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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 3


Agitation

That hypo is heavier than water is the hoariest old chestnut around as far as print washing is concerned. It is used as a selling point by manufacturers whose washers use a downward flow system. A solution of hypo is higher in density than water alone – barely. And if you lower a print straight from the fixer into the washer, you will see flow patterns as the fixer drains down from the surface of the paper. Left alone, however, the fixer will eventually diffuse evenly into the water. Introduce any turbulence into the wash water, and diffusion is quick and complete. And, once in solution, fixer cannot separate itself from the water and “sink” anywhere. If you mixed fixer and water in a bottle and left the bottle on a shelf for months, would you return to find the fixer on the bottom and the water on the top, separated from each other like milk and cream? Obviously not, and this can’t happen in your print washer either.

Let’s now forget for a moment about the rest of the water in the tank. The significant part of the process takes place in the thin layer of water at the boundary between the print surface and the wash water. The rest of the water in the tank merely acts as a reservoir to absorb the hypo diffusing into this layer. However, the rapid removal and replacement of water in this interface layer is fundamental. Paper in a still tank will wash by diffusion alone – eventually – but the strong concentration of hypo in the interface will greatly impede the process. Some degree of agitation is necessary to shift the layer of water at the print surface regularly. This can be assisted mechanically, but usually the simplest solution is providing a good flow-through.
Although a reasonable level of water flow is necessary, excessive flow may waste water and excessive turbulence may work against the washing process. For example, Nova tested an experimental 5-slot 12 x 16 unit running at 6 litres per minute through 3mm jets – a very high volume of flow-through. The unit washed no faster, and the vortex created by this high throughput significantly retarded the washing rate in the central areas of the prints (Figure 4). As the water rate was reduced first to four and then to two litres per minutes, and the jet sized to 2.2 mm, an optimum flow rate was reached. This seems to suggest that any extreme turbulence in a washer of this type is as likely to create problems as it is to solve them. The ideal must be a smooth sweeping of all areas of the print.

If agitation due to water flow is inadequate, supplementary mechanical agitation might help. The SaltHill washer can have an aquarium pump connected to the air input of its multiple venture chambers. This maintains or increases agitation turbulence while at the same time allowing for the reduction of water flow. This appears to be a unique feature. One budget priced washer places prints in a rack, which is supposed to be given an occasional nudge by a hydraulic piston to move it within the chamber. Anything that relies upon water pressure to mechanically actuate it is likely to be temperamental, and this is no exception.






Effect of effluent fix in the wash tank

If a dump/refill system is not used, effluent thiosulphate in the water is being progressively diluted as the wash proceeds. Dilute fixer in the wash water is usually cited as being a major factor controlling wash quality. It is commonly stated that effluent fixer held within the wash chamber or cell is a serious retarding factor to further washing, and that is why most washers are designed to use a high rate of flow-through.

The second part of this article includes some practical tests examining the effect of the presence of effluent thiosulphate. The results show little effect on the rate of diffusion of thiosulphate from the paper in the early to mid stages of the wash, and that is only when the concentration of thiosulphate within the paper is at a very low level that the retarding effect becomes significant. By this stage an “archival” level should have already have been reached. As will be discussed below, thiosulphate retained within the washer is blamed as a significant factor when the dominant one is the quality of the agitation at the surface of the paper.




Washing by exchanges

Translating the “300 ml archival wash” method into practice is to wash prints use a tray with a moderate volume of fresh water, agitating thoroughly, then dumping the water completely and replacing it at regular intervals. Many people use this as a pre-wash technique anyway, and it’s extremely efficient. In a test by David Vestal, a single 8” x 10” double-weight print was washed to a point where it showed no trace of hypo (using the HT-2 test) after only three 20-minute baths in 500 ml of water. SaltHill engineers found that the most efficient wash that they could achieve was a single print in their Vortex washing tray, a pre-rinse device. A single print in a tray with a Kodak tray siphon also achieves an excellent wash with good efficiency.

These techniques become impractical, and too labour intensive, when many prints are involved. Methods of efficiently washing larger numbers of large-sized prints had to be found. This, not the quality of the wash, is what led to the invention of the so called “archival” washers.

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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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