The Cohen Gadol (High Priest) had to take several ritual dips into a pool of water known as a mikveh. But the water was rather cold, and the Mishnah describes how it was warmed:
משנה יומא לא, ב
אִם הָיָה כֹּהֵן גָּדוֹל זָקֵן אוֹ אִיסְטְנִיס מְחַמִּין לוֹ חַמִּין, וּמְטִילִין לְתוֹךְ הַצּוֹנֵן כְּדֵי שֶׁתָּפִיג צִינָּתָן
With regard to the immersion, if the High Priest was old and found it difficult to immerse in cold water, or if he was delicate [istenis], they would heat hot water for him on Yom Kippur eve and place it into the cold water of the ritual bath in order to temper its chill so the High Priest could immerse without discomfort.
Today’s page of Talmud elaborates on this process of warning up the ritual bath:
יומא לג,ב
תַּנְיָא, אָמַר רַבִּי יְהוּדָה: עֲשָׁשִׁיּוֹת שֶׁל בַּרְזֶל הָיוּ מְחַמִּין מֵעֶרֶב יוֹם הַכִּפּוּרִים וּמְטִילִין לְתוֹךְ צוֹנֵן, כְּדֵי שֶׁתָּפִיג צִינָּתָן. וַהֲלֹא מְצָרֵף! אָמַר רַב בִּיבִי: שֶׁלֹּא הִגִּיעַ לְצֵירוּף. אַבָּיֵי אָמַר: אֲפִילּוּ תֵּימָא שֶׁהִגִּיעַ לְצֵירוּף — דָּבָר שֶׁאֵין מִתְכַּוֵּין מוּתָּר
It was taught in a baraita that Rabbi Yehuda said: They would heat blocks of iron on Yom Kippur eve and cast them into the cold water of the ritual bath to temper its chill. The Gemara asks: But by doing so, doesn’t he harden the iron, [which is a labor prohibited on Yom Kippur]? Rav Bibi said: The temperature of the blocks of iron did not reach the hardening point. Abaye said: Even if you say that the temperature of the iron reached the hardening point, the fact that the iron hardened when he placed it in the water is an unintentional act, which is permitted. [His intention was to temper the chill of the water, not to harden the iron.]
This process of hardening hot iron by rapidly cooling it is called quenching. And that is our topic for today.
Why is Iron Quenched?
The quick answer is that when it is rapidly cooled, steel (which is an alloy of iron) is transformed into something called martensite. The rapid cooling, which can be in water or oil or even cold air, locks the carbon atoms into place alongside the iron, forming a supersaturated alloy that is much harder than ordinary steel. But for the transformation to take place the steel needs to be hot enough. As John D. Verhoeven points out in his useful (but expensive) book Steel Metallurgy for the Non-Metallurgist, not much happens to the structure when steel is below about 300F (~150C). For the steel to actually be strengthened, it is often heated to over 1,110F (540C). This is what Rav Bibi was referencing when he mentioned “the hardening point.”
A Quick History of Quenching
One of the earliest references to smelting and blacksmithing comes from the opening chapters of our Bible (Gen. 4:22):
וְצִלָּ֣ה גַם־הִ֗וא יָֽלְדָה֙ אֶת־תּ֣וּבַל קַ֔יִן לֹטֵ֕שׁ כָּל־חֹרֵ֥שׁ נְחֹ֖שֶׁת וּבַרְזֶ֑ל וַֽאֲח֥וֹת תּֽוּבַל־קַ֖יִן נַֽעֲמָֽה׃
As for Zillah, she bore Tubal-cain, who forged all implements of copper and iron. And the sister of Tubal-cain was Naamah.
So it was Tubal-Cain who, according to the Torah, was the archetypal father of all things smelting. Archeologists are not sure where or when steel was created, but it was certainly described in the poems of Homer from around the eighth century BCE:
And as when a smith dips a great axe or an adze in cold water amid loud hissing to temper it—for therefrom comes the strength of iron—even so did his eye hiss round the stake of olive-wood.
But as D. Scott Mackenize noted in his helpful article published in 2007 in Advanced Materials and Processes, the advances in metallurgical technology were located in the Arab World, India, China and Japan. By 500 BCE steel was being produced on a large scale in India. The Crusaders, he wrote, had no steel that was the equal of Islamic metallurgy. One writer from the Crusades described the the quality of a Damascus blade like this: “One blow of a Damascus sword would cleave a European helmet without turning the edge, or cut through a silk handkerchief drawn across it.”
The Italian scholar Giambattista della Porta (ca. 1535-1615) in his books Natural Magic described the temperatures of steel to be quenched:
“When the iron is sparkling red hot, that it can not be hotter, that it twinkles, they call it Silver; and then it must not be quenched, for it would be consumed. But if it be of a yellow or red color, they call it Gold or Rose color; and then quenched in Liquors, it grows harder. This color requires them to quench it. But observe that if all the Iron be tempered, the colour must be blue or violet color, as the edge of a Sword, Razor, or Lancet; for observe the second colors; namely, when the iron is quenched, and so plunged in, grows hard. The last is Ash color; and after this if it be quenched, it will be the least of all made hard.”
Mackenzie noted that his was a critical observation. “He indicates a critical range for quenching, based on the colors of the heated steel. Only when the steel is rose or yellow will the steel be hardened properly. Further, the observation of tempering colors was indicated… it led Della Porta to realize the advantages of the two-stage quench over a direct quench, and reject some of the more exotic quenching baths that was cited in earlier metallurgical literature.
One French traveller to Istanbul at the end of the nineteenth century made this observation:
"...Steel is iron, mixed with charcoal. In Damascus, 10-12 kilograms of iron was required for making one sword blade. Craftsmen mixed this ore with charcoal dust, melted it again and again, until it came to a consistency of their mind.
...Now it was required to quench it in order to give it the necessary strength, and that was the interesting point of the procedure: Europeans quench the steel in water, vegetable oil, or cattle fat, but in the East they were doing it on air. When the craftsmen were done with the processing of the metal, they heated it until totally red, and gave it to a cavalry man waiting on his horse, ready for a ride. The cavalry man rode his horse in the wilderness, waving the blade in the air with crazy screams to make his horse ride faster."
This page of Talmud reminds us that sometimes the sages of the Talmud were keenly knowledgable about manufacturing processes. Rav Bibi knew a thing or two about how to quench steel, and as a result the Cohen Gadol had a warmer dip in the mikveh on Yom Kippur.
“From the earliest times, at the beginning of the Iron Age, quenching has played an important role in the growth of civilization throughout the World. .. While much of the empirical technology developed was used to increase the effectiveness of swords, knives and armor, there has been a technology transfer to other devices important to the arriving Industrial Age. Today, there is a firm grasp on heat treatment, and the mechanism of quenching, enabling special quenchants to be tailored to specific application. It was these original philosophers, alchemists and blacksmiths that are the foundation of the Science and Art of Metallurgy today.”
