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					                                                                                                         Audio Engineering Sxiety Section




 The Diamond as a
                     Phonograph Stylus Material
                                                   E. J. and M. V. MARCUS'"

                         A thorough knowledge of the characteristics of diamonds is re-
                         quired in the preparation of phonograph styli. The authors show
                         photographs which compare wear of different stylus materials.


      PON EXAMINING       the physical prop-                                                   all other respects these stones are
                                                                                                similar to the diamonds used for gem
          erties of groups of raw materials                                                     purposes.
         found in nature, we generally find                                                   (2) Cleavages or Splints: Chips cleaved
  small increments in the values of these                                                       from larger stones, usually during gem-
  properties. Diamond is the outstanding                                                        stone production. Lack the structural
                                                                                                strength of small fine industrials.
  exception to this rule in that it possesses                                                   Sometimes employed for use a s phono-
  durability and hardness valdes which ex-                                                      graph styli due to the comparative ease
' ceed those of the closest comparable ma-                                                      of fabrication.
  terial-sapphire-by      almost &7 hundred                                                   Fabrication of Diamond Styli
  fold.
     Diamond was recognized, even. in an-                                                       There are - t h e e principal methods of
  tiquity, as possessing properties which                                                     shaping diamond for industrial use,
                                                Fig. 2. Diamond cleavages, or chips           namely bruiting, lapping, and sawing.
  have made it the most valuable material                from brger stones.
  (except for some radio-active elements)                                                     All other methods are usually variations
  on earth. These properties are dura-                                                        of the three employing special fixtures.
                                                other parts of the world, the Belgian           All methods use diamond in some form
  bility, beauty, and rarity.                   Congo, Brazil, India, Borneo and Aus-'        to reduce t h s i z e or change the shape of
                                                tralia, are found in alluvial deposits.       another diamond. Diamond is the only
                                                The original volcanic sources remain
                                                                                              material hard enough to cut diamond
                                                undiscovered. Africa produces about 95
                                                                                              effectively. Large stones are sometimes
                                                per cent of the world's production, which
                                                                                              cleaved along cleavage planes which can
                                                has amounted to as much as 13,000,000
                                                carats yearly (one carat = 0.2 gms)  .        be determined by experts.
                                                   The diamond-1;earing volcanic pipes in     Bruiting
                                                South Africa were discdvered acci-               The bruiting operation is similar to
                                                dentally between 1866-1888. These pipes,      freehand wood turning in a lathe, using
                                                called blue ground, contain approxi-          a tool rest. The diamond to be worked
                                                mately 1 part of diamond in 15,000,000        is place$ in a special holder, called a dop,
                                                parts of rock. The soft blue ground is
                                                                                              and put in the lathe chuck. A diamond
                                                mined, crushed and washed in troughs
Fig. 1. Small fie'industrials. Selected                                                       tool placed in a long-handled holder and
                                                containing grease pans. Diamond, having       braced under the arm is worked against
whole diamonds used in manufacture              a great affinity for oil or grease, sticks
of playback styli. Preffrred to cleav-          to the grease in the pans, a& the stones
ages due to greater structural strength.        are then collected and sorted.
Weight before processing 0.2 gm;
      after processing, 0.04 gm.                Classification of Diamond
                                                  There are various types of diamonds,
  For technical purposes, we are most           and each has its specific application. The
concerned with the durability and hard-         most valuable are the large, clear white,
ness df diamond.                                nearly flawless stones which are used for
                                                gem purposes. However, there are other
Formation and Occurrence of Diamond             types used for industrial purposes which
   Diamond is pure crystalline carbon           possess the same physical and chemical
occurring sparinglyf in volcanic rock.          properties but are unsuited for gems.
Carbon, trapped in molten lava and sub-         Diamonds may be transparent, cloudy or
jected to tremendous pr-e        ad-
                                  n.,           opaque, clear white, or colored in various
crystallizes slowly to form diamond.            hues. The qualities of those diamonds
Althobgh it is supposed that all diamonds       suited for use as styli for audio equip-
were formed in this manner, only in             ment will be discussed in this paper:
South Africa do we actually find large          Commercial diamonds are classified in
volcanic intrusio~s "pipes" containing
                   or                           several groups, two of which are:
deposits of diamond. Diamonds found in          (1) Fine Industrials: Diamonds unsuited
                                                  for use as gem stones due to small size,    Fig. 3. Each diamond stylus must be
  * The Tetrad Corporation, Yonkers,N . Y.        color or presence of small inclusions. In   inspected carefully on a shadowgraph.

