Casein Print Photography Guide — Milk-Protein Pigment Process Explained

The casein print — also known as the casein bichromate or casein dichromate print — is a rare and beautiful photographic process in which the image is formed in pigmented casein (a protein derived from milk) hardened by the action of light on a dichromate sensitiser. The process is closely related to the gum bichromate print, but uses casein instead of gum arabic as the colloid binder. Casein produces images with a subtly different quality: finer grain, smoother tonal gradation, and a velvety matte surface that many practitioners find more refined than gum prints. The casein print was used commercially in the early twentieth century for photomechanical reproduction and was adopted by pictorialist photographers who valued its soft, painterly qualities. Today it is one of the rarest alternative processes, practised by a very small number of dedicated printers worldwide. This guide covers the history, chemistry, preparation, coating, exposure, development, and aesthetic character of the casein print.

History of the Casein Print

The principle behind the casein print — like the gum bichromate — rests on Mungo Ponton's 1839 discovery that potassium dichromate is light-sensitive, and Alphonse Poitevin's subsequent finding in the 1850s that dichromated colloids (gelatine, gum arabic, albumin, casein) are hardened by exposure to light. Where the exposed colloid is hardened, it becomes insoluble in water, trapping the pigment. Unexposed areas remain soluble and wash away, leaving the pigmented image. Casein was explored as an alternative colloid in the late nineteenth and early twentieth centuries, valued for its finer particle structure and smoother tonal rendering compared to gum arabic. It was used in the Artigue and Fresson processes — commercial photographic printing processes that produced prints of extraordinary beauty — and was employed by a number of pictorialist photographers in the 1900s and 1910s.

Casein as a Colloid

Casein is the principal protein in cow's milk, comprising about 80% of the total protein content. For photographic use, it is prepared as a solution by dissolving casein powder in an alkaline solution — typically ammonium carbonate or borax dissolved in water. The resulting casein solution is a smooth, slightly viscous liquid that forms a fine, even coating on paper. Compared to gum arabic, casein produces coatings with a finer grain and smoother surface texture. The casein layer, when sensitised with ammonium or potassium dichromate and mixed with watercolour pigment, responds to light in the same way as gum: exposed areas harden, unexposed areas wash away. The fineness of the casein coating gives the casein print its characteristic smoothness and subtlety.

Sensitiser Preparation

Prepare a casein solution by dissolving casein powder in warm water with ammonium carbonate (or borax) as the alkali. The solution should be smooth and free of lumps — straining through muslin or cheesecloth helps. Mix the casein solution with watercolour pigment of your choice (any high-quality watercolour pigment will work — ivory black, burnt sienna, Prussian blue, Indian red). Then add ammonium dichromate or potassium dichromate solution as the sensitiser. The proportions of casein, pigment, and dichromate affect the contrast, colour density, and character of the print. Experimentation is essential to find the balance that suits your aesthetic goals and your negatives.

Paper and Coating

Use heavy, internally sized fine art paper — Fabriano Artistico 300gsm, Arches Aquarelle, or similar — that can withstand repeated soaking and brushing during development. Some practitioners pre-size the paper with gelatine or a dilute casein solution to improve coating evenness and prevent the pigmented sensitiser from penetrating too deeply. Coat the paper with the sensitised casein-pigment mixture using a wide, soft brush (a hake brush works well) under subdued tungsten light. Apply the coating evenly, working quickly before the casein begins to set. Dry in darkness. The coated paper should be exposed within a few hours.

Exposure and Development

Place the contact negative in firm contact with the coated paper in a contact printing frame. Expose to UV light — the sun, a UV light bank, or a UV exposure unit — for a time determined by testing. After exposure, develop the print by floating it face-down in a tray of water at approximately 20-25°C. The unhardened casein-pigment mixture slowly dissolves and lifts away, revealing the image. Development is gentle and gradual — do not agitate excessively. A soft brush can be used to gently encourage development in areas that are slow to clear. Development may take fifteen minutes to an hour or more. The process is self-limiting: once all soluble pigment has been removed, the image is complete. Dry flat.

Multiple Printing and Colour

Like the gum bichromate print, the casein print can be built up in multiple layers — re-coating, re-sensitising, re-exposing, and re-developing with the same or different pigments to build density, extend the tonal range, or introduce colour. Three or four layers are commonly used for full-toned prints. Each layer adds density and richness. Multi-colour casein prints — using different pigments for each layer (for example, cyan, magenta, and yellow) — can produce full-colour images of remarkable subtlety. Registration must be precise for colour work, and each layer must be thoroughly dried before the next is applied.

The Casein Print Aesthetic

The casein print is often described as having a quality somewhere between the gum bichromate and the carbon print. It is smoother and finer-grained than gum, with more delicate tonal gradation. The matte, velvety surface has a quiet, contemplative beauty. The casein print rewards patience and careful technique — the preparation of the casein solution, the coating, the development, are all craft processes that demand attention and care. The rarity of the process today adds to its mystique: a well-made casein print is one of the most beautiful and unusual objects in the world of handmade photography.