אך את הזהב ואת הכסף, את הנחשת את הברזל את הבדיל ואת העפרת. כל דבר אשר יבא באש תעבירו באש וטהר אך במי נדה יתחטא, וכל אשר לא יבא באש תעבירו במים.

However: the gold, and the silver, the copper the iron, the tin and the lead – everything that passes through fire shall be passed through fire, and thereby rendered Tahor, so long as it has been immersed in the waters of a Mikva sufficient for a Nida. And everything which does not pass through fire shall be passed through [boiling] water.[i]

Table of Contents

Historical Background

After prosecuting the war against the Midianites, the Jewish army returns to the camp, laden with spoils of war, and prisoners of war. Moshe was furious that the soldiers had brought back the women who had caused the Jewish men to sin, instead of executing them.

Accordingly, Elazar stepped forward and taught the soldiers what to do with the captured materials. Elazar divides the spoils of war into two categories; that which passes through fire, and that which is not passed through fire.

Halaĉic Precedent

The Rambam at the start of the laws of Vessels[ii] states as follows:

There are seven (7) types of כלים which are Biblically susceptible to Tum’ah, and they are:

Clothing
Sackcloth
Leather Items
Bone Items
Metal Items
Wood Items
and Earthenware Items

As the verse[iii] states מכל כלי עץ או בגד או עור או שק – from all wooden vessels, or a garment, or leather, or sackcloth. And about metal items the verse states אך את הזהב ואת הכסף. And about earthenware items the verse[iv] states וכלי חרס אשר יפל מהם אל תוכו כל אשר בתוכו יטמא ואתו תשברו – and an earthenware vessel into which some of these [Tamay materials] fall into; everything inside it shall be rendered Tamay, and it itself must be broken.

From the oral tradition we learnt that the phrase in the Torah[v] וכל מעשה עזים – and all things made from goats, includes items made from horns, hooves and bones of goats – and the same applies to other types of domestic and wild animals…

The Rambam then goes on to list other materials from which items or vessels made can be susceptible to Tum’ah, or are only susceptible to Tum’ah by Rabbinic ordinance, including stone materials, and earth materials – which are impervious to Tum’ah regardless of what type of Tum’ah or how the item is made.[vi]

The Hermeneutic Framework for the Halaĉic Categorization of Metals

We thus have a clear delineation of taxonomy splitting items made from “earth” or “stone” into a category impervious to Tum’ah as compared to “metal” items, or the other categories provided in the Rambam.

Interestingly, of those categories specified in the Rambam, only the “metal” category includes a listing of materials within that category. The Torah does not specify the details of animal materials from which items made are susceptible to Tum’ah – instead, we know this by the oral tradition. Furthermore, in presenting the list, the Torah provides this to us in the way of פרט וכלל – first providing specific items within the listing and then including a general rule about those items: כל דבר אשר יבא באש. And yet, the Gemara does not discuss the application of this Hermeneutic rule[vii] of Rabbi Yishmael’s. Nor does it explain why the category of “metals” is specified in writing, while that of biological materials is known from the oral tradition.

Furthermore, even the Rambam, whose attention to detail is comprehensive and meticulous does not ever define what a “metal” is. Accordingly, with the advancement of metallurgical and materials sciences, we are faced with questions about various materials, as to whether or not they are “metal”.

Metallic Consequences

The category of “metal” items is not only distinct when it comes to the laws of Tum’ah and Tahara, there are other Mitzvos in which the distinction has a practical difference:

Laws of Bikkurim: First fruits brought in metal baskets or trays are treated differently than those brought in wicker baskets or the like – metallic trays are returned to their owners.
Laws of Kashrus: Metals pots, pans, and other food utensils absorb the taste of the food items they are in contact with – and under certain can revert to their pre-cooked status. Earthenware or glass utensils are treated differently though – as they behave differently.

It would seem that each of these three categorizations have different reasons as to why the are treated differently than other materials, which is why there are different “fires” associated with determining what can or cannot be considered a metal.

Defining the Elemental: What is a Metal?

