RE: [AV Media Matters] metal vrs wooden shelving for VHS tape sto

[rohre@ARLUT.UTEXAS.EDU]

Many years ago I was involved in magnetometer measurements of a potential
tape
library room and associated commercial standard sheet metal shelves.

We found even after banging around the shelves to assemble them into the
snap
in brackets holding the shelves that the field was still of the same
magnitude
as the earth's natural field, 400 mGauss, measured inside a steel framed
concrete tilt walled building.

The metal studs inside sheetrock walls were found to have slightly enhanced
magnetization at the points where power driven screws held the sheet rock to
the metal stud.  Therefore, we chose to "shield" the walls that were outside
walls to hallways with the shelving placed flat to the wall.  This also gave
an extra measure against outside randome influences, such as motors on floor
buffers, etc.  This storeage area served as the active 7 and 9 track digital
tape library for many years, with no problems from metal shelves.

It is true that a sharp blow to metal will induce at least the magnetization
of the earth's field, but this is well below the hazard level to today's
high
coercivity tapes.

I agree that wood shelves have no place in an archive, since that is a
combustible material.  Issues of humidity absorption also could be an issue
with wood, and that is a greater danger than the fire risk, as humidity is
something we constantly fight to preserve older recordings on plastic media
with binders that hydrolyze.

It is good to be aware of materials for a proposed archive, and magnetometer
probes can be obtained to work with various electronic DC current meters.
Thus, an archive that is growing could have its instrumentation to actually
measure proposed materials, and document incident fields in storage areas.
One area I surveyed had a motor generator room on the other side of the
sheetrock wall.  For that case, I added metal sheathing over the wall, with
overlapped seams to add a measure of shielding, and placed two by four
lumber
bolted flat to the wall to add the protection of spacing.  The metal shelves
were bolted to the spacers.  Using the formula for field decline, I believe
we
showed two inch spacing nullified the effect of a strong magnetron test
magnet
held flat against the other side of the wall.  Such a magnet had been
demonstrated to be strong enough to erase digital tapes in use at the time.
If anyone needs precise figures, I could reply privately, after digging out
the archived documents.

However, there are companies that provide even better shielding materials
than
the sheet mild steel I had available.  It has been years since I dealt with
them, but we also purchased mu metal products from Perfection Mica Corp. in
the U.S. Midwest.  They produced sheets and mu metal tape carriers that have
been used in our field transport of research masters to the lab from all
over
the world with great success in protecting the master tapes from magnetic
environmental effects.

Stuart M. Rohre
University of Texas Applied Research Labs, ESL
Analog/Digital Tape Facility