And those in between are in between.  Generally, colors aren't a great guide, because my definition of "cherry red" is going to be different from yours (and likewise from everyone's), and ambient lighting skews things.  Personal experience, especially with a knowledgeable guide present, is going to be your best guide.

Unless you have a glowing version right next to your forge, no, it isn't really that helpful.  That was my original point.

I get about as irritated when people use these things as Jarrod does when people talk about tempering colors.  I have seen them be 200% off with surfaces that had emissivities that were odd.

Then there are the other alloying elements we use that blacksmiths do not. Our steels have more manganese, chromium, vanadium, silicon, tungsten, nickel, and other fun things that can cause problems if forged too hot or too cold.  Problems like turning into the steel equivalent of cottage cheese when too hot, or filling with microfractures when too cold.  With mild steel all you have to worry about is burning or breaking at higher and lower temperatures than we usually use.

What is the cutoff frequency of the lens, the diffraction limit? We can calculate it using the same two parameters we used to calculate the radius of our airy disks. The equation is:

Going back to the original post, one thing nobody has addressed (although Mr. Giagni came close) was the question of whether the steels in the blacksmithing book were the same as those in the bladesmithing book.

Diffraction limited systemformula

You don't see us using IR thermometers because they are both expensive and do not work on glowing steel.  They are calibrated for black body emissivity, and if you throw incandescent heat at them they freak out.  That one does spec that it reads up to 2012 degrees F, which is much higher than I have seen on any other unit, but I have my doubts it does so with any accuracy.  When we need to know the temperature we use a pyrometer setup consisting of a type K thermocouple and something to read it with.

The radius of the Airy disk depends on two important factors: the wavelength of light and the size of the aperture, and we can estimate it with the equation: Ra = 1.22λ/2Na. In this equation Ra is the airy radius, λ is the wavelength of illuminating light, and  Na the numerical aperture.

I'm a little confused - I've been reading "The Backyard Blacksmith" as well as "The Complete Bladesmith" (referred by this forum) and both authors refer to the most effective "temperature colour indication" for mild steel. However, Lorelei Sims says "forging is most effective when the metal has turned yellow"(2100°F)... while our friend Jim Hrisoulas says that "most of the carbon steels" beginners will use should be worked at "the cherry red range" (1500°-1800°F). I'm fairly certain they're considering the same steels so what is up? If I'm just starting with 10 series steel what guideline should I use?

The diffraction limit tells us that there is a limit to the resolution of images we can obtain, no matter how much we improve our optical system. Although this limit is not important for everyday photography or situations in which resolution is already limited by other significant optical aberrations,  it is important to microscopy, telescopic, and high resolution photography.

Abbediffractionlimit derivation

Going back to the original post, one thing nobody has addressed (although Mr. Giagni came close) was the question of whether the steels in the blacksmithing book were the same as those in the bladesmithing book.

Observation and experience are far more valuable than color charts since there are too many variables involved that affect color both to an individual's perception and in individual environments.

Diffractionlimit formula

I had the chance to play with an IR gun that was supposed to work on liquid steel.  You had to determine the emissivity constant for that grade, but once you did that and plugged it into the unit it worked fairly well.  Until smoke or slag got in the way.  Which it did.  Constantly.  And that gun was $30,000.

That is the tempering color chart I was referring to (note that it only goes up to 600F).  The OP was asking about forging temps.  Do yourself a favor and forget that the chart you found even exists.

Diffraction limited systempdf

How does this relate to the limited diffraction?  Imagine a perfect lens with no aberrations, not limited by design or manufacturing flaw. Each point light source will create an airy disk, and when an image is properly resolved the airy disks do not overlap. When the outer rings merge, the sensor will be diffraction limited. The only way to improve resolution will be to increase the numerical aperture. But when the center of each airy disk merges, we’ve reached the limit of resolution; the diffraction cutoff frequency. When you’ve reached the limit of resolution you’ll not be able to resolve further even by increasing aperture.

Diffraction limitedtelescope

As the numerical aperture increases, the radius of the airy disk decreases. In modern optics, the numerical aperture can be as high as 1.4-1.6.  The table below shows the diameter of an airy disk for lenses of different apertures when the lens is illuminated with light at 520 nm.

Diffraction limited systemwikipedia

And before you get to tempering: please ignore temper colors.  They are not accurate enough.  Get a method of using thermal mass and an accurate thermometer.

The diffraction limit is essentially the resolution limit that is imposed on an optical system by the wave nature of light and the process of diffraction.  To understand this, let’s look at what diffraction does when light shines through an opening.

