as a restrainer, and some sodium sulphite as a preservative. 

Next you generally use a stop bath or plain water stop to prevent over development ruining your negative. The stop bath is a very weak acid and is used to neutralize the developing agent. This guarantees two things:

1. You can be sure that you won't have any additional development happening after the developer bath.
2. You won't contaminate your fix with developing agent.

Fixer

On exposed film, after the develop/stop stage you would now see:

• An image consisting of dark areas of metallic silver and light areas of unexposed silver bromide that the developer has not affected. 
• In order to prevent accidental exposure, the unreacted silver ion must be washed from the film. 
• That is accomplished during the fixing process when the film is treated with sodium thiosulfate solution known as photographic hypo. 

If you are a chemistry nerd, the hypo binds the unreacted silver ion, leading to the formation of the dithiosulfatoargentate complex ion

AgBr(s) + 2 S2O32-(aq) → Br-(aq) + Ag(S2O3)23- (aq)

Once the silver ion is removed through the fixing process, the film may be safely exposed to light. Note that this process creates a negative of the image. It is the brightest regions that lead to the darkest portions of the image.

Wash

Now with that introduction out of the way, let's look at the wash phase:

ILFord's Standard Wash Recommendation

Clean the tank with the film under running water and

1. Fill tank with fresh water, turn 5 times 
2. Refresh water, turn 10 times
3. Refresh water, turn 20 times 

and we have a film washed in archive quality After my research, I tend to agree with ILFord's method and here is why:

The Chemistry of Film Washing

The first thing we need to realize is that the physical process we call 'washing' is not really washing in the real world sense of the word. It is not washing like washing clothes or utensils.

• Washing is basically removing the hypo ion complex from the film by the process of diffusion.
• Therefore the driving force for the washing process we have is the difference in concentration of hypo in the film emulsion vs the surrounding water. 
• With the constant fresh water we remove constantly water enriched with hypo from the emulsion surface of the film. 
• The rate of removal slows down because the concentration difference getting smaller with each ion leaving the emulsion - until equilibrium (E) is reached and the transfer stops. 
• Additional washing is absolutely useless.

So the problem boils down to how fast can we reach this equilibrium state? And that will define how efficiently we can "wash" film.

To find out how much hypo we need to wash, let's look at typical numbers and do the math ourselves

Now surface area (in rough estimates) of 

1 (36 exposure) roll of 35mm = 1 roll of 120 = 4 sheets of 4x5 = 1 sheet of 8x10 

which is equal to 80 square inches or 600 square centimeters.

There is published data that takes into consideration film emulsion area, thickness of film and a standard dev tank of 500ml, to arrive at a dilution factor say D.

Now the volume of hypo per liter of fixer is approx 60g, let's say H.

Multiplying HxD gives us the density, which is again something that is listed at approx 0.08 μg/cm2 

The usual value of 1.5μg/cm2 residual hypo is generally allowed for archival use.

Hence clearly theoretically one wash should be necessary and sufficient. [1] Well not exactly. Now there is a lot of science that went into people writing papers on exact time for reaching E and that is around 7mins. I won't bore you with the derivation, you can do your own research. 

What is interesting is some notes, I took reading a few papers, on aspects of film washing I was not paying attention to:

Takeaways:

(1)Take over or residual hypo ion from one wash cycle to another as in tank getting full and reloading of tank with fresh water is important. It is better to pour out water rather than just leave the water tap running.

(2) At any rate 7 mins seemed to be consensus for optimal wash times. I do 10 and will stick to that. If you are washing for 20/30mins, you may want to re-consider.

(3) Pouring water into tank, agitating continuously seemed preferable to just leaving the tank with water getting poured into.

(4) Washing the tank lid/wiping in between wash was also recommended to reduce hold over hypo ion concentration.

Now going back to ILFord's film washing recommendations, it is clear that it is in line with the above recommendations and makes more sense with the info we have now that we understand what is going on.

Hope this was educational and a little bit practical. 

References

[1] Black and White Film Processing, Rolf Suessbrich, Dortmund, Germany
[2] Chemistry of Photography, John Straub's Lecture Notes
[3] ILFord Film Washing Instructions for min water usage.