Geology Reference
In-Depth Information
thick sections (say 5 mm in thickness), sandpapers, and/
or wire mesh and a flat metallic plate with built‐in heat-
ers preferably using rheostats to control the temperature.
Moreover, the hot‐plate technique is inexpensive and
requires very little effort. It involves the removal of
excess ice by melting and with practice good sections can
be made. It has been proven to be very useful for exam-
ining the grain structures of freshwater lake, river, and
glacier ice.
Unlike metals or rocks, the working temperatures for
ice, as pointed out earlier, are very close to its melting
point. As an example, a comfortable working tempera-
ture of −5 °C or 268 K is equivalent to a homologous
temperature of 0.98
T
m
or only 2% below the melting
point,
T
m
= 273 K. Being so close to the melting point, ice
samples provide a very easy method for polishing the sur-
faces by surface melting. Glaciologists, therefore, are in
more envious positions than researchers in geology and
metallurgy. So, why not just melt away the excess material
using warm flat metallic or glass plates? This is the soft
method and precisely what has been practiced by the gla-
ciologists in general. The soft method of hot‐plate tech-
nique is the simplest and could be very quick with
practice. It involves cutting a thick section (2-3 mm thick)
using a good and stable band saw with sharp stainless
steel blade, with tooth spacing of 1 in 6 mm, and removal
of materials from both surfaces by melting. First step is
to remove excess material by putting the thick section on
a warm glass plate resting on a hot (warm-to-touch) plate.
A 15 mm thick large, 300 mm × 300 mm, metallic (prefer-
ably brass to avoid corrosion) plate, heated by insulated
heating wires, provides the necessary thermal inertia for
making sections. Let the bottom surface melt sufficiently
to make it flat. Remove the glass plate together with the
ice plate away from the hot plate, and after squeezing out
the meltwater allow the composite to cool and meltwater
to freeze. Once the ice section is solidly glued to the glass
plate, remove the excess material from the top surface
gradually in small steps, making sure not to melt the glue
and loose the specimen. With practice, the top surface
can be made very flat. The hard method is relatively little
more arduous and time consuming than the soft method.
The hard hot‐plate technique is more involved than
the soft technique described above. It was suggested by
Langway
[1958] and has been used extensively since then.
Recently, a detailed description of this method was given
by
Weeks
[2010, Appendix E], and the reader will gain an
insight of not only the sectioning procedures and precau-
tions to be taken but also the methods to be practiced in
the field for sampling ice blocks. Only a brief description
follows:
1. Saw cut the specimen.
2. Grind the surface with progressively finer sandpaper
or wire mesh sand paper.
3. Hold and press firmly a warm glass plate, large
enough to support the sample, on the specimen surface
and let a water film form between the glass‐ice interface
and refreeze.
4. Using a hand/band saw cut the sample parallel to the
glass plate leaving about a 1 mm thick section of ice
adhering to the glass.
5. Reduce the section thickness from nearly 1 mm to the
desired 0.4-0.8 mm thickness by:
a. Sanding with sandpaper
b. Heated copper or any heated plane surface
c. A modified standard milling machine
d. Preferably a standard biological microtome with a
vacuum plate attachment
Both the soft and the hard hot‐plate techniques have
been used by many investigators since
Rigsby
[1953] and
Langway
[1958] while developing the methods for fabric
diagrams. While developing the classifications of lake
and river ice (presented in section 4.3.1) the hot‐plate
technique was also used extensively by
Michel and
Ramseier
[1971]. As for sea ice, it is rather very messy to
melt or grind with sandpaper at any temperatures com-
fortable to human hands. Moreover, brine drains out of
the sea ice when hot plates are pressed on the ice and dur-
ing the thinning processes. Additionally, the processes of
melting, especially when the final thickness is only about
0.5 mm, undoubtedly disturb and damage the fine sub-
structure of sea ice. Although used extensively in the past
and perhaps the practice is ongoing, the hot‐plate tech-
nique is not suitable for sea ice containing brine pockets,
especially new, young, or even FY sea ice, unless the goal
is to quickly ascertain the type of ice required for many
engineering applications.
A temperature of +15 °C may be warm to human
touch, but it is very hot for ice as it is equivalent to 1.05
T
m
or 5% above the melting point. Ice does not exist at
such temperatures. Geologists or metallurgists will never
expose their materials to such high temperatures and melt
the specimens. They always use the cold‐cut methods.
Their working temperatures are always well below the
melting points of their materials.
6.2.2. Double‐Microtoming Technique
for Thin Sectioning of Ice
The cold‐cut or the solid‐state DMT technique origi-
nally developed for sea ice by
Sinha
[1977a] offers the best
possible choice for making thin sections with near perfect
parallel surfaces. Moreover, the method is capable of pro-
ducing surfaces without any artefacts, such as refrozen
and mushy (brine‐rich) layer of ice at the bottom pro-
duced by hot‐plate methods. The DMT is essential if
investigations are to be carried out on undisturbed lattice
structure of the ice surface, for example, basal and
