Oh...I have no idea.  But, you arent confusing porcelain Prosser buttons
with glass here, right?

Also from the IMACS pages...on Buttons:
http://www.anthro.utah.edu/IMACs/475-Buttons.pdf

"Prosser: Patented in 1849, the process is one combining high fired
clays to produce a glass or vitrified appearance. The most common
varieties are black, white, or calico having an appearance of opaque
pressed glass. The backs have a pebbled or orange-peel surface (Sprague
1983:167-172)."

This site doesn't show any dates for Glass buttons tho:

"Glass: Many different types of blown, molded, and fused glass have long
been used for buttons. Glass has been used for all types of button
construction, and a great range of colors are known. Luscomb
(1967:80-89) discusses over twenty-five different kinds of glass
buttons."

But maybe this reference does. ?
Luscomb, S.C. (Button)
1967 The Collector's Encyclopedia of Buttons. Bonanza Books, New York.

Figure is probably not too helpful:
http://www.anthro.utah.edu/IMACs/475-Buttons.jpg


>-----Original Message-----
>From: HISTORICAL ARCHAEOLOGY [mailto:[log in to unmask]] On 
>Behalf Of Dan Allen
>Sent: Tuesday, October 03, 2006 5:27 PM
>To: [log in to unmask]
>Subject: Re: Milk glass dating
>
>Are the dates for milk glass buttons comparable to milk glass 
>bottles?  I know porcelain " Prosser" buttons were patented in 
>the 1840s.  Where do the glass, especially the four-hole type 
>come into the picture?
>
>Dan Allen
>Cumberland Research Group, Inc.
>University of Memphis