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John,
I wasn't aware that steel was widely used.  I ran into a few War era disks that originally thought were lacquer.  A few of them had one crushed edge from rough handling in a steel cabinet and we determined they had a fiberboard base.  Glass was more commonly used, especially from '42-'46.  Glass was usually carefully marked as such because of the breakability.  If not marked, glass is heavier and has almost no "flex" to it.  Another giveaway is that if a glass disc is gently tapped on the edge, it gives slight ringing sound.
 
Aluminum discs usually show a metallic cross section where the hub was punched for the spindle.  I've heard rumors that some glass discs were bonded to an aluminum hub, though.
 
Hope this helps.
 


 
 
Steve Greene
Archivist
Office of Presidential Libraries
National Archives and Records Administration
(301) 837-1772
>>> John Dawson <[log in to unmask]> 4/23/2012 3:36 PM >>>
Hi everyone,

I was wondering if anyone had tips/resources about determining whether or not a the substrate of a laquer is steel or aluminum.  I have some WWII era discs and some are definitely heavier and sturdier than others.  I am wondering if these are steel. 

John Dawson
Indiana University
Media Preservation Initiative