An initial review of the verse would seem to indicate that the Torah provides a very clear general rule about “metal” items, namely: כל דבר אשר יבא באש – anything that passes through fire. However, this simple understanding of the rule is inadequate for two (2) reasons:

If the rule could be consistently applied to all materials, there would be no need for the Torah to start off by specifying the six materials that are definitively included.
More importantly, the Halaĉa seems to follow the opposite meaning of the rule provided. The simple understanding of the text is that a “metal” is something which can survive a fire, and accordingly, to render it Kosher, must be placed in the fire again. However, the Halaĉa treats a “metal” as something which can be destroyed and reborn – melted and recast through the smelting fire.

This differentiation between the basic understanding of the verse, as speaking of something which survives a fire, vs the Halaĉic categorization of “metals” as being something melted in a fire can be resolved simply:

Elazar was speaking primarily about food utensils – pots and pans, knives etc. – which had previously been used by the Midianites for non-Kosher food consumption. Therefore, he is talking about the typical kitchen fire in specifying that “metal” is something which survives the fire.[1]

On the other hand, the sages were defining what a “metal” is, and therefore, it is the smelter’s forge which provides the defining characteristic:

To qualify as a metal, a material must possess the structural integrity to survive the functional kitchen fire yet possess the fusibility to undergo a reversible liquid phase-change in the forge.

Three different kinds of fire

Looking at this more broadly, and from a very simplistic perspective, we can identify three (3) different purposes as to why metal would be different – which results in the need to test whether something is a metal or not by passing it through fire.

The fire of pride – when a poor person brings his first fruit offerings, the ביכורים, the Kohen is instructed to keep not only the fruits, but also the container in which they are brought. A poor person’s gift is free of ego, as he has nothing. On the other hand, a rich person decorates the first fruits with a fancy metal platter. While doing so is appropriate in order to fulfill the Mitzva of זה א-לי ואנוהו, it still smacks of ego and pride, showing off what כחי ועוצם ידי has done for him. Since ביכורים is primarily the recognition of the exact opposite – that everything belongs to G-d – therefore the metal receptacle is disdained and returned to the owner. In other words, the test of a metal is whether it is valuable.
The tasty fire – When it comes to the laws of Kashrus, there is an interesting physical phenomenon – that materials absorb, under heat[2], part of the food which is being cooked in them. Some materials (like metal) have the ability to exude those same foods when passed through the same medium that it experienced when absorbing the food. Therefore, that which passes through the fire, should pass through the fire, and the rest is passed through (boiling) water. In other words, the test of a metal is whether it can withstand the heat.
The forging fire – when it comes to the laws of Tum’ah and Tahara, the tastiness of the materials is not a factor in determining whether a material is susceptible to Tum’ah. Instead, a new element is introduced – only materials which have been modified by human beings are susceptible to Tum’ah. Therefore, the test of a metal is whether a forging fire is used to change it from its natural state to a useful item.

However, we are still left with our first question – why identify six (6) materials in the category of “metal” instead of providing a single unifying rule: that which is forged through fire.

We must say that the Torah was engaging in providing us with the hermeneutic tools to determine what is a “metal” for the purposes of Tum’ah and Tahara.

Now, while Tum’ah and Tahara are essentially beyond human understanding, we can at least see a pattern of application in its rules, which lets us state that we understand why something would be susceptible to Tum’ah – for a given value of “understanding”. Accordingly, let’s not only take a look at what can be deduced logically from the Torah’s wording, but also at what might give us a hint at “understanding” why these materials are metal, and why they come to exclude materials which are not like them.

One of the “rules” of susceptibility to Tum’ah is that an item must be, or be capable of, being elevated to holiness. Thus, for example, the vast majority of the rules of Tum’ah and Tahara only apply to the Jewish people. A non-Jewish body experiencing the symptoms of death does not convey Tum’ah the way a Jewish body does. The reason for this is because Tum’ah is a parasitic force. It does not receive life directly from G-d but instead must leach its life from that which does. Therefore, Tum’ah does not seek to attach itself to a non-Jewish body, since that body is not capable of attaining the kind of surfeit of holiness that a Jewish body naturally has.

And therefore – only a Jewish body is susceptible to Tum’ah from death[3].

