I have also encountered nineteenth-century utilitarian pottery sherds in that
appeared to be salt-glazed on the basis of their "orange peel" surface texture,
but whose paste was porous enough to stick to the tongue, and so (by one
definition at least) are earthenware rather than stoneware.  These sherds were
from vessels in forms that commonly occur in stoneware, such as jugs and crocks
with slipped interiors.  I have heard the term "salt glazed earthenwares used to
describe them, though I can't remember ever seeing it in print.
 
Question: Temperatures within kilns often vary, based on distance from the heat
source, wind direction, and other factors.  Salt-glazing is supposed to occur as
the salt vaporizes very rapidly after it is introduced to the kiln.  Is it
possible for salt to be introduced into a hotter part of the kiln and glaze
vessels that might be in cooler parts of the kiln and might be incompletely
fired?
 
As to white saltglazed stonewares, I have also seen sherds with a matte finish
that appeared to be "unglazed." I suspect in some cases that this is physical
ware resulting from people walking on sherds that were exposed on yard surfaces.
In those cases there may be some gloss remaining in dimples on the surface.  I
don't know whether there are chemical processes that might have similar effects,
such as, say the high soil acidity common in southeastern New England.
 
As Carl Steen suggests, Parian porcelain is a possibility, particularly if your
sherd is from a figurine or other sculptural form.  Parian was used because of
its textural resemblance to marble, but occurs less frequently on tableware
forms.  Also, the sherd would be porcelain, rather than stoneware.
 
Lauren Cook