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Thread: At what temp do you see dull red glow ?

  1. #1

    At what temp do you see dull red glow ?

    The so-called Draper point is the temperature where one sees an object (regardless which substance) glowing dull red. Many sources say 525C but other 470C. Depends on how dark your environment is.
    I checked more times in a cooling furnace in a dark room the temperature when I was still seeing very dark red glow. I measured 480C.
    In 'normal' ambient light (1000lx), even just molten aluminum (660C) does not glow or very dull red.

    One time I did an experiment melting copper in a flame in bright sunlight (> 40000lx) where I could really see the pink luster of the liquid copper and barely see the orange red incndescence. See here:

  2. #2
    Senior Member
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    Jul 2008
    Suffolk, UK
    THere is a shift in the eye response towards the blue spectrum at low light levels (the intensity of a colour is determined differently as the eye shifts between cones and rods). This would complicate the accuracy at which you can detect the red glow. This along with the obvious contrast issues

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  4. #4
    Senior Member SolarFreak's Avatar
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    Jun 2011
    Northern Kentucky
    I've noticed similar difficulties and variance seeing the red glow of metals varying with the light intensities. Pouring during the day, in full sun, molten aluminum looks like mercury. At night, it's a whole different story. Aluminum looks like blood at night, ingots glow, cats and dogs can be found living together... I'm sure the perception varies from one person to the next, given all other factors are identical.
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    Member oldgoaly's Avatar
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    little harder to see straw and blue on refractory, but reds and yellows are fairly close.
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  6. #6
    Administrator Site Admin Anon's Avatar
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    Straw and blue are oxidation colors for steel and can be a quite accurate method of judging temperature, but of course it only works on clean unoxidized steel in an oxidizing atmosphere. The actual incandescence colors look reasonable for a dull surface in moderate light; my only issue with them is that they're quoted as exact temperatures rather than ranges (and the C values are quoted to the single degree instead of rounded off, which is just silly). As already mentioned, the perceived glow can be very different between a shiny surface and a dark one, and between full sun and nighttime.
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