The Hermeneutic Structure: מיעוט – פרט וכלל

There are various rules[4] in use in our examination of these verses:

The exclusionary word אך

The מיעוט (אך – However): The verse begins with a strict limitation – אך את הזהב ואת הכסף. “Only”. Only the following listed items are included. Any material that is not these items is automatically rejected.[viii]

And the rule of Rabbi Yishmael – פרט וכלל:

The פרט (את… ואת – the Six (6) Metals): The Torah lists six (6) specific materials: Gold, Silver, Copper, Iron, Tin and Led.

In accordance with the Talmudic requirement of צריכא (absolute necessity), nothing in the Torah can be superfluous. Taken together with the introductory term to this list, אך – Only, we can say that the reason each of these six (6) metals are required is because they each serve as a unique identifier to help us exclude materials which we might otherwise think belong in the “metal” category. They function as a stress test.

The כלל (כל דבר – “Everything that comes…”): The general rule does not add new criteria[5]; rather, it introduces an open-ended class of future materials which have not been excluded by the previous list.

The Six Exclusionary Boundary Tests (The פרט)

By analyzing the distinct physical and economic properties of the six (6) archetypal metals, we derive the boundary conditions necessary to define a Halaĉic metal. If an earthly (non-gaseous) material fails even one of these tests, the initial מיעוט of “אך” excludes it.

זהב Gold – The Test of Permanence: Gold survives fire without combusting. In other words, does the fire use the material as fuel, or does the material use the fire to shape itself – in which case, it is a metal.

The Metaphysical Permanence: The Midrash[ix] states: “The world was not worthy of using gold. Why, then, was it created? For the sake of the Mishkan…” In the physical world, everything rusts or atrophies. Gold is the only earthly substance that generally does not oxidize or tarnish at any temperature. By noting that gold was created specifically for the Mishkan, the Sages indicate that gold provides a physical metaphor for the eternity of holiness – holiness is here to shape and be shaped by the world, while remaining unconsumed in the flames of passion and animalistic urges.

That which lacks this permanence of holiness is not susceptible to Tum’ah.

כסף Silver – The Test of Intrinsic Value: Silver is the foundational unit of asset value. The Torah uses this word interchangeably to mean either the material silver, or fungible money. This test therefore excludes all ubiquitous, inherently valueless materials (raw dirt, sand, mud).

The Legal Archetype: The Mishnah[x] states that a woman is acquired for marriage through כסף,or its value. A marriage cannot be effectuated by a handful of sand; it requires an item of intrinsic and obvious value. Therefore, we do not use diamond rings to marry, because although a diamond is obviously valuable, however, its value is not obvious. It requires expert determination. In other words, silver (currency) is synonymous with legal tender, while diamonds are assets with fluctuating value.

That which lacks inherent value is not susceptible to Tum’ah.

נחשת Bronze/Copper – The Test of Synthesis: The presence of נחשת within the list of template “metals” comes to exclude the assumption that a material must be strictly elemental, establishing that synthesized alloys possess independent Biblical status – but only when the result is a single material, and not a clumped together amalgamation.

Amalgamated Textual Proof that the word נחשת can be used[6] to mean an alloy of copper: Pure copper is soft and cannot hold a sharp edge. Yet, Golias is described[xi] as wearing scale armor of נחשת. Similarly, the Mishkan’s Washbasin is described[xii] as being made from polished נחשת mirrors. Pure copper oxidizes and makes a terrible mirror; bronze (a high-tin alloy) takes a brilliant polish.

That which lacks the ability to absorb (due to being too full of its own ego) and, by doing so, improve the whole of the material – whether for protection or to increase light – is not susceptible to Tum’ah.

ברזל Iron – The Test of Malleability (רקוע): Iron requires extreme heat to forge, but crucially, it can be beaten with a hammer without shattering – each beat serving to bend the material into the desired shape. This excludes brittle materials lacking tensile strength (stone, fired pottery).

The Lethal Benchmark: In discussing the laws of an accidental murderer, the Torah[xiii] notes that iron is uniquely endowed with a חזקה (absolute legal presumption) of lethality, regardless of its size.[xiv] Similarly, in building the altar, we are forbidden from using iron in its construction, as the verse[xv] states כי חרבך הנפת עליה ותחלליה. And as was actually the case during the construction of the Base Hamikdash, the verse[xvi] states והבית בהבנתו… כל כלי ברזל לא נשמע – when the Base Hamikdash was being built… all iron utensils were not heard [in its construction]. The hardness of metal is not as it applies to itself, as when it comes to its own shape it is willing to bend. But when it comes to others, it is unyielding and unbending.

