A blended ferrocyanide print process

[Andrew O'Neill]

Hi Andrew,

Welcome back!

The process described in OP works fine and I've tested it on a variety of papers including Schut (courtesy @koraks), Canson Montval 300 gsm, Magnani Toscana 300 gsm, Magnani Velata 200 gsm, Chitrapat 440 gsm, Fabriano Bristol, Strathmore 300 Bristol, Brustro Bristol, Canson XL Marker, Brustro Marker, Strathmore Layout Bond, Copier Bond paper and even the lowly Butter Paper. Colour palette varies somewhat across papers mostly due to the presence of the buffer and other paper additives, but I don't get rogue pigments, smudges, bleeding or prodigious stain.

The only advice I would give at this stage is the following:

1. If your paper can withstand multiple water development steps, give the paper acid treatment to neutralise the buffer. Remove the residual acid in the paper by giving it a good wash in tap water.

1 tsp of Citric Acid/Sulfamic Acid in 1 l of water can be used for 5 X A4 sized sheets.

2. Prepare just enough sensitiser and developer needed for a session:

Sensitiser to coat one A4 size paper:​

Water: 1 ml​

Ferric Ammonium Citrate (green): 0.25 g​

Potassium Ferricyanide: 0.2 g​

Ammonium Chloride: 0.25 g​

Water to make: 2 ml​

Developer for one A4 size paper:​

Water: 5 ml​

Cupper Sulphate: 0.1 g​

Trisodium Citrate: 0.4 g​

3. Coat the paper in relatively dry environment. If that's not possible, dry the coated paper well by blowing warm air on the coated side as well as the other side with a hair drier for a few minutes, and expose immediately. After the exposure is complete, avoid exposing the print to air till it is developed.

Humidity in the paper as well as the environment influences the formation of Prussian Blue from Prussian White during exposure and afterwards. As the formation of Copper Ferrocyanide during the development step critically depends on the availability of Prussian White, controlling humidity helps.

4. Develop in Copper Complex for 2-5 minutes. Longer development is not required.

5. Use plain water for the first wash as this helps in reducing stain in the whites. Citric Acid can be used in the second wash.

Thank you for your continued interest in FerroBlend and happy printing whenever you get back to it!

-- Raghu

Trisodium Citrate instead of Sodium Citrate in the developer?

Edit: I checked on Google, and apparently, they are sort of the same?

 

[koraks]

Edit: I checked on Google, and apparently, they are sort of the same?

They are; citric acid is a trivalent acid, meaning it can deprotonate three times. To make up for the loss of those protons, three electrons need to be borrowed from somewhere. Since sodium can share only one, it takes three sodiums to get the job done. Hence, the stable salt of citrate and sodium will always have these constituents in a 1:3 ratio.

 

[fgorga]

They are; citric acid is a trivalent acid, meaning it can deprotonate three times. To make up for the loss of those protons, three electrons need to be borrowed from somewhere. Since sodium can share only one, it takes three sodiums to get the job done. Hence, the stable salt of citrate and sodium will always have these constituents in a 1:3 ratio.

This is not quite correct. All four forms (i.e. the fully protonated acid, the mono-anion, the di-anion and the tri-anion) of the citric acid can exist in aqueous (water) solution. The exact mix of the various species will depend on the pH.

(The monosodium and disodium salts are also available as solids but usually at much greater cost than either the acid or the trisodium coupounds.)

Shown below is the structure of the fully protonated citric acid; the acidic protons are indicated by the red arrows. The graph to the right of the structure shows the relative amounts of each of the four forms as a function of pH.

Thus if the pH of the desired solution is critical then the difference between using citric acid and trisodium citrate will be important. Using trisodium citrate will yield a solution with a higher pH than one made using citric acid. This difference may or may not be important depending on the application.

Additionally, if the concentration of citrate is important then one must take into account the fact that the molar mass of citric acid (192 g/mol) is significantly less that the molar mass of trisodium citrate (258g/mol); both masses cited are for the anhydrous compounds, adjust as appropriate for hydration (+18 per water).

For the removal of carbonate 'buffer' from paper you definitely want to use citric acid not the trisodium salt. The concentration for this application is not particularly critical except for the fact that a more concentrated solution will have a higher capacity (i.e. will neutralize more paper) than less concentrated solution.

Having no experience with this printing process, I can't say if the difference between citric acid and the indicated trisodium citrate in the developer is critical for this application.

 

[Raghu Kuvempunagar]

Hi Andrew, Trisodium Citrate is what I use for making the developer. It is the most commonly used form of Sodium Citrate and is easily available. If it is not available to you for any reason, please don't spend money on the more expensive forms. You can synthesise by reacting Citric Acid with Sodium Bicarbonate.

 

[Andrew O'Neill]

So.... Sodium Citrate and Trisodium Citrate are the same? If it is, I have heaps of the former.

 

[Raghu Kuvempunagar]

So.... Sodium Citrate and Trisodium Citrate are the same?

For this process, they're.

 

[koraks]

So.... Sodium Citrate and Trisodium Citrate are the same?

Yes. The nuance added by @fgorga is more of academic than of practical relevance in this case (and all photographic instances I'm aware of). The citrate you have on stock will be the trisodium citrate.