Milk Plastic

Measure 240 ml of whole milk into a heatproof jug and write the volume down. Whole milk gives the best yield — skimmed milk works but produces a smaller, more brittle lump.

Heat the milk until a thermometer reads 50-60 °C. Do not boil it. Hot milk curdles faster and more completely; boiled milk gives a stringy, hard-to-handle curd.

Add 60 ml (4 tablespoons) of white vinegar to the hot milk and stir for one minute. The acid pulls the milk down toward pH 4.6, where casein is least soluble.

White rubbery lumps form and the liquid around them turns thin and yellow-green. The lumps are casein; the liquid is whey, which is water, sugar and the whey proteins that do not coagulate at this pH.

Pour the mixture through a fine sieve or a square of muslin. Keep the solids. Let it cool until it can be handled comfortably.

Press the curd between paper towels and keep changing them until no more liquid comes out. Trapped whey is the commonest cause of a piece that cracks or moulds while drying.

Weigh the pressed curd and record it. From 240 ml of whole milk expect roughly 15-25 g of wet curd — the exact figure depends on the protein content of your milk, so it is worth comparing brands.

Knead the curd for two minutes until it stops being crumbly and behaves like stiff dough. Kneading works the protein chains together — an unkneaded piece dries into a crumbly biscuit rather than a solid.

Work in a few drops of food colouring now, while the dough is still damp. Industrial casein plastic was dyed the same way, in the mass, not painted afterwards.

Press the dough into a cookie cutter or silicone mould, or roll it flat and cut shapes. Push it well into the corners; it will not flow to fill them on its own.

Leave the pieces on absorbent paper somewhere dry, turning them once after a day so both faces lose water evenly. Thick pieces need longer and warp if dried fast.

Measure a piece before and after drying. The loss is water leaving the protein network — expect a noticeable shrink, which is why casein plastic was always machined to final size after hardening, never moulded to size.

Sand the edges smooth with fine paper. Dry casein plastic cuts, drills and takes a polish like a soft horn — this is exactly why the button industry adopted it.

Drop one piece in water for an hour. It swells and softens. This is the honest result, not a failed build — untreated casein plastic is not waterproof, and that single fact is what the entire industrial hardening stage existed to fix.

Older than the industry. Casein glue and casein paint are ancient; the idea that the same protein could be a solid material in its own right is late nineteenth century. In 1893 the French chemist Auguste Trillat found that soaking casein in formaldehyde made it insoluble. In 1897 in Germany, Adolf Spitteler and Wilhelm Krische patented a casein material — the origin story attached to it is that the work began with a commission for a washable white schoolboard rather than for a plastic at all.

Galalith. The product was named from the Greek for milk and stone. It was hard, took brilliant colours, could be polished to a deep shine, and did not burn easily — a serious advantage over celluloid, which did. It became the button material of the first half of the twentieth century, and also went into buckles, combs, knitting needles, fountain pens and costume jewellery.

Why it is machined, not moulded. Hardened casein cannot be melted and injected like a thermoplastic. It was made as sheet, rod, tube and button blanks, hardened, then cut, turned and polished to shape. The hardening is slow because the formaldehyde has to diffuse right through the piece: thin sheet takes days, and a block around 25 mm thick can take up to a year of immersion.

What replaced it. Petrochemical plastics after the Second World War were cheaper, faster to mould and genuinely waterproof, and casein plastic collapsed into a niche. It has never entirely gone: it is made from a renewable feedstock and is biodegradable, which is why casein materials keep reappearing in bioplastics research.

What you actually made. The same protein, coagulated the same way, kneaded and dried the same way. What you skipped is the formaldehyde bath — the step that turns a water-sensitive protein solid into a material you could sew onto a shirt and wash. It is documented in the companion blueprint for adults working with proper extraction; it does not belong in a kitchen.