The Casein Manufacturing Process
The 'dry process' for making casein plastics starts with rennet casein.

This was made at a dairy by adding rennet, an enzyme from the stomach of calves, to large vats containing highly skimmed milk. The precipitated casein was then dried at the dairy to form granules which were transported in sacks to the plastics manufacturers. Here the rennet casein granules were ground into a powder fine enough to pass through a 40 mesh sieve.

The following photographs (taken during the 1950s) show the principal features of the process used at Erinoid to make casein plastics.
This grinder was made in Germany around 1890 and started life as a flour grinder. It was used to grind casein from about 1914 until the factory closed in 1980 when it was presented to the Science Museum in London.
After grinding, the casein powder was moistened with water containing any necessary colorants and mixed in Artofex dough mixers, as used for bread. All of the water is absorbed by the casein which swells but appears quite dry - although it will just bind together if squeezed in the hand.
This mix was then fed into the hopper of an extruder which converted it into a continuous rod or strip which was still quite flexible but stiffened somewhat on cooling. Four of these Greenwood and Batley twin machines were delivered to Erinoid at Stroud in 1914. Initially they were gas heated.
This one was stripped down for cleaning, showing the short, powerful screws. Identical twin extruders were used throughout the years.
This extruder was modified to produce rod, striated to imitate horn, for slicing into button blanks.
Extruded sections, or chopped pieces were put into simple picture frame moulds. The manner in which material was placed in the mould determined the colour configuration of the moulded sheet. The moulds were loaded into multi-daylight presses, usually steam heated and loaded to 50 tons pressure. On removal from the moulds, the sheets were still soft and rubbery but after cooling for about half an hour were sufficiently stiff to be placed in racks ready for the next stage in the process.
Immersion in formalin tanks. The length of time they remained immersed depended upon thickness and temperature (it took much longer during winter). Up to 10 months or more for a sheet of 25 mm thickness.
Before about 1930, buttons were trepanned from sheet material by the button manufacturers. Then, the development of suitable machines allowed button blanks to be punched from unformalised sheet, and . . .
. . . later, unformalised rod was also sliced to make button blanks.
This had the advantage that button blanks, being smaller, required less time in the formalin tanks. Larger button blanks were formalised in perforated zinc boxes and small blanks were put into linen bags to be formalised, making them easier to handle.
During formalisation, the material hardened so that it became resistant to water and solvents and was no longer thermoplastic. Soaking in formalin inevitably caused some distortion (this is why very few moulded casein plastics are found). Sheet material required re-pressing to make it flat.
The material at this stage still contained excess moisture which needed to be removed. Rods and buttons were placed in rotary driers . . .
. . . while sheet material was conditioned in stock rooms.
After drying, button blanks were sorted according to thickness by rolling along the tapered gap between rotating cylinders - when the gap was wide enough they dropped through into the boxes.
Rods for knitting pins were packed for despatch after being trimmed to size on centreless grinders.