redsear
To break or crack through brittleness when red-hot; to be red-short or hot-short.
redsear: when hot metal suddenly cracks from brittleness
Redsear describes the catastrophic failure of metal when it is heated to a dull red state, typically between 700 and 900 degrees Celsius depending on the alloy. The metal loses its normal plasticity and becomes brittle, cracking or shattering under its own weight or under minimal mechanical stress. This is the defining characteristic of red-shortness or hot-shortness, material defects that have plagued ironworkers and steelmakers for centuries.
The phenomenon occurs when certain impurities or alloying elements segregate within the metal's grain structure during cooling. Sulfur is the primary culprit in iron and steel, forming iron sulfide films at grain boundaries that weaken the material specifically in the hot-working range. Copper and tin can cause similar brittle behavior in their respective alloys. A metal that will redsear is essentially unreliable for forging, rolling, or drawing operations that require heating.
Detection and consequences
A redsearing metal reveals itself immediately during hot work. A blacksmith heating a billet to forging temperature may find it crumbles or splits before he can even strike it with a hammer. In a rolling mill, the same behavior wastes material and time. Historical quality control was crude: testing bars were heated and bent, and if they fractured cleanly in the red heat range, the batch was condemned or sold as scrap. Modern steelmaking prevents redsearing through careful control of sulfur content (typically held below 0.05 percent in quality steel) and through desulfurization during smelting.
The term itself reflects metalworkers' vocabulary: the metal was described as having a sear or flaw that appeared when it turned red-hot. The word is now almost entirely historical in English-speaking trades, replaced by the more systematic term hot-short or the chemical description sulfur-embrittled. You may encounter redsear in old metallurgical texts, patent specifications from the 19th century, or in discussions of material standards prior to modern steelmaking practice. Understanding it remains useful for anyone reading technical literature from the era before consistent chemical control.