Halacha

הלכה א
כָּל הַכֵּלִים שֶׁיֵּשׁ לָהֶן בֵּית קִבּוּל שֶׁנִּטְמְאוּ אֲחוֹרֵיהֶן בְּמַשְׁקִין לֹא נִטְמָא תּוֹכָן לִתְרוּמָה וְלֹא אָגְנֵיהֶן וְלֹא אָזְנֵיהֶן וְלֹא מְקוֹם אֶצְבַּע הַשּׁוֹקֵעַ בִּשְׂפַת הַכְּלִי. נִטְמָא תּוֹךְ הַכְּלִי נִטְמְאוּ כֻּלָּן. נָפְלוּ מַשְׁקִין טְמֵאִין עַל כַּנֵּי הַכֵּלִים אוֹ עַל אָגְנֵיהֶן אוֹ עַל אָזְנֵיהֶן אוֹ עַל יְדוֹת הַכֵּלִים הַמְקַבְּלִין הֲרֵי זֶה מְנַגְּבָן וְהֵן טְהוֹרִין וַאֲפִלּוּ אֲחוֹרֵי הַכְּלִי כֻּלּוֹ לֹא נִטְמְאוּ:
כסף משנה
1.
Whenever the external surface of a k'li that has a receptacle contracts impurity from liquids, its inner side does not contract impurity vis-à-vis terumah. Nor does the outer side of the k'li's neck, its handles, or the fingerhold on the rim of the k'li contract impurity. If the inner portion of the k'li contracts impurity, all of the above components also contract impurity.
If impure liquids fall on the bases of keilim, their rims, their projections, or handles of keilim with receptacles, one should dry them and the receptacle is pure. Even the entire outer surface does not become impure.

הלכה ב
אֶחָד כְּלֵי הַשַּׂק כְּגוֹן הַשַּׂקִּין וְהַמַּרְצּוּפִין. אוֹ כְּלֵי הָעוֹר כְּגוֹן הַכָּרִים וְהַכְּסָתוֹת. וְאֶחָד כְּלֵי עֵץ אֲפִלּוּ קֻפּוֹת וּמִשְׁפָּלוֹת אִם נִטְמְאוּ אֲחוֹרֵיהֶן בְּמַשְׁקִין לֹא נִטְמָא תּוֹכָן:
כסף משנה
2.
The following law applies to keilim made from goats hair, e.g., sacks and carrying bags, leather keilim, e.g., pillows and bolsters, and wooden keilim, even baskets and dispensers. If their external surface contracts impurity from liquids, their inner portion does not contract impurity.

הלכה ג
פְּשׁוּטֵי כְּלֵי שֶׁטֶף שֶׁאֵין רְאוּיִין לְמִדְרָס כְּגוֹן הַשֻּׁלְחָן וְטַבְלָא שֶׁאֵין לָהּ דֹּפֶן הוֹאִיל וְאֵין מְקַבְּלִין טֻמְאָה מִן הַתּוֹרָה אִם נִטְמְאוּ אֲחוֹרֵיהֶן בְּמַשְׁקִין לֹא נִטְמָא תּוֹכָן. אֲבָל פְּשׁוּטֵי כְּלֵי שֶׁטֶף הָרְאוּיִים לְמִדְרָס כְּגוֹן הַמִּטָּה וְהָעוֹר שֶׁיְּשֵׁנִים עָלָיו וְהַכִּסֵּא וְכַיּוֹצֵא בָּהֶן אֵין לָהֶם אֲחוֹרַיִם וְתוֹךְ אֶלָּא בֵּין שֶׁנָּגְעוּ מַשְׁקִין בָּהֶן מִתּוֹכָן אוֹ מֵאֲחוֹרֵיהֶן נִטְמָא הַכְּלִי כֻּלּוֹ. וְכֵן מִדּוֹת יַיִן וְשֶׁמֶן וְזוֹמָא לִיסְטְרוֹת וּמְסַנֶּנֶת שֶׁל חַרְדָּל וּמְשַׁמֶּרֶת שֶׁל יַיִן אֵין לָהֶן אֲחוֹרַיִם וְתוֹךְ אֶלָּא אִם נָפַל מַשְׁקִין עַל מִקְצָתָן נִטְמְאוּ כֻּלָּן כְּמוֹ הַבְּגָדִים:
כסף משנה
3.
Flat keilim that can be purified by immersion, that are not fit to sit or lie upon, e.g., a table or countertop that does not have a side are not susceptible to impurity according to Scriptural Law. Accordingly, if the external surface of such a k'li contracts impurity from liquids, its inner surface does not contract impurity. If, however, flat keilim that can be purified by immersion are fit to sit or lie upon, e.g., a bed, a hide on which one sleeps, a chair or the like, the distinction between an inner surface and an outer surface does not apply. Instead, whether impure liquids touch their inner surface or their outer surface, the entire k'li contracts impurity.
Similarly, the distinction between an inner surface and an outer surface does not apply with regard to measures for wine and oil, a two-sided cooking k'li, a filter for mustard, and a filter for wine. If impure liquids fell on a portion of the k'li, the entire k'li contracts impurity, as is the law with regard to garments.

