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Blacksmithing and metalworking questions answered.

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Forge and Furnace Heat Colors

Can You Trust Your Eyes?

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.

Fahrenheit  Color  Celsius
3,000°   1,650°
2,900°   1,600°
2,800°   1,540°
2,700°   1,480°
2,600°   1,430°
2,500°   1,370°
2,400°   1,320°
2,300°   1,260°*
2,200°   1,200°
2,100°   1,150°
2,000°   1,100°
1,900°   1,040°
1,800°   980°
1,700°   930°
1,600°   870°
1,500°   820°
1,400°   760°*
1,300°   700°
1,200°   650°
1,100°   600°
1,000°   540°
0-900°   480°
Fahrenheit  Colour  Celsius
Temperature Indicating Cones These are specially formulated clay points that slump or melt at a given temperature. These are used primarily in the ceramics industry. Sets of three are commonly used when the temperature achieved is critical.

Thermocouple and Meter A thermocouple is a bimetallic junction made from special alloys that produce a voltage from heat. They come in various types depending on the temperature range to measure. The type J Nichrome, Alumel thermocouple is very common with a range up to 2,300°F (1260°C). The thermocouple is used with a DC millivolt meter calibrated in degrees to match the type of thermocouple. These are suitable for kilns and heat treating furnaces but forges will burn the junction.

Infrared Meters are commonly gun type devices that are pointed at the heat source and read directly. They come in various ranges depending on the maximum temperature and accuracy needed.

Links & Resources:

Tempil Guide to Ferrous Metallurgy PDF
Tempil Guide to Ferrous Metallurgy PDF

For over half a century the Tempil Guide was published as a wall chart and as handouts but is no longer available from the publisher nor officially online as a PDF.

Shows the heat color temperatures, percentage of carbon, phase diagram, crystal size and describes each feature.

Scale based on Tempil Ferrous Metallurgy chart and others. Temperatures +/- 100°F for discussion only.

2012, 2019 Jock Dempsey,

Copyright © 1998, 2020