The Chiba System A Safer Way to Print with Gum?

I tried gum bichromate a few years ago but gave it up because potassium dichromate is quite toxic. Hazardous chemicals just don't fit with the way I like to work in the studio—messy, intuitive.

Rethinking Gum Bichromate

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The Chiba System was developed by Norwegian photographer Halvor Bjørngård as part of his 2007 master's thesis at Chiba University.

Inspired by cyanotype, he explored how ammonium ferric citrate (a much safer iron salt) and free radical developers like ammonium persulfate could replace toxic dichromate — and still harden gum arabic with UV light.

What's Different?

In classic gum printing:

You mix gum arabic + potassium dichromate + pigment

You expose it to UV light

The dichromate hardens the gum where the light hits

Then you wash it, and the unhardened areas dissolve away

✅ In the Chiba Version version:

You mix gum arabic + ammonium ferric citrate + pigment

BUT — since AFC doesn't harden the gum by itself,

You also add ammonium persulfate (a much less toxic oxidizer)

During exposure, light triggers a chain reaction that hardens the gum in place.So it's still a light-hardened process — just with a different chemistry

Since we're not using potassium dichromate (the traditional chromium-based hardener), we need a new way to “lock in” the image. That's where ammonium persulfate comes in. When exposed to UV light, it triggers a chemical reaction that hardens the gum and binds the pigment to the surface

Toxicity Comparison: Potassium Dichromate vs Ammonium Ferric Citrate & Persulphate

Potassium dichromate is a highly toxic, carcinogenic substance that poses serious health risks—and has a harmful environmental impact. In contrast, the Chiba System uses ammonium ferric citrate, a non-toxic compound approved for food and pharma use. Ammonium persulfate, while not entirely harmless, is far less hazardous; with basic precautions like ventilation and protective gear, it's safe for studio work. By replacing dichromates with iron-based compounds, we reduce health and environmental risks

This recipe comes from Halvor Bjørngård's original thesis. I've tried it a few times and, while I've gotten some promising results, the process isn't always consistent. The outcome can vary, and I've been adjusting parts of the formula. Halvor himself noted that this gum-based method is still in an experimental stage — so some unpredictability is part of the process for now.

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