In 1995 Mike Ware came up with a new formula for the Cyanotype process that addressed a lot of its shortcomings. The process of mixing the chemicals is a little bit more involved than the classic cyanotype process but in my opinion, the results are absolutely worth it. On Arches Platine paper the darks almost go black and it prints in less than half the time.
I made this short video outlining in broad strokes how to make the formula. If you don’t have a magnetic stirrer, you can make a 70c water bath and heat both chemicals to that temperature. But, I suspect that, if you are the kind of person that goes to the lengths of buying potassium ferricyanide - you might have a magnetic stirrer.
The following section is shamelessly ripped from the inventor himself Mike Ware’s article on the new cyanotype process
Please note that all the chemicals are poisonous - tho' not dangerously so!
Measure 20 cc of distilled water from a measuring cylinder into a small pyrex glass beaker, heat it to ca. 70 °C (160 °F), and completely dissolve 10 g of potassium ferricyanide in it, with stirring. Keep this solution hot.
Measure 30 cc of distilled water likewise into another beaker, heat it to ca. 50 °C (120 °F) and dissolve in it 30 g Ammonium Iron(III) Oxalate.
Add 0.1 g of solid Ammonium Dichromate to the Ammonium Iron(III) Oxalate solution and dissolve it. (Alternatively, if you can't weigh out such a small amount, add 0.5 cc of 20% w/v Ammonium Dichromate solution, previously prepared by dissolving 2 g of the solid in distilled water and making up to a final volume of 10 cc). Mix thoroughly.
Now add the hot Potassium Ferricyanide solution to the Ammonium Iron(III) Oxalate solution, and stir well. Set the solution aside in a dark place to cool to room temperature and crystallise - it will take about one to two hours.
Separate most of the liquid from the green crystals by filtration (Whatman no. 1 paper, or even coffee filter paper is ok). The green solid (Potassium Iron(III) Oxalate - about 15 g of it) is disposed of safely (somewhat poisonous - but not dangerously!). The volume of solution should be ca. 62 cc.
Make up the yellow-green coloured solution with distilled water to a final volume of 100 cc. The sensitizer can be made more dilute (e.g. up to 200 cc): it will be faster to print, but yield a less intense blue.
Filter the sensitizer solution and store it in a brown bottle kept in the dark; its shelf life should be at least 4-5 years.