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.
The grooved side is placed down in the pot, with milk poured over. A common feature of these pot boilers was the printed instruction of "this side up" to ensure correct usage and prevent avoidable boilovers.
Like the Edwardian style, this disc would chatter in the pot giving audible warning to the cook. An extra feature was its bubble distribution system (the swirl grooves).
As the milk heated, the hot molecules would flow around the swirl and shoot up through the hole to the surface and would break it, preventing a skin from forming.
Milk boils at 100.2C and with the pot boiler the milk cannot get hotter than 98.9C (at sea level) because the ceramic disc absorbs a lot of the heat.
The milk boiler was most commonly ceramic, but glass and metal discs were also produced. This example was made by Davis Bradley Ltd, of London, and was donated to the museum by Mr H.A. Humphreys.
I am not aware of any New Zealand commercial manufacturers of milk boilers but in Australia Fowler Ware, Wembley and Austco produced similar milk boilers.
Microwaves came into New Zealand homes in the early 1970s and I would suggest that as this new technology was embraced, the market for pot boilers diminished. The efficient ease of heating milk in the microwave was appealing to many, although there is still the risk of boilover.
Take caution out there.