At the Decker Communications blog on April 27, 2017 there was a post by Ben Decker about feedback titled Iron Sharpens Iron. The King James Bible version of Verse 17 from Chapter 27 in the Book of Proverbs says:
“Iron sharpeneth iron; so a man sharpeneth the countenance of his friend.”
This verse typically is interpreted as being positive. For example, a web page at Got Questions.org titled What does it mean that iron sharpens iron? says:
“There is mutual benefit in the rubbing of two iron blades together; the edges become sharper, making the knives more efficient in their task to cut and slice.”
Our common experience is that to sharpen an edge you use a tool like a file or a whetstone to remove material. We might picture that one of those ‘blades’ was a steel file used to hand sharpen the edge of a tool such as a Vermont farmer’s scythe, as is shown above in an image from 1937. The Wikipedia article on scythes says they first appeared in about 500 B.C. Scythes might not have been around when the Book of Proverbs was written, but other edged tools like knives or swords were.
That Got Questions page refers to a 1995 book by Howard and William Hendricks titled As Iron Sharpens Iron - building character in a mentoring relationship. On page 18 it explains:
“The Bible puts it this way: ‘As iron sharpens iron, so one man sharpens another.’ (Proverbs 27:17). Have you been sharpened against the whetstone of another man’s wisdom and character.”
I found a May 21, 2016 blog post by Justin Taylor at The Gospel Coalition titled What If “Iron Sharpening Iron” in the Book of Proverbs is actually Something to Avoid? He referred to a magazine article by Ronald L. Giese, Jr. titled “Iron Sharpens Iron” as a Negative Image: Challenging the Common Interpretation of Proverbs 27:17 that appeared in the Journal of Biblical Literature, Spring 2016, pages 61 to 76 (Vol. 135, No. 1). Dr. Giese concluded:
“The meaning of the verse is therefore along these lines: Just as a hard iron hammer pounds soft iron into something sharp, ready for battle, in the same way a man causes his neighbor to go on the attack (i.e., have a ‘sharp face’).”
If we assume that the Bible is referring to wrought iron, I agree with Dr. Giese that the typical positive interpretation is wrong. He notes that there weren’t iron files back then, so the rubbing interpretation is not valid. But there is one more important detail to complete the meaning for that verse.
Wrought iron doesn’t behave like steel. Steel contains enough carbon that it can be heat-treated to produce hard iron carbide particles. Wrought iron is low carbon iron (plus slag particles), without enough carbon to harden it like steel.
The only way to strengthen (or harden) wrought iron is by work hardening. That means permanently deforming it at room temperature, typically by hammering on the edge supported by an anvil (a process called peening). The Wikipedia article on swords notes this is the same method previously used for bronze swords. You don’t remove material – you just change its shape, which increases its strength.
There is a section by Michael Bussell on wrought iron in the Construction Materials Reference Book, which I found at Google Books. Table 6.8 says that wrought iron has an ultimate tensile strength (UTS) of 53.1 ksi in the hot rolled condition and increases to 86.1 ksi in the cold drawn condition. Similarly, wire in the hot rolled condition has a UTS of 53.1 ksi, versus 92.1 ksi in the cold drawn condition.
Wikipedia says softer European scythes still are peened. What “iron sharpens iron” really means is likely as shown above for peening a scythe blade, a 2014 image from Gilles San Martin. You also can watch the process in an eight-minute YouTube video by Botan Anderson titled Zen and the art of peening a scythe blade.
Writing this post was interesting. I haven’t looked up information about the behavior of wrought iron for ~25 years and that was the only time in my consulting career. It was back when I worked in Columbus, Ohio. After the 1991 fire at Adelbert Hall, a civil engineer I worked with was reviewing documents regarding rebuilding that included measurements of the strength of wrought iron structural beams. He asked me to check if there were any ASTM standards, like those he was used to seeing for structural steels. I finally found a printed copy of the old ASTM standard on the top floor of the main library at Ohio State University. To locate it, I had to look in their old card catalog rather than the current computerized one.