AUDIO     ENGINEERING               JULY, 1950                                                                                         25
Audio Engineering Society Section   .
the spinning diamond in the lathe. This        carbon dioxide, CO,. Diamond can be        ample if we say a piece of wood is hard,
method is quite effective for rapid re-        burnedin oxygen between 700' and 900°
moval and rough shaping of diamond             ,-.
                                               L.
                                                                                          we mean it cannot be sawed or chopped
                                                                                          easily, or that it is difficult to drive a
cones.                                            Diamond is very resistant to strong nail into it. If we say a piece of metal
                                               acids and alkalis. Concentrated solu- is hard, we may mean that it cannot be
Lapping
                                               tions are frequently used to clean t l ~ e filed, sawed, or bent readily. Therefore,
    This is the term used tb describe the
 polishing of diamond surfaces and usu-
                                               stones. A mixture of sulphuric acid an
                                                                                        Q an answer can only be given in terms of
                                               potassium bichromate can oxidize dia- the job being performed by the given
 ally means the generating of a facet on       mond slowly at 200' C. Diamonds are materials. Numerical results for coni-
a stone. Lapping is done on a rapidly          cleaned of foreign mattbr from the mines parative hardness and durability of ma-
 spinning, porous cast iron wheel which        by storing in hydrofluoric acid. They are terials will vary from one set of coh-
has been impregnated with a mixture            cleaned after processing by cooking in a ditions to another. However, a glance
of diamond powder and olive oil. The           concentrated solution of potassium hy- at the table of comparative physical
 stone is held in the dep and placed in a      droxide, usually followed by a second properties shows that diamond has a
removable arm called a talzg. It is then       cooking in sulphuric and nitric acid. tremendous advantage over its next best
placed against the lap or wheel. Diamond       Aqua regia is sometimes used to clean alternative material in resistance to
crystallizes in the cubic system and has       diamonds.                                  abrasive wear, breaking load on a radius,
three equal axes intersecting at 90 deg.          Remarkable as diamond is in its re- indentation hardness, and compressibility.
Lapping can only be done along planes          sistance to chemical action, it is even
which have a definite relation to these        more outstanding in its amazing physical Phonograph Stylus Wear Tests
axes. Under the usual lapping conditions,      properties. The compact arrangement of        Tests have been conducted to deter-
diamond will not polish or cut readily         the carbon atoms in the diamond has mine the comparative durability of vari-
against the so-called grailz or out of         resulted in a n extremely durable and ous stylus materials.
relation to these planes. Therefore, in        hard material.                                When a playback stylus touches a
the polishing of diamond facets it is             The following is a list of physical record groove, only a small area of the
necessary to orient the stone properly         properties of diamond compared with stylus tip actually makes contact with
to obtain the lapping direction. I n polish-   alternative materials :                    the groove walls. The pressure per
ing a cone and radius on a diamond, it
is evident that rotating the diamond cone                                             TABLE I
will encounter grain running in several                      1
                                                                               ).
                                                                                                  Diamond         Alternative Material
directions and that conventional polish-        Resistance to abrasive wear (Rosiwal)            90,000         Sapphire 1,000
ing methods can not be used. It was             Wear resistance, path of turning tool
found necessary to polish at extremely               (Grodzinski)                                 1,250         Carbide 12.5-20.5
                                                Ratio of time required to saw given
high speeds with very fine diamond                   area (Grodzinski)                          100300          Sapphire 1.0
powder to develop a high polish on the          Indentation Hardness (Knoop)                  6000-6300         Sapphire 1600-2000
cone and radius of the stylus. The entire       Initial bearing friction (Shotter)                0.70          Sapphire 1.13-1.60
polishing apparatus must be free of vi-         Breaking load on a radius (Schuler)                25           Sapphire 5
bration and exceptionally accurate.             Compressibility (Williamson)                      0.18         Sapphire 0.381
                                                Surface Finish (Kayser)                           Better than any other material
                                                Index of refraction-sodium                        2.419        Glass 1.426
Sawing                                          Dispersion                                        2.465        Glass 1.532
   Diamond can be sawed with a thin             Note :Moh's scale (1820) which gives comparative but not quantitative hardness values
phosphor bronze saw charged with dia-             of gem materials has been omitted. Moh listed gem materials in order of ability to
mond powder and olive oil. Diamond                resist scratch marks.
impregnated metal saws are also used.
The diamond to be sawed is nicked with            The question is frequently asked,          square inch may amount to several tons.
a sharp pointed diamond and moved into         "How hard is diamond?" This is a             The dynamic forces acting on the stylus
the saw. There are optimum sawing              difficult question to answer, since hard-    tip are several and severe. Hard record
directions, depending upon the crystal         ness is a composite property embracing       materials containing* abrasive have a
structure of the stone.                        many characteristics. When one thinks        greater ability to wear styli, but soft
                                               of something as being hard, he is a t the    record materials often betome imbedded
Chemical and Physical Properties               same time thinking. of a number of           with abrasive p'articles which also can
  Lavoissier discovered that diamond           properties such as resistance to wear,       cause rapid stylus wear. Stylus wear is
consisted of carbon when he burned             ability to resist indentation of a sharp     rapid at first, and then, as the contact
diamond at high temperatures to form           point and non-compressibility. For ex-       area becomes larger and the pressure
                                                                                            per square inch decreases, wear con-
                                                                                            tinues a t a slower rate. This effect is
                                                                                            noticed with new clean records ;however,
                                                                                            as records become worn and the grooves
                                                                                            become progressively loaded with abra-
                                                                                            sive particles, the rate of stylus wear
                                                                                            may continue at a comparatively rapid
                                                                                            rate.
                                                                                               The softer stylus materials have a
                                                                                            great tendency to load the record grooves
                                                                                            with abrasive particles, as shown by G.
                                                                                            A. Briggs. I t is a fallacy that styli made
                                                                                            of soft materials cause less record wear.
                                                                                            Brigg's photomicrographs show steeI
                                                                                            flakes imbedded in the groove walls after
 Fig. 4. Showing different 1.0-mil styli after fifteen plays on 12-inch Vinylite            only one playing of a new shellac disc
 records with an 8-gram pickup. Left, diamond; center, sapphire; right, osmium.             by a steel needle. Another photomicro-

