See Mason. Barring that, see my previous post in another thread on washing which gives the equations and data from Mason. The article that Lee refers to is correct to a point. The amount of hypo approaches an equillibrium value, but that value is far lower with flowing fresh water according to the same type of math as in the article.
You see, the bath has to be sufficiently large, and by that time you are sometimes passing the break even point in water conservation.
Well, read Mason or the other post.
Couldn't find the 1979 Mason edition that you recommend in another thread as having the wash info. There is only one copy of the 1975 edition in paperback on Amazon for $499.00 plus $3.99 shipping.
Originally Posted by Photo Engineer
Also couldn't find a thread here searching on "Mason" that had the equations you posted. Will keep looking and post back here if I find it.
P.S. Found the 1975 Mason for $120 + shipping on Abe Books. No 1979 edition found. Still can't find the thread with Mason's equations.
Last edited by Lee L; 11-03-2008 at 08:05 PM. Click to view previous post history.
Interesting. Whenever PE writes, I listen, and now he has me thinking about the washer he must be using. I guess if I had a film or print washer I really trusted to flow fresh water across the surface, sure, I'd be all for the continuous flow model. But, in my little world, I have no trustworthy washer, and in that little world the fill and dump method is the rock solid dependable proceedure. I never have fully trusted the minimalist Ilford sequence, as stated, and have always washed using many more complete changes of water than Ilford's recommendations.
Originally Posted by Photo Engineer
PE, what you write makes me wonder where one goes to know one's getting a really good washer. It seems every washer comes festooned with happy verbiage that proclaims it's "archival" credentials, but how many really get you there expeditiously? I suppose almost any washer would eventually bring fixation by-products down to some given level. How does one find a washer that is dependably efficient? With the high cost of what appear to be well made washers, I don't want to end up with a million dollar dog.
That is not correct. Common sense itself dictates other wise.
Originally Posted by Photo Engineer
The volumes of fresh water each wash vs the remaining volumes
of contaminants is very lopsided in favor of the fresh water and
the more lopsided with each succeeding wash.
A somewhat typical example: A tank is drained of fixer. Within
remains only that fixer which adheres internally to tank and
film; perhaps 20ml +/- remains.
Add 500ml of fresh water then let soak with some agitation.
As a matter of practice the first soak need not be any more
than a rinse; 2 minutes will do. The great bulk of the
remaining fixer is removed.
With the second 500ml dose of fresh water we are primarily
cleaning the emulsion itself. Perhaps a 2ml equivalent of full
strength fixer remains. As a matter of practice the second
soak should be protracted and with some little agitation.
Fresh water diffuses inward, fixer outward. Good thing
emulsions are of gelatin and very thin.
The third soak with some little agitation should be yet
more protracted. Dan
How long would you suggest for a complete film washing time?
Now consider running water. It is always zero in chemistry and as it passes over the film, the exchange is roughly double that in single baths and it can get to zero in the film (or a very very tiny amount).
Everytime I find a film or paper that I like, they discontinue it. - Paul Strand - Aperture monograph on Strand
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Well guys, I'll post it again. BTW, I have re-read Mason and Jacobsen along with the Kodak technical manual. And Dan, your common sense is not correct here. Common sense told mankind that the earth was flat too, and the sun revolved around the earth!
Here are the equations for washing:
Diffusion rate governed washing dX/dT = k(a - X) where a = thiosulfate in the emulsion, X = loss in concentration over T which is time. This is read as the change in concentration per unit time.
This is an exponetial when integrated as in:
k = 1/T * Ln( a / a - x) where Ln = Log base 2
So, if 80% washes out in 4 minutes, then 20% remains and 80% of that will take another 4 minutes. No matter what method is chosen, the residual silver complexes and hypo must have fallen to a level that gives the best image stability. Now, this equation only works at the instant of immersion when there is NO fixer or complexes in the wash water such as in freely running water with agitation.
As washing proceeds in still water or agitated non running fresh water, the equation becomes:
dX/dT = k[(a - x) - w] where w = the amount of salts built up in the wash water at any given time! The larger w becomes, the slower the wash becomes and in standing water (say any of the 5 or so changes you use) it becomes larger with time and is NEVER zero. In running water it can be made equal to zero.
In addition, with FB paper, this equation does not apply at all due to the cellular nature of FB paper and the washing is very sluggish and can take up to several hours. So, this works only with film and RC paper.
Mason goes on (as does Jacobsen) to describe the ideal wash being fresh water introduced into a final tank which overflows into the preceding tank and soforth for a series of tanks connected together. Jacobsen gives the diagram of this countercurrent wash which has been used for years by commercial photofinishers.
Kodak simply says "wash the film in running water for 30 minutes..... at a rate sufficient to achieve about 12 turnovers / hour...". For FB paper it is 1 hour.
Refernces: L. F. A. Mason of Ilford "Photographic Processing Chemistry"
C. I. Jacobsen of Pavelle "Developing"
Kodak B&W Darkroom Dataguide (2 suggestions - one for normal and one for archival use. I have quoted the archival above)
It seems it should be that way on the surface (no pun intended).
Originally Posted by dancqu
But the time for the emulsion to come to equilibrium with the wash water is independent of the amount of fixer left in the emulsion. Once the emulsion has the same fixer concentration as the wash water then it is time to change the water - soaking any longer won't improve matters one whit.
One never waits for things to go all the way to equilibrium, 90% of the way there is plenty good enough. I use 5 - 10 minutes for each water change for no particular reason other than it feels right, is convenient, and yields zero color on the residual hypo test.
4 or 5 changes of water - assuming a generous ratio of water to prints and some agitation and shuffling - is all that is needed for fiber base w/ KHCA.
A word on residual hypo test: the solution is silver nitrate, and unless all the silver nitrate is then washed out of the print it will stain as the print dries - this doesn't indicate there is fix left in the paper - turning brown in the light is what silver nitrate does.
I'm making this one a sticky, since this is a question that we've all thought about, and there are some good posts here.
I've thought about this a bit more.
Look at "w" above in my post as "waste products" and then consider a bacterial culture that becomes self limiting due to production of waste. This is what happens. The wash becomes self limiting as "w" increases with time and the wash water is agitated but not changing. Therefore, the diffusion outward is slowed. If the water is running, "w" can be kept at close to zero.
Now, this does not mean that it cannot work. As I said above, there are two levels, "good enough" and "archival". Kodak and others recognize both. Your negatives should always be made archival, but prints are optional depending on use. Also, hypo eliminators and wash aids enter into this.
In the final analysis, I repeat "trust but verify".
Thanks David, the other thread like this has apparently been lost in the mist of time.
Thanks for re-posting the Mason and Jacobsen info and equations. That was very helpful.
My Summitek Cascade print washer works with a very low flow rate on a principle like that of the countercurrent flow wash you mention in your post. It does 12 changes/hour for sequential 11x16 compartments at a 250 ml/minute flow rate.
Last edited by Lee L; 11-03-2008 at 09:06 PM. Click to view previous post history.