That which lacks the ability to bend but not break – especially when it comes to dealing with others – is not susceptible to Tum’ah.

בדיל Tin – The Test of Adhesion: Historically, Tin was primarily a subservient material used for soldering and plating. When tin is melted and wiped over copper to plate it, it doesn’t just dry on the surface like paint or wet mud. At the atomic level, the liquid tin actually diffuses into the solid copper, creating a completely new boundary layer called an intermetallic compound. They become one contiguous structure. This is known in metallurgy as “wetting.”

Now, imagine trying to “plate” a copper pot by dipping it into melted stone (lava), melted salts, or liquid glass (without highly specialized modern fluxes). What happens? The liquid might temporarily wrap around the copper, but because it cannot form an atomic bond, it simply dries into a rigid, brittle crust. The moment the pot cools or is tapped, the thermal shock causes the crust to fracture, shear, and flake off. It is a purely superficial, mechanical shell.

This gives us the following exclusion – materials which do not form a separation layer by integrating with the underlying material are not metals.

The Etymology of Separation: The word בדיל – Tinis connected to the root ב-ד-ל meaning “to separate.” In the ancient world, copper pots which can become toxic when used with food, were coated with tin. The tin literally separates the poisonous base metal from the food – but it does so by bonding with the poisonous base metal.

That which lacks the ability to separate while simultaneously integrating – but instead always remains separate, both from the bad and from the good – is not susceptible to Tum’ah.

עופרת Lead – The Test of Fusibility/Phase Change: Lead is a very soft material, which easily undergoes a reversible phase change from solid to liquid. This excludes refractory materials that crack or remain unalterably solid under heat.

The Etymology of Dust: The word עופרת is derived from the root ע-פ-ר meaning “Dust”. Lead reflects no shine (light), has no resonance (sound), and acts like dense mud. The Torah famously uses lead as the metaphor for absolute dead weight[xvii]. But when it comes to describing holiness, dust represents humility, as Avrohom stated[xviii] ואנכי עפר ואפר – and I am [like] dust and ashes.

That which lacks humility is not susceptible to Tum’ah.

The Grouping

In our verse אך את וזהב ואת הכסף, את הנחושת את הברזל, את הבדיל ואת העופרת [spelled phonetically, with commas added for emphasis] we see that the Torah also does not group the listing as a single itemized list. Instead, the Torah groups them into three (3) distinct groups: Gold & Silver, Bronze/Copper & Iron, Tin & Lead. Perhaps, having reviewed the exclusions derived from each of these materials, the following can be said as to why the Torah has them grouped in this manner:

Gold & Silver – Treasury Metals (the Metals of Capital)

In antiquity, gold and silver were unique because they were often found in their native, pure state (unlike iron or aluminum, which require intense smelting to separate from ore). Because of their permanence and rarity, human beings do not use them to build bridges or forge swords. We use them as the ultimate store of value. They represent Capital—the economic engine of civilization.

חשיבות – This establishes that metals have intrinsic value.

Bronze & Iron – Industrial Metals and Alloys (the Metals of Industry)

Bronze and iron were so critical to the development of technology and industry that ages of civilized development are literally named after them: the Bronze Age and the Iron Age. These are not materials for hoarding in a treasury; these are materials whose value is based on their function; more specifically industry and defense (or offense). They are extremely hard materials requiring technological intervention to produce and use – blast furnaces and alloying – and of course hammering.

ריקוע – This establishes that metals are malleable and forgeable.

Tin & Led – an Amalgamation of Fusibility & Subservience (the Metals of Process)

Tin and led share a striking characteristic: they have remarkably low melting points. They can even be melted in a standard campfire. They are so physically similar that the ancient Romans didn’t even have separate words for them. Pliny the Elder famously classified tin as plumbum candidum (white lead) and lead as plumbum nigrum (black lead).

Generally, these metals are not typically used alone, due to their softness and low melting points. Instead, they are fused onto other materials (like a tin lining of a copper cup) or added to other utensils (like led weights to a fishing line). They are the unobtrusive functional materials that are added to the whole to bind to other materials and to make other technologies work.

