10.3.
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333
amount of chromium that must be employed, the images tend to have
rather coarse grain. Gold is hardly ever used nowadays because of its
tendency to aggregate in large grains.
Nickel has a very small grain size but of course lacks great density.
It tends to amalgamate with tungsten so that its evaporation is not reliable.
Paladium, and to a lesser extent platinum, were for a long time the
metals of choice for shadowing. Platinum has by far the better grain
size. It has been found that a very good compromise results from blending these two metals. Usually a 4:1 ratio of platinum to palladium is
used. Wire alloyed in this way is commercially available. The density of
this alloy is 19.4 (platinum alone is 21.45). This gives it an advantage
over almost anything else which might be used except tungsten (19.3) and
uranium (18.7). The former is too difficult to handle for its extremely
high melting point precludes finding a convenient way of evaporating
it under slow controlled conditions.
In recent years uranium has become a metal of choice for much refined
shadow-casting. Its density, of course, is very great so that very thin and
therefore sharp films suffice to give contrast. Its grain size is small. When
used for very high resolution work to bring out the finest order of surface
detail, the film thickness is kept minimal, only a few atomic diameters
thick. Uranium should be etched momentarily in concentrated nitric acid
just before use to remove oxide films.
Bradley (1959) demonstrated that platinum and carbon, when evaporated simultaneously, yield shadows capable of very high resolution.
The carbon in essence prevents the formation of platinum crystallites
so that the grain size is perhaps smaller than with any other known
material. If an aperture in a screen is used between the source and the
target, extremely sharp shadows can be obtained. Bradley described a
method of preparing his own platinum-carbon rods in which powdered
platinum was blended with powdered carbon. This is a somewhat arduous and complicated technique but blended rods are now commercially
available (Ladd Research Industries, Inc.). Simpler methods, but less
controllable, involve drilling the tip of a carbon rod and inserting a piece
of platinum wire, or wrapping the wire around a cylindrically machined
carbon rod tip.
For much work, great precision in the amount of metal deposited is
not required, and visual estimation is adequate. A piece of white porcelain can be laid beside the specimen, at the same distance from the
SHADOW-CASTING
333
amount of chromium that must be employed, the images tend to have
rather coarse grain. Gold is hardly ever used nowadays because of its
tendency to aggregate in large grains.
Nickel has a very small grain size but of course lacks great density.
It tends to amalgamate with tungsten so that its evaporation is not reliable.
Paladium, and to a lesser extent platinum, were for a long time the
metals of choice for shadowing. Platinum has by far the better grain
size. It has been found that a very good compromise results from blending these two metals. Usually a 4:1 ratio of platinum to palladium is
used. Wire alloyed in this way is commercially available. The density of
this alloy is 19.4 (platinum alone is 21.45). This gives it an advantage
over almost anything else which might be used except tungsten (19.3) and
uranium (18.7). The former is too difficult to handle for its extremely
high melting point precludes finding a convenient way of evaporating
it under slow controlled conditions.
In recent years uranium has become a metal of choice for much refined
shadow-casting. Its density, of course, is very great so that very thin and
therefore sharp films suffice to give contrast. Its grain size is small. When
used for very high resolution work to bring out the finest order of surface
detail, the film thickness is kept minimal, only a few atomic diameters
thick. Uranium should be etched momentarily in concentrated nitric acid
just before use to remove oxide films.
Bradley (1959) demonstrated that platinum and carbon, when evaporated simultaneously, yield shadows capable of very high resolution.
The carbon in essence prevents the formation of platinum crystallites
so that the grain size is perhaps smaller than with any other known
material. If an aperture in a screen is used between the source and the
target, extremely sharp shadows can be obtained. Bradley described a
method of preparing his own platinum-carbon rods in which powdered
platinum was blended with powdered carbon. This is a somewhat arduous and complicated technique but blended rods are now commercially
available (Ladd Research Industries, Inc.). Simpler methods, but less
controllable, involve drilling the tip of a carbon rod and inserting a piece
of platinum wire, or wrapping the wire around a cylindrically machined
carbon rod tip.
For much work, great precision in the amount of metal deposited is
not required, and visual estimation is adequate. A piece of white porcelain can be laid beside the specimen, at the same distance from the