Wedgwood scale

1783 Wedgwood scale

The book of science

Tom Sharp

Josiah Wedgwood thermometry Wedgwood scale

Wedgwood scale

A kiln would boil mercury and melt glass, so how do you measure how hot a kiln gets inside? Josiah Wedgwood made a temperature gauge and scale based on how much pipe clay shrinks as it heats. * In the kiln, Wedgwood placed a half-inch disk of dried pipe clay on an inclined metal plate between two metal bars marked with scale gradations spaced a half-inch apart at the top and three-tenths of an inch apart at the bottom. The clay as it dried slipped between the marked bars and where it rested marked the hottest temperature.

The scale

The onset of red heat, 1,077 °Fahrenheit, was zero degrees Wedgwood, and 240 steps added 130 degrees each up to 32,227 °Fahrenheit. This worked for his pottery business; however, although Wedgwood tried to calibrate his scale using the expansion of silver with increasing temperatures, his starting temperature and his temperatures for when metals melted were too high.

Workable degrees

I’m with Goldilocks; too hot is too hot; whatever feels right is, I say, peachy; and if it’s cold as ice it’s surely not nice.

Wedgwood’s pyrometer was horribly inaccurate; however, at least it didn’t melt or have its parts weld together in high temperatures. His metal plate with the scaled bars was not necessarily put into the kiln with the disk of clay, because the disk of clay would not expand again as it cooled.

See also in The book of science:

Readings in wikipedia: