• Chemistry 17.05.2013 No Comments

    Another little chemistry project: Photographic prints using the cyanotype and salted paper process.

    Cyanotype

    Cyanotype Print

    Cyanotype Print

    The cyanotype process (or blueprint) is used to produce blue contact prints from transparent negatives. The blue color is the water insoluble pigment Iron(II,III) hexacyanoferrate(II,III) which is normally produced by reacting an Iron(II) salt with a ferricyanide salt or a Iron(III) salt with a ferrocyanide.

    To make photographic prints, the reaction has to be catalyzed by light. This is done by utilizing a light sensitive compound which releases Iron(III)-Ions upon exposure. The original cyanotype process uses an Ammonium iron(III) citrate solution, which has several disadvantages. An improved process[1] replaces the citrate with Ammonium Iron(III) Oxalate, which can be produced from simple chemicals in a two step synthesis.

    Synthesis of Ammonium Iron(III) Oxalate

    In the first step oxalic acid reacts with ammonia to form ammonium oxalate:

    Ammoniumoxalatsynthese

    Oxalic acid has a molar weight of 90.04 g/mol but is a dihydrate (two water molecules attached to each acid molecule) which results in a molar weight of 126,10 g/mol. Ammonia has a molar mass of 17,03 g/mol and was available as a 25% (13.4 Mol/L) solution which has a density of 0.91 g/mL. Those numbers allow to convert the molar ration of 1:2 to a weight ratio of 1 g : 1.35 g. I used 1.77 g of oxalic acid and 2.3 g ammonia solution. To react, the oxalic acid is added to the ammonia solution. The resulting solution is evaporated and the remaining solid ammonia oxalate monohydrate is collected. Ideally, it should weigh 2.00 g (14.04 mmol).

    The next step converts the Ammonium Oxalate to Ammonium Iron(III) Oxalate with the help of Iron(III) Chloride. As both reactants are solids, solutions have to be prepared.

    Ammoniumoxalatoferratsynthese

    Ferric chloride has a solubility of 920 g/L, while of the oxalate only 45 g will dissolve in 1 L of water. Having produced 14.04 mmol of the oxalate, we need to add 4.68 mmol of the chloride. When weighing the substances, the fact that Fe3Cl is a hexahydrate, has to be kept in mind. The molar mass of 162.21 g/mol has to be corrected to incorporate the mass of six water molecules, resulting in mass of 1.26 g. The 2.00 g of oxalate and 1.26 g of chloride are dissolved in 44.34 mL H2O and 1.37 mL H2O respectively.

    Vial of Ammonium Iron(III) Oxalate

    Vial of Ammonium Iron(III) Oxalate

    The solution, which should be of a light green color, now contains 2.00g or 4.68 mmol of Ammonium Iron(III) Oxalate and 14.04 mmol of Ammonium Chloride. Unfortunately, I could not come up or find a reaction to separate the two components. But I later verified, that the Ammonium Chloride has no negative effect on the print process.

    Preparing the Sensitizing Solution

    The sensitizing solution is later applied to a piece of paper to make it sensitive to light. It contains three parts Ammonium Iron(III) Oxalate and one part Potassium hexacyanidoferrate(II) by mass.

    The previously prepared 2.00 g of oxalate are added to 2 mL of water, heated to approx. 50°C and stirred until dissolved. In a second container, 0.67 g of Potassium hexacyanidoferrate(II) are added to 1.2 mL of water, heated to 70°C and stirred until dissolved.

    Subsequently, the two solutions are mixed together while continuously stirred. The solution is then filtered, filled up with water to a volume of 12 mL and, after it has cooled, stored in a dark place.

    Preparing the Paper and Making the Print

    The sensitizing solution should be stored in a dark place until needed. It can be applied to paper by placing a few drops on the sheet and dragging them over the surface with a glass rod. This is best done with minimal lighting. When the paper has soaked up the solution evenly, it is hung up to dry.

    The negative can be printed on sheets of overhead transparency. For best results, the image should be black and white and printed with high toner density.

    The dry sensitized paper is attached to the negative and then exposed either by sunlight or by ultraviolet light. I produced good results with UV tubes from a tanning lamp (which I also use to expose circuit boards). The exposure time is about 10s-20s with UV light. With sunlight, you can see that the exposure is finished, when the transparent areas of the negative have sufficiently darkened.

    To finish the print, the paper is rinsed with tap water until all green/yellow color is washed out and a blue and white image remains. After drying the paper, the print can be framed.

    Salt Print

    The salted paper process uses different chemicals and a different method to sensitize the paper. While the cyanotype is based on iron, the salted paper uses silver in the form of silver chloride. This salt is sensitive to light and decomposes to black elemental silver upon exposure. This results in a brown and white image.

    Preparing the Paper

    As the name implies, the paper is treated with a Sodium Chloride solution (2 g NaCl per 100mL water). After the paper has been soaked in the solution, it is dried. Meanwhile, a solution of silver nitrate is prepared (1 g AgNO3 per 10mL water). All steps involving the silver have to be carried out in a dark room to not expose it prematurely.

    The silver nitrate solution is then spread over the salted paper using the same method as with the cyanotype sensitizing solution. When the silver nitrate comes in contact with the sodium chloride in the paper, it reacts to sodium nitrate and the light sensitive silver chloride:

    Silberchloridsynthese

    The exposure follows the same procedure as with the cyanotype. Afterwards, the print has to be fixed to inhibit the further exposure of silver chloride. Due to the very low solubility of silver chloride in water rinsing is not enough.

    Instead, the print has to be soaked in a mixture of sodium thiosulfate, ammonia and water (10:2:100 ratio by weight). The print is soaked in this solution for about 15 min until all the unexposed silver is removed.

    The finished print can then be rinsed, dried and framed.

    Salt Print and Cyanotype

    Salt Print and Cyanotype

    Choosing the Paper

    The right paper plays an important role in making a good looking print. Standard printer paper does not take up the solution very well and wrinkles when drying. I had good results with index cards and water colour paper.


    [1] : The New Cyanotype Process (http://www.mikeware.co.uk/mikeware/New_Cyanotype_Process.html)

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