Recent I was asked this “What material has the best emissivity?”.
It is a tricky question to answer as first I need to figure out what is meant by “best”. Best could mean, best highest, or best lowest.
Emissivity it not just a material property, it is affected by other factors.
The object with the highest emissivity is called a “blackbody”, this would have an emissivity of 1. Blackbodies however are theoretical objects. The nearest true object to this would be a blackbody simulator, these are used for calibration and would have a typical emissivity of 0.996, so almost black (not to do with the colour black). This is an object constructed to have a high emissivity, and is typically a box pierced with a hole. Normally the highest emissivity that we come across is paint, which can have an emissivity up to about 0.96.
The object with the lowest emissivity would be a flat plate made of polished metal. Polished metals can have an emissivity as low as 0.01.