This little gadget was a popular tool in the 20th century.
Its simplicity is deceiving as the design is scientifically driven. This ceramic disc is a milk boiler/pot saver/pot minder/milk saver/boilover preventer/milk guard - an object of many names and all descriptive of its use.
Its main purpose is preventing milk boiling over on the stove top. As many of us will know, you can look away for one short moment, but if it is at the crucial time the boilover happens suddenly and you are left with a big mess.
This milk boiler design is a modernised version that was made in the 1940-50s.
Before that there was an Edwardian style, which was produced at the start of the 20th century. The Edwardian style stood tall in the pot and had a waved foot. It would chatter when the milk was coming close to the boil, warning the cook.
The new and improved disc style reduced storage requirements and offered an additional service.