Letter Re: A Useful Heated Steel Color Emission Chart

Dear James:
I came across this table in a reference book and thought it may be useful to everyone. Note: This chart should not be used as a guide to combating fires. Remember all fires are dangerous, and you should call the fire department, if that is a possibility, when you see flames. All degrees are in Fahrenheit below.

Yellow 450 degrees Fahrenheit
Brown to Purple 550 degrees Fahrenheit
Blue 600 degrees Fahrenheit
Faint Red 900 degrees Fahrenheit
Dark Cherry Red 1,100 degrees Fahrenheit
Full Cherry Red 1,400 degrees Fahrenheit
Salmon 1,600 degrees Fahrenheit
Lemon 1,800 degrees Fahrenheit.
White 2,200 degrees Fahrenheit
Sparkling White 2,400 degrees Fahrenheit

Regards, – Mikael

JWR Replies: Of course all the usual torch and metal-working shop safety rules apply.

That chart, BTW, is handy companion piece to the Combustion Temperature Reference that was posted previously in SurvivalBlog. I recommend printing out hard copies of both posts for your shop reference binders. Keep in mind the standard provisos that the true measurement of the volatility of a stored material is its “flash point”, which in most cases is considerably lower than the flame point figures noted in the Combustion Temperature Reference.

Also, when using color as a reference for gauging the temperature, keep in mind that the ambient light available can skew the color observed. Holding up a piece of metal in the dim light of a blacksmithy will not show the same color as holding up the same piece of metal heated to the same temperature in bright daylight. This can lead to heat-treating errors. This was best illustrated in the classic book Hatcher’s Notebook.” In it, Colonel Julian Hatcher recounted the story of the “Low Number Springfields”, that many shooters in the current generation might not have heard: Here it is in a nutshell: The smiths at the Springfield and Rock Island Armories were manufacturing Model 1903 Springfield rifles. One of the steps in the process was heat-treating the receivers to a certain color of redness. This was before the days of precise industrial pyrometers–back when heart treating was judged “by eye”.) It was found that some of those receivers failed–due to the heat treating being of insufficient hardness. The Board of Inquiry discovered that some receivers that were heat treated on overcast days, lacked sufficient heat treating (and blew up dramatically when fired), while those made on sunny days had the specified strength. This was because on overcast days, the heated receivers showed the correct “color” when they had not yet actually reached the requisite temperature. This failure in process control was of course soon corrected, but ever since, “low number Springfields” have not been trusted for full-pressure pressure .30-06 loads. (The manufacturing transition BTW, was with Springfield Armory M1903 rifles that had serial numbers below 800,000 and Rock Island M1903 rifles with serial numbers below 285,507.) Just an interesting historical tidbit…