FerroBlend: A Blended Ferrocyanide Printing Process

An introduction to FerroBlend for the experienced alternative photographic artist. FerroBlend’s inventor Raghavendra Udupa (also known as Raghu Kuvempunagar on online forums) shares the process, which is a recent iron-based alternative photographic process that blends cyanotype and cuprotype chemistries to produce expressive duo-tone prints, extending the creative range of non-silver image-making.

Writer / Raghavendra Udupa
Photography / Raghavendra Udupa, Sabir Ahmed, Debanjan Dasgupta, Ramesh Adkoli, Rajkumar Krishna

WARNING: This is for experienced and advanced users. Read the MSDS (Material Safety Data Sheet) before attempting to use chemicals and use the correct safety equipment. Hazardous chemicals such as Ammonium Dichromate are used, though this is optional as a restrainer.

[This guide assumes prior experience with the Classic Cyanotype printing process, which can be found here on AlternativePhotography.com, on Mike Ware’s webpage, in the cyanotype beginners guide or more advanced books like Christina Z. Anderson: Cyanotype.]

FerroBlend is a recent addition to the siderotype family of alternative printing processes that use light‑sensitive iron salts. The process draws inspiration from two widely practiced non‑silver methods — Cyanotype and Cuprotype (learn the history, and the working process here and here)  — and seeks to extend the creative possibilities they offer by combining their image‑forming chemistries within a single print.

While Cyanotype and Cuprotype are both straightforward and economical, each produces a relatively narrow range of colour tones on its own. Cyanotype is associated with the blue hues of Prussian Blue (ferric ferrocyanide), while Cuprotype produces the warm reddish‑brown tones of Hatchett’s Brown (cupric ferrocyanide). Achieving additional tones in these processes normally requires a separate toning stage, often involving toxic metal salts or botanical toners. FerroBlend takes a different approach. It is designed as a duo‑tone process that forms both Prussian Blue and Hatchett’s Brown within the same image, resulting in split‑tone prints with cooler shadows and warmer highlights. The name “FerroBlend” reflects the blended formation of these two distinct metal ferrocyanide pigments in the print.

The light‑sensitive compound in FerroBlend is ferric ammonium citrate, the same photosensitive salt used in both Cyanotype and Cuprotype. In practical terms, the FerroBlend sensitiser is identical to a conventional Cyanotype sensitiser made from ferric ammonium citrate and potassium ferricyanide in suitable proportion. Coating and exposing a FerroBlend print, therefore, follow the same working methods as Cyanotype. The principal difference lies in development. Instead of acidified water, FerroBlend uses a dedicated developer that introduces copper ions and supports the simultaneous formation of both Prussian Blue and Hatchett’s Brown pigments.

During exposure under ultraviolet light, ferric ions in the sensitiser are reduced image‑wise to ferrous ions. These react with ferricyanide to form a mixture of Prussian Blue and Prussian White. When the exposed sheet is placed in the FerroBlend developer, part of the Prussian White reacts with cupric ions to form the reddish‑brown Hatchett’s Brown pigment, which builds selectively in image areas. Because residual ferricyanide can also react with copper and cause unwanted staining, the developer includes a chelating agent — sodium citrate — to keep excess copper in solution and help preserve clean highlights.

Initial experience suggests that FerroBlend prints show greater resistance to fading than traditional Cyanotype prints. However, as this is still a comparatively new process, systematic long‑term studies on the archival stability of FerroBlend prints remain to be carried out.

Paper Choice for FerroBlends

Unbuffered papers are generally preferred because they do not contain calcium carbonate, which introduces alkalinity that may interfere with iron‑based processes. Buffered papers may still be used if the buffer is first neutralised with a mild acid before coating. Even unbuffered papers can benefit from a short rinse in slightly acidified water prior to coating. After this rinse, wash the paper in neutral-pH water to remove any residual acid.

