Materials thatcannot be koshered are materials that absorb but do not emit. When, for example, hammets is cooked in an earthenware pot, the pot absorbs the hammets. There is no way to cause the pot to emit what it absorbed.

Which materials fall within this category? Earthenware and chinaware.

Corningware, made since 1995, is like chinaware, not like glass, and cannot be koshered.

Earthenware

What is the source that earthenware cannot be koshered?

The Torah teaches us that the meat of the sin offering, after a prescribed time, could not be eaten. Thus, if a small amount of it were absorbed in a vessel, that vessel would have to be koshered, lest the food that was absorbed be emitted into the next item that is cooked in the vessel.

The Torah states:

And if it were cooked in a copper vessel, it must be purged and rinsed in water

Vayikra, 6:21

Regarding vessels made of earthenware, however, the Torah states:

And, an earthenware vessel in which it (the sacrifice) was cooked must be broken…

Vayikra, 6:21

…because it cannot be koshered.

Chinaware

Chinaware falls under this category as well.

If, however, the chinaware vessel (or earthenware vessel, for that matter), absorbed its hammets—not when the food or the vessel was hot, but, instead, by the process known as kivush—which involves a liquid remaining in a vessel for 24 hours ormore— the vessel may be koshered by milui ve-irui (filling and pouring). For the steps to be followed in koshering a vessel by milui ve-irui, click here.

Enamel

Enamel is a substance that is used to coat the metal of roasters, ovens and many other items.

In the trade, what the layman refers to as enamel, is actually called, porcelain enamel.

It is not, however, made of porcelain. It is, rather, powdered or liquefied glass that is applied to metal. As follows:

Porcelain Enamel refers to a glass coating on metal (steel, cast iron, aluminum, copper, etc.) where the glass is fused to the metal at high temperature. It can also be called “vitreous enamel,” glass-on-steel,” “glass-on-metal,” etc. Over the years the term “Porcelain Enamel” has developed to mean a very hard, glass coating to differentiate it from paint (organic coatings) which are soft and not nearly as durable. It is not actually “porcelain” which is actually a narrow compositional range of ceramic body made into vases, bowls, plates, etc. and may or may not be glazed.

            Correspondence from Porcelain Enamel Institute

Some of the doubts regarding this substance, in halacha, emanate from the fact that the components of  this substance were, for some time—unknown. Apparently, because artisans did not want to reveal their trade secrets, they would not let the public know how enamel was made. (This is evident from a Responsum by the Hatam Sofer, 1762-1839.)

Even though, however, we know that enamel is made from glass, we cannot give it the halachic status of glass. Since the glass of the enamel is fused to metal, the combination of the two takes on the status of a new material, whose halachic status is doubtful.

Despite this, based on a safek sefeka (double doubt), some enamel utensils can be koshered. The method: hagalah, performed three times. (To review the Steps in Performing Hagalah, click here)  

The safek sefeka (double doubt) is as follows…

The first doubt: Some hold that enamel is like earthenware. Some hold that enamel is like glass. 

The second doubt: Even if we hold that enamel is like earthenware, some hold that earthenware can be koshered by performing hagalah three times.  (Others do not.)

Consequently, if  an item is one, such as a pot, that would be koshered by regular  hagalah if it were made of metal, we permit koshering it, if it is made from enamel, by performing hagalah three times. (One need not wait 24 hours from hagalah to hagalah, but one should change the hagalah water each time.)

On the other hand, if the item is one, such as an enamel stovetop or an enamel hot plate, that would be koshered by hagalah by pouring if it were made of metal, we have a different conclusion: It cannot be koshered.

Reason: Although triple hagalah is mentioned in halacha, there is no mention of triple hagalah by pouring as an effective way of koshering. 

To see a comprehensive list of various utensils and the methods of koshering them, click here