One step up from this [putting your hand in a hot oven to test its heat] was the paper test. This was much used by confectioners in the nineteenth century. The point here was not to gauge the fiercest heat as you stoked a fire up, but the subtle gradations of gentler warmth as they oven cooled down, suitable for baking cakes and pastries, whose high butter and sugar content made them much more liable to burn than bread. Each temperature was defined by the color a sample of white kitchen paper turned when put on the oven floor. First, you had to put a piece of kitchen paper inside the oven and shut the door. If it caught fire, the oven was too hot. After ten minutes, you introduced another piece of paper. If it became charred without burning, it was still too hot. After ten minutes more, a third piece went in. If it turned dark brown, without catching fire, it was right for glazing small pastries at high heat: this was called 'dark brown paper heat.' ... A few degrees below dark brown paper heat was 'light brown paper heat, suitable for baking vol-au-vents, hot pie crusts, timbale crusts, etc.' Next came 'dark yellow heat,' or moderate temperature, good for large pastries. Finally there was the gentle heat of 'light yellow paper heat,' which ... was 'proper for meringues.' A variation was the flour test, which was the same but with a handful of flour thrown on the oven floor; you were meant to count to forty seconds; if the flour slowly browned, it was right for bread.
from Consider the Fork: A History of How We Cook and Eat,
by Bee Wilson