In a list of halakhic rulings that appears on our daf (=page), the Gemara pronounces that we follow the ruling of Rav Zevidwith regard to kunai – glazed pottery. Rav Zevid ruled that glazed vessels are permitted – i.e. they are not absorbent and are thus permitted for use, even if non-kosher food was stored in them, if they are white or black, but are forbidden if they are green. This, however, applies only to those that have no cracks; if they have cracks we must assume that they absorbed the non-kosher food and are forbidden for use.
It appears that our Gemara is discussing the common method of covering a simple earthenware vessel – which is porous and therefore considered to be absorbent – with a protective glaze that would keep the vessel from absorbing food or liquid. This is done by pouring the glaze – made from liquid suspensions of various powdered minerals and metal oxides – on the piece and then firing it in a kiln.
The quality of the glazing and how well it will keep the pottery from absorbing things depend on the temperature of the kiln and the elements that are used in making the glaze. One of the most popular elements used in glazes – even today – is lead, to which various minerals are added to give the glaze its color. Some types of glaze are more susceptible to cracking, which would allow the pottery to absorb even after being covered with glaze. The different colors of the glaze that are mentioned in our Gemara indicate different methods of sealing vessels, some of which are better or more reliable than others. Some of them give the pottery the qualities of glass and are considered totally sealed, while others are considered to be porous even after undergoing this process.