In the forge, ceramic studio and welding shop we often judge how hot things are by eye. Anything glowing red to yellow is obviously very hot (over 1,000°F or 540°C). But just HOW HOT is it? In all these fields there are processes where temperature is critical. In commercial shops we use special tools to measure high temperatures. These include temperature indicating crayons, ceramic cones, thermocouples and infrared devices.
In many situations these devices are not available, ready or affordable. So we judge the temperature by eye. In some situations and with practice we become fairly close and if nothing else consistent. The biggest problem with judging temperature by eye is the ambient light.

As ambient light varies so does our ability to judge temperature by color. The brighter the light the less color there is to see as bright light masks the heat color with the surface color. In direct sunlight a high heat anywhere from 2,000 to 2,600°F appears nearly white and anything less than 1400°F may be imperceptible. For this reason I tell students to go by feel more than eye. This also helps to adapt to a range of steels where the lower forging range is much higher for some steels than others.

Prior to the ready availability of temperature sensing tools heat treaters often sat in nearly dark rooms and let their eyes adjust to the lighting before judging heats. In modern shops and when working outdoors the smith often keeps a shaded box that he can thrust a piece of steel into and observe the color.

Charts such as found in heat treating references and on this page are only accurate to +/- 200°F (110°C). Combine this with variations in ambient light and the error is much greater. In situations with consistent lighting, good references and experience we can do better but not a great deal. If temperature is critical we need measuring devices.

A2 Magnetic Point. Ferrous metals become non-magnetic at this point, about 1,410°F (765°C). The A3 hardening point is a curve and is as high as 1,675°F (913°C) and as low as 1,350°F (732°C) (the A1 point). This means the non-magnetic point can only be used for annealing and hardening plain carbon steels with carbon content between 60 and 80 points.

Temperature Indicating Markers (crayons) These come in a range between 110°F and 2,000°F (1093°C). They are used in two different ways. They are tested against the heated part and if they skid from melting then the temperature has been reached. They are also marked on the part to be heated and the lines observed melting as heat increases. Often a set of three is purchased to bracket the desired temperature.