Recommended papers – Rinse the paper in a slightly acidified water bath (e.g., a 1% citric acid solution) prior to sensitising:

• Hahnemühle Platinum Rag
• Bergger Cot 160
• Legion Revere Platinum

Recommended papers – Neutralise the buffer by rinsing in an acidified water bath (e.g., a 5% citric acid solution) prior to sensitising:

• Magnani Toscana 300 gsm hot press
• Lana Lanaquarelle 300 gsm hot press
• Canson Montval 300 gsm
• Canson XL Watercolour 300 gsm

Recommended papers – Since the paper does not respond well to acidification, pre-coat the paper with a 20% ammonium chloride solution (2ml for an 8”x10” sheet):

• Strathmore 300 Bristol
• Fabriano Bristol
• Canson XL Marker
• Strathmore Layout Bond

Light Source

Any ultraviolet light source suitable for Cyanotype — including sunlight — may be used to expose sheets sensitised for FerroBlend. Exposure times are typically similar to those used for standard Cyanotype printing.

Chemicals Used in FerroBlend

In addition to the materials already used for Cyanotype, FerroBlend requires a small number of additional chemicals that are commonly available from photographic or laboratory suppliers. The quantities listed below are sufficient for approximately one hundred 8 × 10 inch prints.

• Ferric Ammonium Citrate (required): 25 g. MSDS for Ferric Ammonium Citrate
• Potassium Ferricyanide (required): 20 g. MSDS for Potassium Ferricyanide
• Copper Sulphate (required): 75 g. MSDS for Copper Sulphate
• Sodium or Potassium Citrate (required): 50 g. MSDS for Potassium Citrate
• Citric Acid (required): 100 g. MSDS for Citric Acid
• Ammonium Carbonate (optional): 100 g. MSDS for Ammonium Carbonate
• Ammonium Chloride (optional): 50 g. MSDS for Ammonium Chloride
• Ammonium Dichromate (optional): 1 g. MSDS for Ammonium Dichromate

WARNING: Before use, review the hazards associated with these chemicals and ensure you understand their safe handling and proper disposal
procedures.

Chemical Solutions

Preparing stock solutions in advance usually makes printing more consistent and convenient than mixing dry chemicals each time. Store all solutions in well‑sealed, clearly labelled bottles.

Please note that only the solutions marked “required” are essential for the process. The solutions marked “optional” are intended for creative control or for troubleshooting specific issues.

Sensitiser

Capacity: 100 prints of size 8 × 10 inches

Solution A (required):

• Water: 75 ml
• Ferric Ammonium Citrate: 25 g
• Water to make: 100 ml

Solution B (required):

• Water: 75 ml
• Potassium Ferricyanide: 20 g
• Water to make: 100 ml

Restrainer (optional):

• Water: 50 ml
• Ammonium Dichromate: 1 g

Sensitiser Additive (optional):

• Water: 10 ml
• Citric Acid: 4 g

Developer

Developer (required):

Capacity: 20 prints of size 8 × 10 inches

• Water: 75 ml
• Copper sulphate: 2 g
• Sodium citrate or Potassium citrate: 10 g
• Water to make: 100 ml

Developer Additive (optional):

Capacity: 100 prints of size 8 × 10 inches

• Water: 200 ml
• Ammonium chloride: 50 g
• Water to make: 250 ml

The developer additive functions as a creative control, enabling the printmaker to adjust the warmth of the print. Because its effect also varies with the choice of paper, some experimentation is often necessary. A practical starting point is 0.5 ml of additive for every 5 ml of developer, increasing the amount as needed based on the visual result.

Clearing Bath (required):

Capacity: 5 prints of size 8 × 10 inches

• Water: 1000 ml
• Citric acid: 2 g

Toner (optional):

Capacity: 10 prints of size 8 × 10 inches

• Water: 500 ml
• Copper sulphate: 5 g
• Ammonium carbonate: 10 g

Add the ammonium carbonate gradually while stirring. The solution will effervesce, so use a container with sufficient head‑space.

The toner generally brightens the highlights and shifts the blue tones in the shadows toward black. It provides an additional creative control for the printmaker.

