Gum Over Platinum — The Ultimate Combination Print in Alternative Photography

Gum over platinum — sometimes abbreviated as gum/Pt or gum-over-Pt/Pd — is one of the most luxurious and technically demanding combination printing processes in alternative photography. It layers the rich, precise tonal base of a platinum or palladium print with one or more gum bichromate layers to add colour, texture, and painterly depth. The platinum/palladium base provides an unmatched tonal foundation: a full range from luminous highlights to deep, neutral or warm shadows, with the extraordinary permanence and delicacy that noble metal prints are famous for. The gum bichromate layers add colour (any watercolour pigment), selective emphasis, and a hand-painted quality that transforms the photograph into something that hovers between photography and painting. This guide covers the rationale, materials, workflow, registration, aesthetic control, and the unique beauty of gum over platinum printing.

Why Combine Gum and Platinum?

Platinum/palladium prints are renowned for their tonal beauty, but they are monochrome — limited to the grey, warm brown, or cool grey tones of the platinum and palladium metals. Gum bichromate prints offer colour freedom (any watercolour pigment) and painterly texture, but they struggle with shadow depth and fine detail — gum layers tend to be soft and atmospheric rather than sharp and detailed. The combination of both processes gives you the best of both worlds: the platinum base provides the structural detail, shadow depth, and tonal precision that gum alone cannot achieve, while the gum layers add colour, warmth, and expressive texture that platinum alone cannot provide. The result is a print of extraordinary richness — detailed, colourful, permanent, and unmistakably handmade.

Materials

You need: platinum/palladium printing chemicals (ferric oxalate sensitiser, platinum and/or palladium salts, potassium oxalate developer); gum arabic; potassium dichromate or ammonium dichromate; high-quality watercolour pigments (tube or dry); a UV exposure unit or sunlight; a contact printing frame or vacuum frame; fine art paper that can withstand multiple wet processing cycles (Bergger COT 320, Arches Platine, or Hahnemühle Platinum Rag — the paper must handle both the acidic platinum chemistry and the alkaline gum clearing baths without disintegrating); digital negatives calibrated separately for the platinum exposure and the gum exposures; registration pins or a pin registration system for aligning multiple gum layers precisely over the platinum base.

Workflow: Platinum First, Then Gum

Step one: make the platinum/palladium print. Coat the paper with platinum/palladium sensitiser using a brush or glass rod. Dry. Expose through a calibrated digital negative under UV. Develop in potassium oxalate. Clear in successive baths of dilute hydrochloric acid or citric acid. Wash thoroughly and dry. The platinum print provides the full tonal structure of the image — from luminous highlights to deep, neutral shadows. Step two: size the platinum print for gum (if needed — most Pt/Pd-compatible papers are already sized, but an additional gelatin hardening may help adhesion). Step three: coat the platinum print with the gum bichromate emulsion — a mixture of gum arabic, dichromate, and watercolour pigment. Dry in the dark. Step four: expose under UV through a registered digital negative (which may be the same negative as the platinum exposure, or a different negative optimised for the gum layer). Step five: develop in water — the unexposed gum washes away, leaving the pigmented gum in the exposed areas. Dry. Repeat steps three to five for additional gum layers if desired.

Registration

Precise registration of the gum layers over the platinum base is critical — even small misalignments produce double images and blurred edges. The standard approach uses a pin registration system: two or more registration holes are punched in the paper and in each negative using a precision hole punch. Registration pins (metal or plastic pins mounted on a board) hold the paper and negative in exact alignment for each exposure. This system ensures that each gum layer falls precisely on top of the platinum base and any previous gum layers. For prints smaller than 20x25cm, a simpler approach — taping the negative in position and using registration marks — can work, but for larger prints and multi-layer work, a pin registration system is essential.

Colour and Tonal Control

The gum layers add colour in a subtractive manner — pigment absorbs certain wavelengths and reflects others. A single warm-toned gum layer over a neutral platinum base adds warmth and richness without obscuring the platinum's tonal detail. Multiple gum layers in different colours (for example, a warm ochre layer followed by a cool blue layer) build complex colour relationships. The density and hue of each gum layer is controlled by the pigment concentration, the dichromate concentration, and the exposure time. Lighter gum layers preserve the platinum base's influence; heavier layers add more colour and progressively obscure the metal image. The creative choice — how much platinum shows through, how many gum layers to add, what colours to use — is entirely yours, and the variations are infinite.

Paper Considerations

The paper must survive the acidic clearing baths of the platinum process and the multiple wet-dry-wet cycles of gum printing without degrading, cockling, or losing surface quality. Heavy cotton rag papers (300gsm or more) perform best. Bergger COT 320 is the standard choice — it handles both processes excellently. Arches Platine and Hahnemühle Platinum Rag also work well. The paper should be pre-shrunk (soaked, dried, and flattened) before the first coating to minimise registration errors caused by paper expansion and contraction during wet processing. Even with pre-shrinking, slight dimensional changes can occur — another reason why a pin registration system is essential for precision work.

The Gum Over Platinum Aesthetic

Gum over platinum prints are among the most beautiful photographic objects in existence. The platinum base gives the image structure, detail, and tonal depth that pure gum cannot achieve. The gum layers add warmth, colour, and a painterly quality that pure platinum cannot provide. Together, they create prints of extraordinary richness — detailed yet atmospheric, precise yet handmade, permanent yet alive with the subtle variations that mark every hand-printed work. The process is slow, demanding, and unforgiving of mistakes — but the rewards are prints that no other process, analogue or digital, can replicate.