Mixing Day: Preparing Platinum Sensitizer Solutions
4/12/2026
Yutaka Suzue
I have been planning to mix my stock solutions for platinum printing. Today I finally had a chance to mix them. I’ve been using kits from Bostick & Sullivan, but this time I wanted to prepare them from chemicals, as I usually do for Kallitype solutions and developers.
I’ve been researching various sources, and I decided to use the recipe from Christopher James’s book The Book of Alternative Photographic Processes. Other sources from Mike Ware’s website and some Japanese sources were also very helpful for understanding the processes.
The Chemicals
Almost all of the chemicals arrived from Bostick & Sullivan. Some of them, such as ferric oxalate, are the same chemicals I have been using for Kallitype, and I have some in stock. I pulled them out and lined them up together. They are ferric oxalate powder, oxalic acid dihydrate, potassium chlorate, and the star of the show, potassium chloroplatinite (K₂PtCl₄).
Ferric oxalate powder, 100 g, from Bostick & Sullivan.
Oxalic acid dihydrate, 100 g. Added in small amounts to help the ferric oxalate dissolve.
Solution A — Ferric Oxalate (Sensitizer)
Solution A is the main sensitizer: ferric oxalate dissolved in water with a small amount of oxalic acid. The recipe calls for 16 grams of ferric oxalate and 1 gram of oxalic acid in 55 ml of distilled water. Kallitype sensitizer uses 20 grams of ferric oxalate in 100 ml, so this Solution A looked very thick and muddy. Unlike Kallitype sensitizer, I needed to add oxalic acid to stabilize the ferric oxalate complex. In Kallitype, silver nitrate is itself acidic in solution, so it does not need to lower the pH.
16.00 grams of ferric oxalate powder. The bright yellow-green color is striking.
1.00 gram of oxalic acid dihydrate, weighed separately.
I measured 55 ml of distilled water in a graduated cylinder. The water needs to be warm, around 120°F, to help dissolve the ferric oxalate.
55 ml of distilled water in a graduated cylinder.
I poured the water into a Pyrex measuring cup, then slowly added the ferric oxalate powder, stirring as I went. The powder turned the water into a beautiful saturated yellow liquid. It took some patience; ferric oxalate does not dissolve instantly. I had to keep stirring, giving the crystals time to break down. Again, this seemed much muddier than Kallitype sensitizer. Actually, I don’t know why the platinum process requires a denser ferric oxalate solution than Kallitype. I tried to find the reason why, but I could not find it.
Solution A beginning to dissolve. The deep yellow color of ferric oxalate in solution.
Solution B — Ferric Oxalate with Potassium Chlorate (Contrast Agent)
Luckily, Solution B is almost identical to Solution A. I just repeated the same process continuously, but with one critical addition: potassium chlorate. This acts as a contrast agent. By varying the ratio of Solution A to Solution B in the final sensitizer mix, you can control the contrast of the print. More B means higher contrast. In fact, I have a plan to test the drop chart from The Book of Alternative Photographic Processes, p. 323.
Another 16.00 grams of ferric oxalate, this time for Solution B.
Potassium chlorate, 10 g bottle. A strong oxidizer — handle with care.
I weighed out 0.3 grams of potassium chlorate. At this tiny quantity, I don’t know how much precision matters. The scale read 0.30 g, which felt about as accurate as I could manage. Then I measured 1 gram of oxalic acid again, the same as for Solution A.
0.30 grams of potassium chlorate. A very small amount for contrast control.
1.00 gram of oxalic acid for Solution B.
The workspace: three coffee filters with weighed chemicals, the Pyrex cup, and measuring spoons.
Adding the second batch of ferric oxalate to the Pyrex cup for Solution B.
Solution C — Potassium Chloroplatinite (The Platinum Salt)
This is the very expensive one. Potassium chloroplatinite is the compound that actually deposits metallic platinum onto the paper during exposure and development. The formula is K₂PtCl₄ — that is platinum in its +2 oxidation state with four chloride ligands. In fact, I confused it with potassium chloroplatinate (K₂PtCl₆). My original memo had a typo, and I needed to check my chemical again. I needed to spend a bit of time studying these, and Claude AI told me that it is essentially insoluble.
Potassium chloroplatinite (K₂PtCl₄), 10 g from Bostick & Sullivan
The powder itself is a nice reddish-brown color. I poured the entire bottle, and it weighed 10.13 grams — slightly over the 10 g target. I just hoped it was within a reasonable margin. This dissolves in 50 ml of distilled water.
10.13 grams of potassium chloroplatinite. The rich reddish-brown color is distinctive.
The fine texture is apparent.
I poured the water into the Pyrex cup where the potassium chloroplatinite was placed. It did not dissolve as quickly as I expected, and I needed to stir it for quite a long time, about 10 minutes or so. The water temperature must have been around 100°F. It was turning into a deep, warm reddish-brown. Anyway, I could see that the powder was dissolving more and more, with less remaining at the bottom of the Pyrex cup.
Solution C: the deep reddish-brown.
Stirring with gloves on. The color is beautiful and a little intimidating.
Bottling and Storage
Once all three solutions were fully dissolved, I transferred them into amber glass dropper bottles, labeled each one with the contents and today’s date (4/12/26), and placed them into storage. The amber glass protects the light-sensitive ferric oxalate from degradation.
Solutions A, B, and C, labeled and ready.
Conclusion
The whole process took about two hours, mostly because I was being meticulous about double-checking my notes and books.
Now the solutions need to rest and mature. As far as I understand, most sources say ferric oxalate has a shelf life of about 4 months to a year. I hope I will have enough print ideas, so I can consume them much sooner. It is known that the platinum salt solution should remain stable almost indefinitely. This is good because it is so expensive. I also need to keep in mind that the platinum salt solution needs to be stored at room temperature since it could crystallize.
Next step: preparing a digital negative and making my first test print. That is going to be a whole other adventure.