26                                                                                    AUDIO ENGINEERING                   JULY, 1950
                                                                                                     Audio Engineering Society Section




Fig. 5. 2.5-mil styli after 1000 plays on 10-inch Vinylite records with a 1%-ounce pickup. From left to right: diamond;
                      sapphire, front view; sapphire, side view; osmium, front view; osmium, side view.


graph shows a fibre needle completely        we know that eventually even the dia-           ing the resistance of various materials
abraded after one playing. These con-        mond will begin to show wear.                   to corrosion. As the hardness of the
ditions lead to excessive record wear and       The results of the micro-groove test         powder used for the sand-blast was in-
poor response.                               were rather surprising. We found that           creased in going from quartz to sapphire
   Our tests were conducted, on Vinylite     after only 15 plays or 534 hours playing        to silicon carbide, the differences in the
records, both standard groove 78 r.p.m,      time, both the sapphire and osmium tips         hardness values for the test materials
and microgroove 33% r.p.m., using three      were very badly worn, and the test              become smaller. W e observed corre-
stylus materials, diamond, sapphire, and     was stopped at that time. Again the
                                                                                             sponding results in our stylus wear tests.
osmium. The following conditions were        osmium wore out faster than the sap-
observed for all styli under test:           phire. Photomicrographs and tracings            As we have already shown, the wear

