Recently I posted a video about making cyanotype prints so, for those of you who want to check that technique out, here is how you make the sensitiser liquid that I used.

Firstly, the recipe that I’m using is Mike Ware’s new cyanotype process. It has several advantages over the old and better known process. If you didn’t watch the video yet, the main benefits are:

  • A more stable solution – the stock lasts up to 5 years once mixed.
  • Faster developing time
  • Better contrast
  • More durable image

If you are interested in the chemistry and science behind the recipe, you should check out his site where he describes it in some detail.

Ingredients

First let’s talk about the chemicals you will need. These are commercially available and you probably won’t get put on a watch list for ordering them. You don’t need very much of any of them, but you’ll find it hard to order them in quantities smaller than a kilo or so. Which is enough to keep you going for the rest of your life probably. None of the chemicals are significantly dangerous, but you should most certainly wear protective gear while working with them. At a minimum, wear gloves for all of it and a dust mask while grinding up the potassium ferricyanide. There isn’t a fume danger that would require a respirator but you should work in a well-ventilated area. Although there is a cyanide component, it’s not in a particularly toxic form. Still, take care to keep it away from children, pets and food preparation. The ammonium dichromate is the most dangerous chemical that we use and you should take extra care in handling and storing it. It’s a hexavalent chromium compound, which means that it’s carcinogenic and that it oxidises rapidly when heated. If it is stored in too warm conditions (over about 40°C), it can spontaneously combust.

To make 100ml of sensitiser (enough for 40-50 A4 sized prints), you will need:

  • 30g Ammonium Iron(III) Oxalate. This is a green crystal and has the formula (NH4)3[Fe(C2O4)3].3H2O. It’s sometimes listed as Ferric Ammonium Oxalate.
  • 10g Potassium Ferricyanide. This is a red crystal that you need to grind up into a yellow powder. Its formula is K3[Fe(CN)6]
  • 0.1g Ammonium Dichromate. This is an orange crystal with the formula (NH4)2Cr2O7. As it’s hard to measure such a small amount, I recommend making a solution with 2g of the ammonium dichromate made up to 10ml with distilled water, and then using 0.5cc of that solution.

You will also need distilled water, a coffee filter paper, a brown glass jar to store the finished sensitiser in, and glassware to measure and mix the chemistry with. Optionally (and recommended), you will need a second 100ml bottle (which can be clear glass or plastic), and 40g of anhydrous citric acid.

Sourcing

I bought the ammonium dichromate and the potassium ferricyanide in 1kg bottles locally from a lab supplies store. The ammonium iron(III) oxalate was trickier to get hold of, and I ended up buying it via Alibaba from a Chinese chemical firm. The companies I was trying to get quotes from were mostly interested in supplying industrial quantities of chemicals and it took a bit of hunting before I found a place that would sell me a single kilo for a reasonable price. This was also by far the most expensive chemical, I ended up paying about $100 for a kilo of it. Some suppliers quoted me almost $400 for a kilo! While that’s quite a lot, the kilo that I have is going to last me pretty much forever. Also, to put that in perspective, Photographer’s Formulary make a new cyanotype kit that makes 100ml and costs about $25. So, if I make more than 400ml, I’m coming out ahead. Also, also, I can make extra and sell it to friends and local photographers to offset that cost too.

Somewhat confusingly, there are two chemicals that are usually described as either ammonium iron oxalate or ferric ammonium oxalate, they have different chemical formulas and different CAS numbers (a CAS number is a universal reference number for chemicals). The stuff you want has the CAS number of 13268-42-3.

Method

To begin then. You should do all of this under tungsten lighting. Not daylight or fluorescent lighting as the compounds are sensitive to UV light.

Make a bain marie by pouring boiling water into a large pan and floating your mixing bowl in the middle. Put 30ml of distilled water into the mixing bowl and let it warm up. Top up the hot water as needed or put it on a stove at a low heat. The distilled water should get to about 50°C or so. Add 30g of the ammonium iron(III) oxalate and let it dissolve.

Next, add either 0.1g of the solid ammonium dichromate or 0.5cc of your 20% solution (depending on how you chose to do that part).

While that is all dissolving use a pestle and mortar to grind up 10g of the potassium ferricyanide into a yellow powder. This will take a few minutes. When you can’t see any more red colour, then it’s done.

Add that to the hot solution in your bain marie and stir it thoroughly until you see green crystals start to develop. Take the mixing bowl out of the bain marie and put it somewhere dark for an hour or so to cool down to room temperature.

When it’s cooled, use the coffee filter paper to strain the mixture into your amber bottle, then use distilled water to make it up to 100cc – you should have about 30-35ml of sensitiser before you add the extra water.

Your sensitiser is now done. Store it in a dark dry place in the brown bottle and don’t expose it to daylight more than you have to. For best results, leave it to settle for about a day or so before you start to use it. Once it’s ‘ripened’, you will get better results with a more intense blue.

I also recommend making a 40% solution of citric acid while you are in the chemical mixing zone. Anhydrous citric acid is very easy to find as it’s a common food additive. You should be able to get it from baking supplies stores. Put 40g of it in a separate bottle (this one can be clear as the citric acid doesn’t react to light) and then make it up to 100ml with distilled water. You add a few drops of this to the sensitiser on the paper to increase contrast and facilitate clearing. Don’t add it directly to the sensitiser in the bottle as it will significantly reduce the shelf-life of the mixture.

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