הלכה ד
הַמַּרְדֵּעַ וְהַמַּלְמָד יֵשׁ לָהֶן אֲחוֹרַיִם וְתוֹךְ. כֵּיצַד. נָפְלוּ מַשְׁקִין עַל הַחַרְחוּר לֹא נִטְמָא מִן הָעֵץ הַסָּמוּךְ לַחַרְחוּר אֶלָּא שִׁבְעָה טְפָחִים בִּלְבַד וּשְׁאָר הָעֵץ טָהוֹר. וְכֵן אִם נָפְלוּ מַשְׁקִין עַל הַדָּרְבָן לֹא נִטְמָא אֶלָּא אַרְבָּעָה טְפָחִים הַסְּמוּכִין לַדָּרְבָן. נָפְלוּ חוּץ לְשִׁבְעָה וְחוּץ לְאַרְבָּעָה הֲרֵי זֶה כִּכְלִי קִבּוּל שֶׁנָּגְעוּ מַשְׁקִין בַּאֲחוֹרָיו וְלֹא נִטְמְאוּ אֶלָּא מִמְּקוֹם מַגָּעוֹ וְלַחוּץ:
כסף משנה
4.
A concept paralleling an external surface and an inner surface apply with regard to a plow.
What is implied? If impure liquids fall on the lance, only seven handbreadths of the beam adjacent to the lance become impure and the remainder of the beam is pure. Similarly, if impure liquids fall on the metal peg, only the four handbreadths of the beam that is adjacent to the metal peg become impure. If the liquids fell beyond the seven handbreadths and the four handbreadths, the situation resembles a receptacle whose outer surface was touched by impure liquids, in which instance, only the place where the liquids touched and the outer surface contract impurity.

הלכה ה
כִּיס שֶׁבְּתוֹךְ כִּיס שֶׁנִּטְמָא אֶחָד מֵהֶן בְּמַשְׁקִין לֹא נִטְמָא חֲבֵרוֹ. בַּמֶּה דְּבָרִים אֲמוּרִים כְּשֶׁהָיוּ שִׂפְתוֹתֵיהֶן שָׁוִין. אֲבָל אִם הָיָה הַחִיצוֹן עוֹדֵף וְנִטְמָא הַפְּנִימִי. נִטְמָא הַחִיצוֹן נִטְמָא הַחִיצוֹן לֹא נִטְמָא הַפְּנִימִי וּבְשֶׁרֶץ בֵּין כָּךְ וּבֵין כָּךְ אִם נִטְמָא אֶחָד מֵהֶם לֹא נִטְמָא חֲבֵרוֹ:
כסף משנה
5.
When there is a pouch with an inner pocket and one of the two contracted impurity due to contact with impure liquids, the other does not contract impurity.
When does the above apply? When their rims are on the same level. If, however, the external one overlaps the inner one, and the inner one contracts impurity, the external one also contracts impurity. If the external one contracts impurity, the inner one does not contract impurity. If the impurity stems from the carcass of a crawling animal, in all instances, if one contracted impurity, the others also did.

הלכה ו
הַחוֹקֵק רֹבַע וַחֲצִי רֹבַע בְּעֵץ אֶחָד וְנִטְמָא אֶחָד מֵהֶן בְּמַשְׁקִין לֹא נִטְמָא הַשֵּׁנִי אַף עַל פִּי שֶׁהֵן עֵץ אֶחָד. כֵּיצַד. נָגְעוּ מַשְׁקִין טְמֵאִין בְּתוֹךְ הָרֹבַע הָרֹבַע וַאֲחוֹרָיו טְמֵאִין וַחֲצִי הָרֹבַע וַאֲחוֹרָיו טְהוֹרִין. נָגְעוּ בַּחֲצִי הָרֹבַע מִתּוֹכוֹ חֲצִי הָרֹבַע וַאֲחוֹרָיו טְמֵאִים הָרֹבַע וַאֲחוֹרָיו טְהוֹרִין. וּכְשֶׁהוּא מַטְבִּיל מַטְבִּיל אֶת הַכּל. נִטְמְאוּ אֲחוֹרֵי הָרֹבַע אוֹ אֲחוֹרֵי חֲצִי הָרֹבַע בְּמַשְׁקִין הֲרֵי אֲחוֹרֵי הַכְּלִי טָמֵא שֶׁאֵין חוֹלְקִין אֶת הָאֲחוֹרַיִם:
כסף משנה
6.
When one carves out a measure of a revi'it and half a revi'it into one piece of wood and one of them contracted impurity from impure liquids, the other does not contract impurity even though they are on the same piece of wood.
What is implied? If impure liquids touched the inner surface of the revi'it measure, that measure and its outer side are impure, the half-revi'it measure and its outer side are pure. If the liquids touched the inner surface of the half-revi'it measure, that measure and its outer side are impure, the revi'it measure and its outer side are pure.
When one immerses this piece of wood in a mikveh, it should be immersed in its entirety all at one time. If the outer surface of the revi'it measure or of the half-revi'it measure contracted impurity from liquids, the entire outer surface of the piece of wood contracts impurity, because an outer surface is not divided.

הלכה ז
קוּמְקוּמוֹס שֶׁהָיוּ אֲחוֹרָיו טְמֵאִין וְהָיָה מַרְתִּיחַ אֵין חוֹשְׁשִׁין שֶׁמָּא יָצְאוּ מַשְׁקִין מִתּוֹכוֹ וְנָגְעוּ בַּאֲחוֹרָיו וְחָזְרוּ לְתוֹכוֹ אֶלָּא הֲרֵי הַמַּשְׁקִין שֶׁבְּתוֹכוֹ טְהוֹרִין לִתְרוּמָה: בְּרִיךְ רַחֲמָנָא דְּסַיְּעָן
כסף משנה
7.
When the outer surface of a kettle was impure and it was boiling, we do not suspect that maybe liquids flowed out from it, touched its outer surface, and then receded back inside it. Instead, the liquids that are inside it are pure with regard to terumah.

טהרה הלכות כלים פרק כח
Taharah Kelim Chapter 28