From: Patent Tactics, George Brock-Nannestad

Hello Mike,

to your very thorough discussion of glass vs. metal I can add the following 

Audio Record Vol. 7, No. 3, pp. 1-2, March 1951

 - this is the magazine of Audio Devices, and they discuss the advantage of 
having the much smoother surface of glass to use as a base, and they sound 
clearer. Surface roughness graphs of iron, aluminum and glass are shown (and 
if they have not cheated), the glass is very much smoother. They claim that 
their process is much improved over the wartime ones.

Audio Record may be found on the internet archive.

Best wishes,


> From: Steve Greene <[log in to unmask]>
> > John,
> > I wasn't aware that steel was widely used. 
> It was the ONLY metal used for home discs during the war.  It wasn't
> used much for professional discs because of the weight of professional
> thickness grades.
> > I ran into a few War era disks that originally thought were lacquer.
> They're all lacquer except for some celluloid coated discs, mostly
> fibreboard.
> > A few of them had one crushed edge from rough handling in a steel
> cabinet
> > and we determined they had a fiberboard base.
> Fibreboard was widely used for the most inexpensive grade of home discs
> from long before the war thru long after the war.  They were both
> lacquer coated or covered with a clear celluloid sheet. Almost all of
> the soldier voice records mailed to family were made this way. Fibre was
> rarely used for professional discs because they are too flexible in
> large sizes, as well as having a LOUSY surface!
> > Glass was more commonly used, especially from '42-'46.
> The first Presto and AudioDisc glass discs were shipped in April 1941 so
> they can show up before Pearl Harbor day.  They restarted using aluminum
> in the fall of 1944, although stocks of glass lasted as late as 1948. 
> There were companies which had a service of stripping your old aluminum
> discs and re-coating them, so that is an explanation of some aluminum
> discs showing up during the war.
> > Glass was usually carefully marked as such because of the breakability.
> > If not marked, glass is heavier and has almost no "flex" to it. 
> Presto said it was experimenting with glass at first because of the lack
> of flex which was an important problem with using lacquer for disc
> mastering instead of wax.  That is why CD masters are glass.  The first
> Presto glass discs were twice as thick as ordinary discs, and they were
> very heavy.  But I have always felt that a regular glass disc was
> slightly lighter in weight than aluminum.  When I was heavily into
> working with ETs I usually could tell glass when first picking one up.
> > Another giveaway is that if a glass disc is gently tapped on the edge, it
> gives slight ringing sound.
> Aluminum discs also ring.  But sometimes you can hear that there is a
> crack in the glass when the ring is a thud.  Most glass discs are
> translucent, but some are almost opaque.  Hold them up to the light --
> although it is safer to have a light below the discs so you don't risk
> holding them up high! 
> The edge and the center holes of glass discs are usually an indication
> because except for AudioDiscs the holes are drilled before coating.  If
> the discs are made by spraying or dropping the coating while the discs
> are on a conveyor belt, the holes will be coated.  If the discs were
> made by dipping -- as almost all home discs were -- there is no lacquer
> in the center inch or so, so the substrate shows thru.
> > 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
> Almost all glass AudioDiscs had a fibre center hub about 2 1/2 inches in
> diameter which can have holes that look metallic because the metal on
> the turntable spindles can rub grey coloring onto the holes if the discs
> were slip-cued.  If there are no paper labels, those center cores are
> easy to spot, but if there are well-applied paper labels you might not
> notice the outer diameter of the fibre core.  AudioDevices cut the large
> hole in the glass and placed the fibre center in before coating, so the
> lacquer coating is what holds the core in place, although it sometimes
> falls out.  I've never seen a glass disc with an aluminum center hub.  
> Mike Biel  [log in to unmask]  
> >>> 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