היתוך – This establishes that metals are fusible and bondable.

The Rabbinic Bridge: Glass and Glazed Ceramics

This exclusionary framework illuminates the Gemara’s categorization of Glass. Glass is extracted from earth (sand) and undergoes a reversible phase-change in a forge, similar to עופרת. However, once cooled, it lacks malleability; if struck with a hammer, it shatters – unlike ברזל. Consequently, the term אך excludes glass from the definition of a metal; Biblically, it is stone/earth.

Yet, because it has one of the attributes of metal – it mimics the phase-change of עופרת, it presents a Halachic anomaly. The Gemara[xix] records Rabbi Yosi ben Yoezer’s decree that glass vessels contract ritual impurity: “Since they can be repaired by melting them down, they are similar to metal vessels.” The Sages recognized that glass straddles the thermodynamic boundary and applied Rabbinic (but not Biblical) metallic stringencies to it.

This complex interaction extends to composite items. The Shulĉan Aruĉ[xx] addresses earthenware glazed with lead or glass. Despite the metallic surface, they retain the status of earthenware and cannot be koshered via הגעלה – boiling water. The extreme heat required to fuse the glaze fundamentally alters it, causing it to adopt the porous characteristics of the base clay. A thin veneer of metal cannot override the structural Halaĉic identity of the dominant base material.

Application to Modern Chemical Extraction

Furthermore, the פרט וכלל algorithm in the Torah is not limited to materials widely known at the time. The rules extend even when confronted with modern materials.

Aluminum is tightly bound in bauxite ore. Its extraction requires the modern Bayer process to refine bauxite into alumina, followed by the Hall-Héroult process (electrolysis at roughly 960°C).
Titanium is extracted from rutile ores via the Kroll process, requiring chlorine gas, carbon at extreme heat, and reduction with magnesium in an argon atmosphere.

A non-religious observer might argue these chemically engineered materials cannot be Biblically considered metals, since they were not “known” when the Torah was written. However, the Torah never dictates the method of extraction, only the properties of the final material. When produced, they navigate the six-part gauntlet: they are not a temporary material (זהב), they hold market value (כסף), they can be alloyed (נחשת), they possess high malleability (ברזל), are fusible (בדיל), and change phase-state under heat (עופרת).

Modern Poskim and the Methodological Divergence

While the פרט וכלל derivation is logically airtight, modern Halaĉic authorities approach newly discovered materials using a completely different methodology, differing in three distinct ways:

Precedent vs. Derivation: Modern Halaĉic authorities (Poskim) do not use the thirteen (13) Rules of Rabbi Yishmael. The rule[xxi] אין דורשין פסוקים בזמן הזה prohibits original hermeneutic derivations in our times. Therefore, authorities like the Tiferet Yisrael (on Platinum) and Rabbi Moshe Feinstein (on Aluminum) bypass our verse entirely and rely on the Rambam’s operational definition: any earth-extracted material that can be melted and hammered is a metal.
Societal Naming (לשון בני אדם): Poskim frequently operate on societal consensus – based on the concept of[xxii] ואף על פי שאינם נביאים… בני נביאים הם. If people (especially the Jewish people) universally categorize a new substance like Titanium as a “metal,” Halaĉa generally absorbs that reality without stress-testing it against Biblical verses.
The “Plastic Problem”: Plastics undergo a reversible phase change (like עופרת) and have structural integrity (some even as hard as ברזל). Yet, Rav Moshe Feinstein[xxiii] and the Tzitz Eliezer[xxiv] rule plastics are not metals. They introduce two (2) of the constraints we noted above: the lack of true forging (ריקוע – they are molded, not hammered), and their non-mineral origin (synthetic petroleum derivatives, are considered a בריאה חדשה – a new formation). Thus, they default to the Halaĉic category of Wood/Earth.

The Linguistic Proof: The Etymology of מתכת

As we noted above, the word מתכת does not appear anywhere in the תנ”ך as a noun for “metal.” The abstract genus of “Metal” is a philosophical categorization that flowered in Mishnaic Hebrew as a word to include all materials that survive the פרט וכלל test.

