Kodachrome

The dye colours are cyan, magenta and yellow — the subtractive primaries — and each records the opposite additive colour: cyan records red, magenta records green, yellow records blue. Write that pairing down; it is the single thing people get wrong.

Bloom 12 g gelatin in 80 ml cold water for 10 minutes, then warm to about 50 °C until clear.

Divide the warm gelatin into three equal portions.

Tint one cyan, one magenta, one yellow with food colouring. Keep them pale — you are stacking three, and density multiplies.

Pour the cyan layer thinly onto watercolour paper or a flat sheet. Let it gel 10 minutes.

Cast a thin gelatin separator over it. The patent's Figure 1 labels this an “inert layer containing yellow dye”, not a clear one: it is a yellow filter that stops blue light reaching the layers below, which would otherwise fog them. Tint this separator faintly yellow and note why.

Add the magenta layer, then another inert separator.

Add the yellow layer last, on top. You now have the Kodachrome stack in order.

Dry flat, then hold to the light. The stack should read as a dark neutral where all three overlap — subtractive mixing removes, it does not add.

Cast three small two-layer test patches: cyan+yellow, cyan+magenta, magenta+yellow. Predict each result before you look. Record predictions and outcomes.

Make a second stack with the yellow layer at the bottom. Compare against the first under the same light. Kodachrome puts blue-sensitive on top because blue light is absorbed first and never reaches the lower layers otherwise.

Photograph a bright window through one, two and three layers at fixed camera settings. Compare brightness. Each layer subtracts.

The patent's method treats each layer differently, selectively bleaching and redeveloping one stratum without disturbing the others. Draw the three-layer stack and mark which layer is being worked at each stage. Kodachrome development ran to roughly 27 steps.

US 2,113,329, “Color photography”, Leopold D. Mannes AND Leopold Godowsky Jr., assigned to Eastman Kodak, filed 27 February 1935, granted 5 April 1938. Two inventors — do not credit one.

The date everyone quotes is the wrong one. Kodachrome is universally dated to 1935; that is the filing date and the market launch — the first product, a 16 mm movie film, went on sale 15 April 1935. The patent grant is 1938.

Mannes and Godowsky were working musicians, not staff chemists, when they began. Kenneth Mees, Kodak's research director, hired them in 1930 and gave them a laboratory. Their solution used a controlled-diffusion bleach process and a development procedure of roughly 27 steps — which is why Kodachrome could never be home-processed and had to be returned to a Kodak laboratory.

Why dye-in-development was the breakthrough. Putting dyes or couplers into the emulsion before exposure degrades sensitivity and shelf life. Forming the dye at the moment of development, in place, alongside the silver image, keeps the unexposed film simple and stable. Kodak also filed in Europe, including the German applications DRP 723388 and DRP 631324.

Kodachrome's archival stability was exceptional — properly stored transparencies from the 1940s still hold their colour, outlasting many later films. Processing ended in 2010 when the last laboratory closed, which is the practical death of a film: not when it stops being made, but when nothing can develop it.