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Mysteries Of The Vortex (Part One)
Mysteries Of The Vortex (Part One)
Author: Martin Reed (Silverprint)
Published by Les McLean
19th November 2008
Default Page 2


Diffusion and related issues

In Practice, the exponential decrease illustrated above works reasonably well for the first minutes of the wash, but slows progressively as the thiosulphate level in the paper drops. Looking at it from a more practical angle, the events that occur from the start of the wash cycle are not so linear.

First, a high level of fixer is sluiced from the surface and adjacent paper layers, resulting in a very rapid drop in thiosulphate level. This takes place within the first few minutes. Next, normal diffusion proceeds and a fairly linear progressive decrease in thiosulphate level dominate over the next half hour or so. However, getting out the very last trace of thiosulphate takes much longer. Thiosulphate and their silver complexes are larger molecules, which are prevented by friction from washing easily out of the paper structure.

The dense layer of baryta in commercially manufactured photographic paper seems to be especially significant in slowing thiosulphate removal. I observed this firsthand when doing some tests on washing emulsion hand-coated onto 300 g/m2 Cranes Parchment paper. This achieved a level of 0.015 g/m2 thiosulphate (using HT-2 testing) after only 10 minutes, faster than double weight baryta paper. It is virtually impossible to remove all thiosulphate from a paper, even with a wash duration of several hours, but this is possibly not a drawback (see “Desirable level of residual thiosulphate”)
The three stages of washing mentioned earlier, it should be stressed, are not three consecutive steps, but are effects happening concurrently. The net result is a graph of thiosulphate content declining with wash time taking the general shape shown in Figure 3.




There are a number of treatments that can improve one or more aspects of the washing process, including improvements in agitation and the use of washing aids – and especially, in the control of fixing time.



Leave it out

The purpose of most of the time, water and energy spent in the washing stage is to remove thiosulphate from the paper base, where it never fulfilled any function in the first place. Although it can certainly be removed, why not look at ways of limiting the uptake of fixer in the paper?

Major research by Ilford culminated in the 1981 publication of their archival processing sequence, based on careful control of the quality and timing of the fixing stage. The sequence uses remarkably short washing times, enabled by using a very short fixing time in concentrated ammonium thiosulphate fixer. Originally 30 seconds, this short fixing step prevents the fixing agent from penetrating significantly into the core of the paper. The paper can then be well washed using a pre-rinse, hypo clearing, and a final short wash. The thiosulphate level achievable using the Ilford sequence is, on average, 0.2 ug/cm2 (.002 g/m2), comparable with the ANSI archival standard. This is exceptionally good, way below the levels detectable with the HT-2 test, and heading for a point at which it is beyond the capability of the laboratory Methylene Blue test to detect. In practice, results may be modified by the wash water hardness, but will usually be well within any reasonable standards for archival processing.
The original Ilford archival sequence, with all processing times at 20C (68F) is:

  1. FIXING ( Ilford Universal Rapid Fixer 1 + 3): 30 seconds
  2. FIRST WASH ( Good supply of fresh running water): 5 minutes
  3. HYPO CLEARING RINSE (Ilford Galerie Washaid 1 = 4): 10 minutes
  4. FINAL WASH (Good supply of fresh running water): 5 minutes

The significance of the short fixing time is seen when looking at the levels of residual thiosulphate in the paper as the fixing time is increased. There is relatively little rise up to 60 seconds, with levels increasing significantly thereafter.

Code:
Fixing time                 Micrograms
in seconds    Thiosulphate per sq.cm.
    30             0.14
    60             0.16
   120             0.60
   240             1.00


The 30 second time, although ideal, is difficult to apply while ensuring good coverage of all areas of larger prints. Ilford now recommends 60 seconds as optimal, because it allows more control without significantly increasing retained thiosulphate levels (0.16 Vs. 0.14 ug/cm2)


While the results from using this sequence are unquestionable, it flies in the face of traditional techniques, which have always tended towards an attitude of “more fixing is better fixing”. Although all the steps are short, they must be controlled accurately, and the fixer capacity must be monitored carefully – both of which involve considerable attention. Having an assistant to carry out the processing is one answer. The washing steps don’t seem suited to a washer designed for conventional long washes; the entire sequence is probably best done in trays, where short washes can be carried out with good agitation, dumping the tray contents several times during washing.

On the other hand, the fixing step is the only real critical step of the Ilford Archival Processing Sequence. The hypo clear and wash stages have been optimised by Ilford for minimum residual thiosulphate in the shortest time, using the least water. These can be adjusted considerably – there is no reason why a more extended wash should not be used. Perhaps the basic problem is that Ilford presents the sequence as a package, but the rigid requirements needed to follow their steps create practical problems in use. Photographers may also be unwilling to substitute materials, lacking in confidence that other processing chemicals will not work as well as the chemicals Ilford recommends. Another problem is that many photographers cannot easily test the effectiveness of the short fixation on other makes and brands of papers. However, the Ilford archival processing sequence is the best and most efficient way to reduce residual thiosulphate in FB papers.

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  #1  
By Mike O'Pray on 20th November 2008, 12:40 AM
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Many thanks for this Les. Now it's set out and easily accessible.

Mike
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  #2  
By Argentum on 20th November 2008, 10:44 AM
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Thanks for that Les, I've snaffled it and converted into pdf for local reference.
Now if part 2 was only in text format instead of scanned magazine pages I could do the same but with a much smaller file size.
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  #3  
By Ag-Bromide on 21st November 2008, 08:57 PM
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I hope Martin Reed makes it available as a PDF for downloading (unless I`ve missed it on Silverprint`s site).
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  #4  
By Argentum on 21st November 2008, 09:13 PM
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Part 2 is available from the web site but its scanned pages from a magazine which makes it 16MB.

http://www.silverprint.co.uk/pdf.asp

mysteries of the vortex part 2
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  #5  
By Dave miller on 21st November 2008, 09:41 PM
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We will be posting part two here shortly.
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  #6  
By Andrew Bartram on 26th November 2008, 08:56 PM
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What a fantastic resourse this User Group is! I have long mourned the demise of "Darkroom User" Magazine and regularly re-read the original "Silverprint Manual" that went on to be the original AG+ Periodical (now sadly too much devoted to digi stuff I don't want to think about).
Some of the above was indeed published in the Silverprint manual.
Thanks Les for making it available
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  #7  
By Martin Aislabie on 9th December 2008, 01:46 PM
Thumbs up FB Washing Explained

What a well argued piece of work, with some back-up data to support his case.

So often these articles are a mixture of old wives tales and opinioneering.

Thanks for posting it Les

The only thing that puzzles me - why Martin doesn't have this article on his web site along side Part2 ?

Martin
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  #8  
By Les McLean on 9th December 2008, 03:19 PM
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Quote:
Originally Posted by Martin Aislabie View Post
What a well argued piece of work, with some back-up data to support his case.

So often these articles are a mixture of old wives tales and opinioneering.

Thanks for posting it Les

The only thing that puzzles me - why Martin doesn't have this article on his web site along side Part2 ?

Martin
When I spoke to Martin to ask his permission to use the article he asked me to let him have the scans I made from the original publication for he had lost his copy of the first part.
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  #9  
By Dave miller on 9th December 2008, 05:32 PM
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Quote:
Originally Posted by Les McLean View Post
When I spoke to Martin to ask his permission to use the article he asked me to let him have the scans I made from the original publication for he had lost his copy of the first part.
He could just post a link to this site.
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