While the noun is absent, its root is not. מַתֶּכֶת is derived from[xxv] נ-ת-ך meaning “to melt, to fuse, or to pour out“. Grammatically, it is a verb that drops the letter Nun when the prefix letter Mem is added; מתכת.

Literally, מתכת means: “That which is melted.“ When the Sages coined a word to encapsulate the laws of metal vessels and impurity, they looked at the common denominator of the Biblical list. They named the category after the ultimate exclusionary stress-test: Its behavior in fire.

[1] This is a problem. If Elazar is talking about a kitchen fire, how did the sages extend this to include a smelting fire? Perhaps we can say that the sages were able to do so, because later on the Torah provides a list of the golden items taken in the spoils, which included things which could not be used as kitchen implements: [Bamidbar 31:50] איש אשר מצא כלי זהב אצעדה וצמיד טבעת עגיל וכומז – all of which are women’s ornamental Jewelry and undergarments, which would not be placed in a kitchen fire. Therefore, since those were included in the אך את הזהב and the כל דבר אשר יבא באש, it might mean that definition of “metal” is something that can be smelted, while the definition of metal which needs to be koshered is one in which it has been used in fire. This needs more thought.

[2] Although this isn’t limited to actual fire. The heat from pressure, or the heat due to chemical acidity, can just as easily perform the same function.

[3] There are other parallels to different types of Tum’ah that follow a similar pattern.

[4] We are excluding the possibility of deriving something from the use of the word את, which only one of the sages in the Gemara is identified as using that word to derive inclusionary points: See Gem.

[5] As we discussed previously, the requirement to survive or be smelted, as derived from the statement by Elazar, is not necessarily related to the categorization of the material – as it can be used to apply the rule as to how to render something Kosher. Therefore, it is only the existence of the term כל דבר which adds a general rule. In other words, the Torah could have started the explanation on how to Kosher something with the phrase אשר יבא באש. The addition of כל דבר serves only to create a general rule applicable to “metal” materials – that they pass through fire [and are not excluded by being unlike the six (6) metals specified].

[6] In fact, in his commentary to the Ĉumash, Rabbi Adin Even-Yisroel [Steinsaltz] almost exclusively uses the term “bronze” to translate the word נחשת.

[i] Bamidbar 31:22-23

[ii] הלכות כלים 1:1-2

[iii] Vayikra 11:32

[iv] Vayikra 11:33

[v] Bamidbar 31:20

[vi] הלכות כלים 1:6

[vii] See Rabbi Yishmael’s Rules – #5

[viii] Gem. Rosh Hashana 17b, & elsewhere. See also Rashi Shemos 31:13

[ix] Rabbah Shemos 35:1

[x] Gem. Kiddushin 2a

[xi] Shmuel I 17:5-6

[xii] Shemos 38:8

[xiii] Bamidbar 35:16

[xiv] See Gem. Sanhedrin 76b

[xv] Shemos 20:22

[xvi] Melaĉim I 6:7

[xvii] Shemos 15:10 – צללו כעופרת, in describing Egyptians who drowned immediately, without being tossed by the waves, and dropped to the bottom of the sea.

[xviii] Beraishis 18:27

[xix] Gem. Shabbos 15b

[xx] Oreĉ Ĉaim451:23

[xxi] Based on Gem. Nida 19b & Pesaĉim 66a – some of the thirteen (13) rules require explicit oral tradition. Furthermore, the Gemara Bava Metzia 86a stated רבינא ורב אשי סוף הוראה – these were the last of the sages who could rule directly from the Torah, without relying on the teachings of prior sages. This was codified by the Rambam in his Sefer Hamitzvos. However, the Ramban famously argues based on the fact that derivation from the Torah is explicitly provided for by the Torah, as stated in the verse ואת שעיר החטאת דרש דרש משה. Therefore, it cannot be excluded by the sages. However, the mental capacity required to properly use these methods were not present amongst our people since the era of the Gemara.

[xxii] See Sefer HaChinuch 330:5. See also Gem. Pesaĉim 66a, where Hillel ruled: אלא הנח להן לישראל: אם אין נביאים הן – בני נביאים הן

[xxiii] Igros Moshe, Oreĉ Ĉaim 2:92

[xxiv] Vol. 7:37

[xxv] Yeĉezkel 22:20