As Alan said, the inaccuracy gets worse once the surface is incandescent. There are optical pyrometers out there that are supposedly good for glowing materials, but I haven't had the opportunity to use one yet.  The ones I have seen for sale are quite expensive.

No, they are not.  Blacksmiths use mild steel, low carbon in in the 0.18-0.25-ish %.  Bladesmithing steels are in the 0.60-1.20% carbon range, which makes a huge difference in behavior.

No; the passage of light through the atmosphere introduces significant distortions that limit performance to a lower resolution than provided by diffraction limited observation.

Diffraction limited systemapplications

For visible light microscopy, the diffraction limit— the smallest detail that can be resolved— is about 200-250 nanometers.

No, they are not.  Blacksmiths use mild steel, low carbon in in the 0.18-0.25-ish %.  Bladesmithing steels are in the 0.60-1.20% carbon range, which makes a huge difference in behavior.

Ahhh... thanks! That would explain the difference..they both used the same terminology and didn't seem to contradict each other at any other point. Odd though, I went back and looked and while Jim mentioned 'carbon steels', Lorilei never mentions what particular steel she is working with in that section.

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Lastly, I don't have a lot of equipment and tools, but one thing I do have access to is a Temperature Gun as shown here: http://www.reedinstruments.com/product/reed-r2007-dual-laser-infrared-thermometer        I'm kind of shocked I've not seen it used before in any of the online videos or instructionals, but I thought, as a beginner, it might help me pinpoint these 'colours' more effectively...at least starting out.

You don't see us using IR thermometers because they are both expensive and do not work on glowing steel.  They are calibrated for black body emissivity, and if you throw incandescent heat at them they freak out.  That one does spec that it reads up to 2012 degrees F, which is much higher than I have seen on any other unit, but I have my doubts it does so with any accuracy.  When we need to know the temperature we use a pyrometer setup consisting of a type K thermocouple and something to read it with.

Spend some time initially with your steel getting used to what it tells you. For instance. Heating it up initially use a magnet as the steel heats. When the steel stops attracting the magnet, with 10xx series you are in the 1415-1425 F range. Get to know what that color looks like to your eyes in those conditions. Next you want to watch for the decalescence effect. This will be of great importance in the hardening process. Get to know what color, again to you, that occurs at. The next milestone is when sparks come off the blade. In technical terms, for the purpose of forging on  an homogeneous bar, it is called "too frigging hot !" You are burning the steel. Don't do that again.

Any lens has a finite aperture, and when light passes through an aperture— no matter its size— diffraction will occur. We call a diffraction pattern an Airy disk, and it is disk shaped. There is a bright area in the center surrounded by concentric rings,  and each ring decreases in brightness as you move out. This Airy disk is the key to the diffraction limit, as it is the Airy disk which is the smallest point to which you can focus a beam of light. We sometimes call this the ‘minimum spot size’ of an optical system.

Yep!  Lorelei is using mild steel, which over here is usually A36, a grade of remelted scrap that does not have a carbon content specification.  It just has have a minimum tensile strength of 36 Kpsi.  The carbon can vary wildly, but there's never enough for a good blade.  We can also get 1018 and 1020 if we're willing to pay extra for clean, easier to weld mild steels.

diffraction-limitedspot size formula

This article— part two of our series on “Avoid Optical Pitfalls”— will look at what the diffraction limited is, how to calculate it, and how it can affect the resolution of high performance optics.

Many years ago I was a test engineer in an elevator manufacturing plant.  I got into an argument with a design engineer over a new product were were making.  There was a shiny (reflective) copper plate in the product that I was concerned was getting too hot.  This guy came out with his IR temp gun and told me there was no issue, that it was only 120F.  You could see the heat (and smoke) coming off of it, so I told him to touch it if he felt his measurement was accurate. (He declined)  The guns are tuned for something that has an emissivity near 0.95.  (A perfect black body is 1.0) A shiny surface will be much lower.  If I remember right, I back calculated that the copper must have been closer to 0.3, but that was 20 years ago.

The "IR guns" do have their place, and I actually own two of them.  I keep one in the kitchen to check the temp of my cast iron frying pans before the food goes in, and I use another to check the shell temps of my pet tortoises.

To put it in simple terms forging should happen in that zone between ""tfh" and when it cools to the point it stops moving readily under the hammer. Your reading should have told you the effects of forging too hot or too cold.

There’s a mistake people often make when assessing the performance of a microscope or telescope— forgetting the diffraction limit.  It is a theoretical limiting factor governing the maximum obtainable resolution of an optical system. When we leave it out of our calculations we may end up with very unrealistic expectations.

"The Backyard Blacksmith" mostly deals with mild steel which can be forged a lot hotter than the steels used for knives.