potassium iodide - what was mr crawley's intention?

how is using ultra diluted doses of iodide meant to improve/induce?/increase edge effects? i still don't understand how this is meant to work - go slow as i'm not the sharpest! ... and why not another restrainer, say , pot. bromide?

i was reading another link where pot. iodide was used in combination with pyrocat hd. the acutance, to my understanding, of pyrocat hd is in it's tanning/emulsion hardening process - a different acutance strategy. the strange thing is, i read somewhere that modern emulsions aren't succeptable to this technique ( ultra dilute iodide in fx-1 ), yet in the previous link, mr finch claims ( i have no reason to doubt ) that he's using 35mm fuji acros!

in this link things get even more interesting. both "acutance" formulas use a sh*tload ( 100g/L - grain mush) of sodium sulfite in both baths and the ultra diluted pot. iodide!!



thanks,

I wrote to Geoffrey Crawley mentioning that he recommended the inclusion of a very small amount of potassium iodide in FX-1 whch he said produced adjacency effects with some but not all films at the time he published the formula (1961),asking if it still produced adjacency effects with modern films.
He replied that there is indeed no point in adding the iodide to FX-1 with modern films.The original formulation with potassium iodide was designed for the era of 'acutance' films.
The question raises the point of what constitutes a modern film and if there are any films on the market that still show acutance era response to potassium iodide.
IMO it is quite possible there are. Efke 25 & 100, Plus-X, Tri-X and HP5 all give stronger edge effects on Rodinal stand development than do the tabular grain films.I think only a real expert with a microscope could answer the question now, as in 1961.

Most modern films contain much higher proportions of Silver iodide in their overall Silver halide content, while older films typically contained around 2.5% some modern films can contain up to around 15%.

For this reason adding very small amounts of KI to a developer will have no significant effect, as there is likely to be far more iodide available in solution from the film itself, as it develops.

Ian

I doubt if iodide in a developer ever worked to improve adjacency effects. To do so, it should be released imagewise. When found in developers it is used as a buffer to modulate imagewise edge effects by iodide.

PE

My summary of G W Crawley's explanation BJP Jan 6 1961:
To produce marked adjacency effects on (1961) slow and medium speed films 0.00005 g/L potassium iodide may be added to a suitably balanced metol developer,FX-1.The effect probably works by shortening the inductance period thus 'setting' fine surface detail early on.The effect on the inductance period of similar concentrations of oxidation products of some developers has been known for some time.The presence of iodide ion may enfeeble the developing agency and predispose it to produce adjacency effects.The effect is only obtained with films of a particular grain structure (this includes Adox 17),not others(including Adox 14,21).

I note that Adox 25 and 50,also the Efke, are still on the market ,maybe somewhat changed,so anyone so inclined could repeat the work using these two films.

It would be hard to tell without side by side comparisons. I did not see the article and have never seen any comparisons done anywhere on this subject, only on the effects of iodide in the emulsion itself. The effect when in the emulsion is very emulsion dependant.

I find it hard to believe that the iodide effect Crawley describes can overwhelm an iodide containing emulsion, and therefore perhaps these emulsions he mention were bromides or extremely low iodide. OTOH, we all know of the bromide effect itself which is rather strong during development.

So, I remain unconvinced that it is a generic effect but rather one confined to certain films and with a mechanism that is yet in doubt.

PE

Yes. The only films surviving from the acutance era where Crawley mentioned an effect of potassium iodide in FX-1 are the two derivatives of Adox 17.These are sold today as Adox 50 and Efke 50.

In the Jan 6th 1961 article, part of a series of eight, Crawley is talking about 1 part Iodide in 20,000,000, he also seems to think it was also used in the Kodak High Definition developer, which I think had just been released in the UK around the same time.

Many developers which are designed for continuous use with replenishment improve considerably when seasoned due to the build up of bromide & iodide, amongst other things, but the iodide level is far higher than Crawley advocates.

Perhaps films like Adox KB14/R14/PL14 (modern EFKE 25) have an extremely low iodide content in their emulsion and his addition makes a far greater difference.

Ian

Ian;

When Kodak uses iodide in a developer it is there to preseason the developer, particularly if it is intended for a replenished process. Iodide has a very severe restraining effect so the developer is designed to be pre-restrained so that it can have added strength at the start to, in effect, counter film iodide. The level chosen is set at an 'average' value for capacity.

Thus, you find it in all C41 and E6 developers and for this exact reason.

PE

So I think I'm gathering that unless I'm using Adox 50 or Efke 50, mixing up a batch of FX-1 without the Potassium Iodide will give results that are at least close to the actual formula? And that even with Efke/ Adox 50 the difference may be indistinguishable?
Think I'll try it- I already have all but the KI.

I missed a film from Crawley's article. As well as Adox 17 he considered that Plus-X was affected by potassium iodide.Kramer also found an effect with Plus-X:
http://www.groups.google.com/group/rec.photo.darkroom/msg/ac305030bbeb70e7

Somewhere many years ago I had an illustrated article with photographs showing the resolving power of FX-1 and Plus-X, it may have been in the BJP, but it also referred to Pan-F as well as Adox Kb14. I think the photographs showed the magazine test shots mention in Crawley's articles.

I did test my 5omm f1.8 SMC Takumar using Adox Kb14 and FX-1 with the KI at the time, 1975/6, and the resolving power of the lens/film combination was outstanding, unfortunately I didn't test without the KI. In the last part of the article, BJP, January 27th 1961, Page 49, Crawley suggests testing with and without the KI at all apertures, I only read the original article for the first time - this year .

Crawley's film groupings in the Jan 27th 1961, BJP, article are a little strange and his article seems to indicate FX-1 works better with Adox Kb17 not the much finer grained Kb14.

Ian