I'm not sure how the disodium version of EDTA is superior, other than it has 2 sodium versus 4 sodium. Using equimolar amounts of each substance should yield similar results.
The Ziatype process uses ferric ammonium oxalate instead of the traditional ferric oxalate. The fao version is much more water soluble than stand ferric oxalate, so it is generally much easier to clear Ziatypes when compared to traditional Pt/Pd methods. Since citric acid is considerably less expensive than di- or tetra-sodium EDTA we supply it with the Ziatype kits, and then pass the savings along to our customers.
The effect you notice when adding oxalic acid to the ferric oxalate mix has little to do with changing the pH of the coating solution. Oxalic acid has a symbiotic relationship with the ill-defined ferric oxalate. Long story short: adding oxalic acid converts ferric oxalate from a non-stoichiometric form to a stoichiometric form. The stoichiometric form is more soluble in water, thus easier to clear.
Thanks for reading.
Last edited by Dana Sullivan; 01-19-2007 at 11:04 AM. Click to view previous post history.
You have a point concerning the FAO versus FO: my comment was based on my experience with New Chrysotypes which also use FAO. Mike Ware notes in the chrysotype instructions on his webpage
Originally Posted by Dana Sullivan
that disodium EDTA, not tetrasodium EDTA should be used in the first (developing) bath, because the tetrasodium version is alkaline (It is used for the further clearing baths. I have both varieties; it is true, they have a different ph. What is more, I have old chrysotype prints which I made long before Mike placed his instructions on his webpage, developed in tetrasodium EDTA, and I have noticed a stain on two of them - which has appeared only after years!
But FAO or FO, iron stain seems to me iron stain, and as far as I know, it is better to develop and clear kallitypes with acidic baths. It might not matter using tetrasodium EDTA if the developer was slightly acidic, but then it sometimes might, perhaps depending also on the paper, and on how much is left to clear. So, while I am not certain, I think it is better to be on the safe side. By the way, your point about the hardening action of acids seems to me not relevant here: shouldn't sizing agents in a paper all be hardened already?
Citric acid at about 2% or 3% is a much safer clearing agent for kallitype than EDTA tetrasodium IMO. Anytime you introduce an alkaline agent in the process before the iron salts are cleared you run a risk of setting them in the paper. This may not happen in all circumstances but I believe the risk is very real, and expecially if the development instructions call for washing between the developer and clearing agents.
Originally Posted by Lukas Werth
On the other hand, I would not recommend disodium EDTA as it may bleach the silver and cause a reduction in Dmax. It may be possible to use it with very low dilutions, but certainly not at the same dilution you might with a POP palladium process like Ziatype or Ware-Malde.
Yes, I have always cleared my kallitypes with citric acid; actually, according to your suggestions in the article on the unblinking eye which corroborated what I read elsewhere. I didn't know that disodium EDTA could work as a silver bleach, but it is anyway much more expensive than citric, and therefore not really relevant for this purpose.
Originally Posted by sanking
By the way, I remember I also succeeded to bleach a kallitype when clearing it with citric acid by leaving it much longer in the developer/clearing bath than recommended: I tried to be on the safe side, but it backfired. With the right paper and the right timings, there should be no problem clearing a kallitype with citric acid.
Since my Kallitype experience is limited, I'm using Dick Stevens book as a guide & reference. His recommended clearing bath is tetrasodium EDTA because its more soluble & more alkaline than disodium EDTA. The chelating action of either enables the removal of the iron from the print, which is the major cause for staining.
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When I first started to print with kallitype I tried tetrasodium EDTA but found it to be less effective in clearing with the papers I was using than a number of other acidic clearing agents so I switched to citric acid, which is dirt cheap and clears very well when fresh.
Originally Posted by doughowk
In my experience most of the clearing problems in kallitype result from either depleted developer, old and exhausted clearing bath, and/or intermediate wash in alkaline water between developer and clearing.