Sounds like you have a canning jar lid liner--one of the glass discs that
fitted into a zinc (zinc alloy, actually) screw-on top, and sealed the top
of the jar by applying pressure (when the cap was screwed down) to a red
(usual color) rubber gasket.  Even though these were always milk glass,
they were often called "porcelain" because of the superficial resemblance,
esp. in color, and because "porcelain" is a more prestigious word--better
for advertising purposes.  The place to start with canning jars is Julian
H. Toulouse's "Fruit Jars," Crown Publishers, New York, 1970.  Out of print
(and, nowhere available for less that about $150.00), but you might find it
in a university library or a big public library.



At 03:48 PM 9/29/99 -0500, you wrote:
>Our lab has come across a milk glass lid liner artifact that bears the
>inscrption "BOYD'S GENUINE PORCELAIN LINED CAP " around the edge, and in
>the center there is a "5." We are trying to figure out why something
>that is obviously milk glass is labeled as porcelain.  Can anyone help
>us with the identification of this inscription?
>
>Thanks,
>Laura Benson
>