dk nkf - Nordisk Konservatorforbund Danmark

for defects. Also, from each new stamper a number
of records were played repeatedly to determine the
number of reproductions this particular record would
tolerate before its quality became unsatisfactory.
For many years the absolute minimum was 50
playings[1,2,9].
The wax was manufactured in an industrial plant or
in a dedicated department of large record companies.
The raw materials were stearic acid, metal salts, and
alkaline solutions, and the materials were boiled in
a similar way to soap manufacture. To the molten
mixture were added various natural waxes, such as
Montan and Carnauba wax in order to improve the
interaction with the cutting stylus during recording.
The molten mixture was poured into circular cans
and left to cool; due to shrinkage the cast wax could
be easily removed. The top surface was finely shaved
to obtain a mirror-like surface. The recording took
place in a recording machine which was essentially
a vertical lathe, in which the cutting tool (the cutting
stylus) was transported across the surface and at the
same time vibrated by a cutter-head. The vibrations
were representations of the sound. The groove was
given a cross section that experience had shown
would give a good interface to the steel needle later
used for reproduction in the finished record. The
swarf (chips) was sucked off so that it would not get
entangled in the machinery.
The recorded wax had a very delicate surface, and
it was carefully carried or shipped to the record
factory galvanotypic department. The surface of the
wax was made conductive by careful brushing with
graphite or by vacuum sputtering, and a negative
was created by electrodeposition of copper. In order
to obtain a fine-grained (small crystal) deposit on
the modulated grooves, the current was low at first,
but then increased to have a fast growth of thickness.
The copper and wax were then separated, and the
negative (the father) was given a semiconductive
release layer and a copper positive (the mother)
was formed galvanotypically from that. The release
layer ensured that the original negative could be
separated easily and be retained as the original. It
was frequently silvered or nickeled. The wax was
shaved again (refacing) for later re-use. The original
thickness of the wax of ca. 40 mm could be reduced
122
to ca. 20 mm with no adverse effect on its cutting
qualities. From the mother a series of stampers (sons)
could be similarly formed, and whereas copper was
used in the beginning, nickel and chromium plated
nickel became the norm to increase durability[13].
2-10 thousand records could be pressed from one
stamper before it was worn so much that the quality
suffered. Polishing of the original negative was only
rarely performed, because it obliterated the high
frequencies that were recorded.
The record material was provided to the presses
in the form of “biscuits” of material that was
pre-heated on a hot-table. The compound for the
record material consisted of ca. 80% mineral filler,
mostly a mixture of natural minerals, such as slate
dust or barytes, or fillers obtained industrially,
such as barium sulphate. As opposed to concrete
manufacture, where a wide distribution of particle
dimensions is important, in record manufacture
which is dependent on the wetting of the particles
by the binder, uniformity is aimed for. This is
obtained by comminution and screening. The binder
was shellac with addition of resins and waxes, all
depending on the actual properties of the batch of
raw materials used. A further constituent of the
record material was record scrap, i.e. the material
left over from manufacture and old records received
back (this was prevalent during the two world
wars). All constituents were ground and then mixed
on rollers or kneaders (masticators) and rolled hot
in the form of slabs. The slabs were cooled, broken
up, and then ground to a uniform powder that was
hot rolled to make up the biscuits. It was important
to maintain a very low moisture content, because
the heat of the presses would otherwise generate
steam. All machinery was subjected to heavy wear,
because the record material was abrasive[3,6].
An alternative to the shellac compound record
was the much cheaper phenolic record, which had
a commercial life from ca. 1931-35. “Hit-of-the-
Week” and Durium were single-sided reddish carton
sheets impregnated with a dark brown phenolic
resin that was cured as it received the impression of
the stamper. Two recordings were on one side, and
apart from popular tunes, several language courses
used this format.