                                .     TABLE I 1
                                                                                             ratio between diamond and sapphire
                                                                                             when played on Vinylite records is far
                                                                                             above the Rosiwal 90-1 ratio, while this
                               Standard Groove                      Microgroove
                                                                                             difference is not quite as great when
 Record Material             10" Vinylite                12" Vinylite
 Record Player               Garrard Changer             Webster 3 speed changer             these materials are used on shellac
 Cartridge                   Astatic L 70                G. E. RPX-041                       records.
 Pick-up weight                  1Goz.       -               8 gms.                              Other results of our stylus and record
 Radius of stylus            2.5 Mils                    1.0 Mil
                                                                                             wear tests showed that wherever there
   Test methods were made to conform         were made at the completion of the test.        was excessive stylus wear it was always
to the severe playing conditions usually     The qiamond showed no sign of wear              accompanied by excessive record wear.
encountered with home-type equippetit,       after 15 plays. The microgroove diamond         The badly worn styli are unable to track
and both tests were repeated twelve times    was continued in use for a total of 100         properly and have a tendency to chop off
to eliminate the possibility of erro_r.      plays or 37 hours. A slight flat was            the high-frequency crests in the grooves.
   The results of the standard groove test   noticed on the diamond at the end of 37         I t was observed that stylus pressure,
showed that after 1000 plays, or approxi-    hours.                                          pick-up arms, and cartridges have a
mately 50 hours playing tive, wear on           Apparently, stylus wear is far more          great effect on stylus and record wear.
both the sapphire and osmium tips was        rapid on microgroove than on standard            Considerable variation in rate of stylus
very severe. Considerable wear was           groove records. A rough estimate is             wear was noticed in making wear tests
noticed as early as 100 plays. There was     about three times faster. The rate of            on different types of equipment. For
complete conformity to groove shape          wear of the 1.0-mil osmium and sapphire          example, one professional arm caused 1.0
after 1000 plays. The diamond stylus did     styli caused by microgroove playing is           mil styli to wear out about five times
not show any wear after 1000 plays.          so rapid as to make these materials un-         faster than did other arms designed for
Shadowgraphic tracings were made after       suited for continuous use on microgrbove        amateur use.
each 100 plays and showed the progres-       records.                                           Quality of response deteriorates gradu-
sion of wear on the sapphire and osmium         'To return now to the hardness values         ally as stylus wear increases. This de-
tips. Photomicrographs were taken of         given in the table, we can see that it is        terioration is noticed more readily on
the stylus tips after 1000 plays. Two        necessary to conduct tests under the con-        good quality audio equipment. On home-
views were photographed, one parallel        ditions of use before hardness values can        type equipment, with its narrower fre-
to the groove position ahd the other         be assigned to any materials.                    quency range, stylus wear is usually not
perpendicular to the groove.                    If we could accurately weigh the stylus       noticed audibly until it has become very
    Superimposing the osmium shadow-         tips before and after the tests, it is quite    bad. The gradual wear over a long period
graph tracing over the sapphire tracing,     certain we would find a difference be-           of time occurs slowly, and therefore the
we found the same amount of wear for         tween the diamond and sapphire in ex-            listener does not readily notice the
sapphire a t 1000 plays as for osmium at     cess of the 90-1 ratio given by Rosiwal.       -change. H e cannot remember how his
400, or a 10-4 ratio. Since the amount          According to Ridgway and Eppler,              equipment sounded six months before
of material removed from the diamond         the differences in relative mechanical           when the stylus was new. This has the
 was so slight as to escape notice a t 500   corrosion hardness become smaller as             unfortunate effect of causing excessive
 times magnification, the ratio of resist-   the hardness of the material used for the        record wear by continuing the worn
 ance to wear could be written : Diamond     corroding or abrading increases. They            stylus in use.
  co, Sapphire 10, Osmium 4. However,        used a sand-blast technique in determin-                            [Continued on page 411

AUDIO ENGINEERING                JULY, 1950                                                                                         27
                   THE DIAMOND
                          [fronz p a g ~ 271

      record wear by continuing the worn
      stylus in use.
                          BIBLIOGRAPHY
      G. A. Briggs : "Sound Reproduction,"
        Wharfedale Wireless Works, Bradford
        Road, Idle, Bradford, Yorkshire, Eng-
        land.
      W. F. Eppler: "Relative Mechanical Cor-
        rosion Hardness of Svnthetic Corundum."
                                         ,
        Industrial Diamond < ~ e v i e wJune 1945,
        pp. 121-125.
      P. Grodzinski: "Notes on the Cleaving and
        Sawing of Diamond," Mineralugical
        Magazine, London, Sept. 1944, pp. 46-50.
        Diamond Tools, N.A.G. Press Ltd. Lon-
           don. 1944.
               7-- - --
      J. F . - ~ a ~ s e r
                       : Zgzdz~strialDiamond Review,
        Vol. 4, 1944, pp. 2-4.
      F. Knoop: Research Paper 1220, 1939; 11%-
        dustrial Dia~nond Review, Vol. 1, 1940,
        pp. 292.
      R. R. Ridgway: "Hardness Values for
       ,Electrochemical Products," Trans. Elec-
        trochenz. Soc.. Vol. 63. 1933. DD. 369-393.
      A. Rosiwal : ~ k a d . d. W'iGnschaften,
        Wien, 1893, No. 11 and 1917.
      Schuler and Dimpker: Zeitschrift f. Zn-
        struinentkz~nde.Vol. 55. 1935. D. 63.
      G. F. Shotter : ~ ' e t e r fnstrument Jewels
                                and
        and Pivots, 2nd report, 1944, p. 110.
        British Electrical & Allied Ind. Res.
        Assoc.
      A. Williamson : Journal Franklin Institute,
        Vol. 193, 1922, p. 491.




AUDIO ENGINEERING                   JULY, 1950