[78-L] Glass Compound 78s

[Michael Biel]

78records at cdbpdx.com wrote:
> > Greetings. I am wondering how to easily identify glass compound 78s.

I assume you are referring to a glass-base lacquer coated recording
disc.  This is a disc that is a sheet of glass with a plastic coating on
it that the recording is directly cut into.  

> > I was playing a record from 1942 and I noticed the sunlight was sparkling
> > off the record in a rainbow of color, as if the light was being reflected
> > through a prism.

This could happen with an aluminum base lacquer disc as well because
that base is reflectively shiny as well, but when you describe it being
translucent you can only be describing a glass base disc. 

> > I examined the record closely and discovered I could see brightly lit
> > images through it. It has an unfortunate crack and the edges of the
> > crack seem to be flaking off like glass. 

The lacquer coating can flake off like this from any base material,
aluminum, steel, or glass.  The dark coating is not itself glass.  If
part of the clear glass base is crumbling, then the base is more than
just cracked, it is smashed.

> > Could this be a glass compound record? Are there any sure-fire
> > ways to identify glass compound records besides cracking them
> > and looking to see if the edges are flaking off?  Thanks! CDB

As dl mentioned, you often -- but not always -- can see light shining
thru the translucent discs.  If you do not want to hold the disc up to
the light, you can shine a strong light up thru it.  Once you have
identified a glass-base disc it should always be stored in a sleeve with
a second aluminum-base disc with it, and the disc handled only with that
second disc supporting it.  Archives with large collections of glass
base discs should have a rule that an aluminum disc be slid into the
sleeve and both discs be taken out together.  I was lucky when I worked
with the NBC Chicago collection at Northwestern to have a large amount
of unimportant aluminum based discs to store with our glass.

As dl mentioned, professional grade discs were not marked on the discs
themselves, only on the sleeves which often can get switched so that a
glass base sleeve could house an aluminum disc, but most home-grade
glass base discs are marked on the manufacturer's label.  But glass was
rarely sold to the home recording market.  They used steel base and
fibre base discs.  Many radio and recording studio labels did mark the
discs as glass, but not always because the labels were sometimes
prepared by a secretary, not the recording engineer.  The first glass
Presto discs were very thick and had a metal grommet in the center hole.
 AudioDevices' AudioDisc brand had a fibre-center that was about
2-inches in diameter, and the edge of how this fits into the larger
glass hole can sometimes be seen or felt.  Later in the war Presto had a
printed label that said glass base, but I am not sure if the discs were
shipped with this label on them or if it was to be applied by the user.

As to whether glass discs were more flexible when new, I doubt that
because one of the reasons glass was used was because it was less
flexible than aluminum.  One of the selling points of glass prior to the
war was that it was ideal for use for mastering because it remains
flatter than metal when being electroplated.  The first glass discs were
shipped in early April 1941, prior to our entry into the war, and before
aluminum rationing.

Mike Biel   mbiel at mbiel.com

-------- Original Message --------
Subject: Re: [78-L] Glass Compound 78s
From: David Lennick <dlennick at sympatico.ca>
Date: Tue, June 16, 2009 12:25 am
To: 78-L Mail List <78-l at klickitat.78online.com>

Very simple..assuming that it's sturdy enough for you to handle without
it 
breaking (and those things are VERY thin), hold it up to the light. If
you see 
translucent bluish, yep..it's glass based. As for the flaking, that's
not 
reversible.

If you find this somewhat risky, you can tape the edge lightly with your
ring 
finger. The sound will be different from tapping an aluminum disc.

Not all manufacturers identified glass lacquers, except on the sleeve,
which 
was pretty dumb. Amazing that 65 years ago, these things were able to be

shipped..carefully, yes, but they frequently made it intact. They must
have 
been a bit more flexible when they were new.

dl