Working Procedure for printing FerroBlends

Step 1: Prepare all stock solutions as described above.

Step 2: Prepare the paper for coating. If using buffered paper, pre‑acidify it before coating.

Caption: Canson Montval 300 gsm watercolour paper, pre‑acidified.

Step 3: Mix equal parts of Solution A and Solution B immediately before coating to prepare the sensitiser. Approximately 2 ml of sensitiser is sufficient for one 8 × 10-inch sheet when applied with a brush. If desired, add 0–2 drops of restrainer per 1 ml of sensitiser to help achieve cleaner highlights or to work with negatives of a slightly lower density range. Coat the paper as you would for Cyanotype.

Caption: After sensitisation — yellow throughout without any signs of fogging.

Step 4: Allow the coated sheet to dry at room temperature for 30–60 minutes. Gentle warm‑air drying on both sides of the paper can help ensure uniform hydration.

Step 5: Place the negative in contact with the coated surface (optionally separated by a thin clear sheet) and expose to UV light until the highlights appear greenish and the deepest shadows show tonal reversal.

Caption: After exposure — highlights appear light green, midtones blue, and deep shadows show tonal reversal; overall solarised appearance.

Step 6: For development, use about 5 ml of developer for an 8 × 10-inch print. If needed, add 0.5–1 ml of developer additive to every 5 ml of developer. Apply half the developer evenly with a brush, then apply the remainder. The solarised appearance fades quickly, and warm tones begin to build from the highlights. Development is typically complete within about 10 minutes.

Caption: Immediately after brushing on the developer, the solarised appearance disappears, with clear highlights and well‑formed midtones and shadows.

Caption: Ten minutes after development — duo‑tone print with increased warmth.

Step 7: Rinse the print in approximately 250 ml of the clearing bath (single‑use) for two minutes.

Step 8: Wash the print face‑down in gently running, neutral‑pH water for about 10 minutes.

Caption: After final wash — subtle changes in colour tone, especially in the highlights.

Step 9: Optional toning: Use 50 ml of toner per 8 × 10-inch print (or dilute 1:1 with water if preferred). Pour the toner over the print and rock the tray continuously for two minutes to ensure full coverage. Rinse the print in neutral‑pH water for two minutes and repeat twice more. The toner may be reused once or twice if employed within a short period of time.

Step 10: Wash the print again for 10 minutes in gently running, neutral‑pH water.

Step 11: Dry the print on a rack or by hanging it until it is completely dry.

Troubleshooting

Poor contrast → Check the density range of the negative (ideally 1.8–2.1).
General stain → Pre‑acidify buffered papers if not already done.
 → Confirm that sodium/potassium citrate — not citric acid — was used in the developer.
 → Check workplace relative humidity; 40–60% is ideal.
 → Add more developer additive (this also increases warmth).
Highlight stain → Add restrainer to the developer.
Shadow bleeding → Add 1–2 ml of 20% potassium ferricyanide (same as sensitiser Solution B) to the developer.
Weak warmth → Increase the developer additive in the developer.
 → Pre‑coat the paper with 1–2 ml of 20% ammonium chloride (same as developer additive) before sensitising.
Excessive warmth → Add one or two drops of sensitiser additive to the sensitiser at the time of coating.

Acknowledgments

Since the first publication of the process in March 2025, the working method has been refined based on user feedback to make it more robust. Appreciation is extended to members of the Photrio community for their generous feedback and interest in the process. Special thanks to Andrew O’Neill for producing an excellent demonstration video on The FerroBlend Process and to Koraks for his blog post on FerroBlend Fade to Pink — A Brand‑New FerroBlend Process. Sincere thanks are also due to fellow printmakers Ramesh Adkoli, Debanjan Dasgupta, Rajkumar Krishna, and Sabir Ahmed for their support and encouragement.

Raghavendra Udupa is an analogue photography enthusiast from Mysuru, a historic city in southern India. He enjoys exploring alternative photographic processes and spends much of his creative time experimenting with photochemical techniques.