A boiler (an earthenware vessel used to heat water)

that one coated in mortar

or potters' clay,

that which touches mortar

is tamei (as the mortar serves to insulate and/or protect the vessel),

and [that which comes into contact] with potters' clay

is tahor (as the potters' clay does not adhere well, and will crumble and fall off the boiler).

A kettle that develops a hole

and is repaired with pitch

Rabbi Yosi rules tahor

as is it not able

to hold hot liquids

like cold liquids (pitch will melt if exposed to hot water for an extended period);

and so he would say

regarding utensils [coated or repaired] with pitch:

copper utensils (which are meant to be put on a fire) coated with pitch

are tahor (as the pitch will inevitably melt off),

but if [intended] for wine (which is not heated)

are tamei (as the pitch deemed an integral part of the vessel).

פרק ג משנה ח

A barrel that developed a hole

and was repaired with pitch

in an excessive amount,

that which comes into contact with the necessary [pitch]

is tamei,

[that which comes into contact with] the excessive [pitch]

is tahor.

Pitch

that was dripped onto a barrel (on its exterior, which serves no purpose),

that which touches it is tahor.

A funnel

of wood or earthenware

that was plugged with pitch,

Rabbi Elazar ben Azaryah

rules [either] is tamei,

Rabbi Akiva

rules if of wood is tamei

and tahor if or earthenware,

Rabbi Yosi

rules both to be tahor.