[Avodah] Modern knives, and tevilas keilim

Micha Berger micha at aishdas.org
Thu Jan 11 15:37:06 PST 2018

On Sun, Jan 07, 2018 at 01:02:48PM -0500, Akiva Miller via Avodah wrote:
: It seems that many of the kitchen knives that I see for sale these
: days are labeled as being made of something called "ceramic"...

Wiki <https://en.wikipedia.org/wiki/Ceramic_knife> says:
   A ceramic knife is a knife designed with a ceramic blade typically
   made from zirconium dioxide (ZrO2; also known as zirconia).[1] These
   knife blades are usually produced through the dry-pressing and firing
   of powdered zirconia using solid-state sintering. Zirconia is second to
   diamond in toughness. It is 8.5 on the Mohs scale of mineral hardness,
   compared to 4.5 for normal steel and 7.5 to 8 for hardened steel and 10
   for diamond. The resultant blade has a tough edge that stays sharper
   for longer when compared to conventional steel knives. The ceramic
   blade is sharpened by grinding the edges with a diamond-dust-coated
   grinding wheel.

: Anyway, I strongly suspect that these knives are non-metallic and thus
: exempt from Tevilas Keilim.
: To help insure that this thread stays on Avodah, I will add the
: following: To my knowledge, plain unglazed earthenware is clearly
: exempt from tevila, but many (most?) require tevila for *glazed*
: earthenware, because the glaze is considered like glass. I suspect

The question would be whether sintered zirconia is zekhukhis. So, I
clicked the link on "sintering" at the above page to get to
    Sintering is the process of compacting and forming a solid mass of
    material by heat[1] or pressure[2] without melting it to the point
    of liquefaction.

    Sintering happens naturally in mineral deposits or as a manufacturing
    process used with metals, ceramics, plastics, and other materials. The
    atoms in the materials diffuse across the boundaries of the particles,
    fusing the particles together and creating one solid piece. Because
    the sintering temperature does not have to reach the melting point
    of the material, sintering is often chosen as the shaping process
    for materials with extremely high melting points such as tungsten
    and molybdenum....

So, it's like making glass, but instead of heating to the melting
point, it gets the particles to merge by other means. There is ceramic
instering (see the appropriate subsection of the above wikipedia entry),
except "[a]ll the characteristic temperatures associated with phase
transformation, glass transitions, and melting points, occurring during
a sinterisation cycle of a particular ceramics formulation". So, they
arent' really ceramic, they are sintered into a single whole in a
way cheres, which is poorous, isn't.

It would seem that if you want to apply the rationale of zekhukhis,
these ceramic knives would qualify, but since many/most posqim seem
nervouse about even including pyrex, I think a typical pesaq would be
to treat it with the chumerous of both zekhukhis and cheres.

Tir'u baTov!

Micha Berger             Man is equipped with such far-reaching vision,
micha at aishdas.org        yet the smallest coin can obstruct his view.
http://www.aishdas.org                         - Rav Yisrael Salanter
Fax: (270) 514-1507

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