MAKING THIN SECTIONS BY HAND
Shortened and customized for our Department from the original by
Richard Hill
copyright 1999
M.A.P.S. Digest
Used in compliance with US copyright laws, one copy for education purposes.
Abstract
A method of manually making thin sections is presented. Simple techniques for maintaining parallelism
that avoids direct measurement is described in detail and a number of examples are shown.
Introduction
The techniques described here are simple but can lead to remarkable results. This was gleaned from
several sources and tried over the last few years. (Allman & Lawrence 1982, Kummel & Raup 1965)
Patience is required for rushing the job will inevitably lead to ruin of the specimen and a waste of the
time put in to the work. Materials, for the most resourceful, can literally cost nothing and at most will
cost little.
Procedure
Overview.
The task is to find a good specimen, grind and polish one face of a small cut out slab, glue that to a glass
slide. Next, grind down and polish the other face of the specimen so that it is parallel to the first side
and only about 0.03-0.05mm thick.
Cutting the specimen out.
Back in the lab cut off a piece of one end of the rock. Cut another piece off the main rock as thin as you
can, 2-4mm will do but 1-2mm is better. If it breaks then save the pieces. The important part is that you
have a little slab with roughly flat, parallel sides. Trim the slab to be the size of a cover slip.
Materials for grinding.
For the next steps a selection of grinding powders will be needed along with some other items:
•
220 carborundum or silicon carbide
•
400, 600 carbo. or silicon carbide or F and 4F pumice powder
•
Polishing powder
•
A piece of twill or denim.
•
Microscope slides.
•
Epoxy (20 min. or longer) or PaleoBond glues.
•
Some pieces of brass shim stock or equivalent, preferably in several thicknesses and at least
twice as long as the narrowest dimension of the microscope slides being used.
More of the coarse powders will be needed and less of the fine which should please your wallet. These
can be found at most any rock shop. There is no need to buy better grades and these are only
recommendations. Substitutions can be made as to powder composition and grade. At minimum a
coarse, medium and fine grade is required. Diamond polishing paste while not only expensive is too hard
for this work. Titanium oxide is too slow and soft. Cerium polishing powder offers a good compromise
and is a very fast polisher especially if attention is paid to the last grinding step to make sure it is "fined"
thoroughly. In our lab we use stannic oxide, aka tin(IV) oxide, SnO2.
Grinding tools.
You will also need some glass blanks. The glass should be at least 1/4" thick and about 4" square. The
glass will likely not be truly flat when you get or cut them. Check them by bouncing light off the surface
at a very low angle look to see if any light passes under the edge of a good ruler laid across it on edge
Make sure it is a good quality ruler (available at good hardware stores). Ideally, if the glass is dead flat,
no light will be seen below the ruler especially after the glass is ground a bit. Anywhere a thin thread of
light can be seen is a low spot. With a freshly cut piece of glass it is probable that the very edge will be
higher than the middle. This is due to a problem called the Twyman Effect. There is a lot of tension
stored in a polished glass surface and when it is cut the surface of the glass relaxes almost always
resulting in a "turned-up-edge". This can be ground off easily using the instructions below.
Truing the glass blanks.
Glass blanks can be brought into flatness by grinding several against each other.
To start take two of the glass disks and put one on some newspaper on a good flat surface. I prefer
working in an old baking pan or cafeteria tray to confine the slopped abrasive and run-off water. To
begin a "wet" put a "charge" of the 220 grit on the bottom glass disk with a little water. Grind another
similar sized disk on top back and forth with a stroke that causes and overlap of about a quarter of the
disk diameter. Keep rotating the top piece every few strokes and the bottom piece, in the opposite
direction, every minute or so. As you grind the top piece will become concave and the bottom convex.
So if you started with a piece that was ground down in the center and thus concave and put that down
as the bottom piece, eventually it will be flat again by this process. You will have to check with the ruler
every few minutes. To make them initially flat should only take five to ten minutes.
Grinding
Take the little slab, place it on the glass flat (aka glass tool) with some abrasive and water from a rinse
water bowl or pan that you should have nearby. Use only one finger on the back of the piece and grind
with a medium to light pressure and move in a zig-zag or circular motion around the glass tool (see
diagram at end of paper). Be sure to move about the glass plate as best you can so you don't grind a low
spot in one area unless you are trying to grind down a specific area of the tool. Grind it until all the saw
marks are gone. Use a 10x handlens to inspect the piece as you grind it. If you had a steady hand on the
saw and the cut marks have little relief, you could start with the 400 or 600 abrasive size.
After each abrasive be sure to inspect the surface to see if the pits left by the abrasive are uniform and
that all the bigger pits from the previous abrasive have been removed. Be sure to wash up the work
completely between abrasives. One grain of a coarser grit in the finer grades will ruin your hard work. All
this is particularly important if you are only using only the three minimum grades of abrasive for
grinding. I use the cafeteria tray with newspaper in the bottom and toss out the paper after every grade
plus wash the tools, specimens, and change the rinse water. Once all the saw marks have been ground
off the specimen and you are sure on inspection that the piece is still flat and the grind pattern on the
plate is even, you can go on to the next abrasive. Go from 220 to 400 to 600 and then to the polish.
Remember while grinding and polishing to push the piece around in the abrasive with a medium to light
pressure. Don't rush the grinding or you will break the specimen. If in the finest stage the piece kicks and
sticks, try using a thicker slurry of abrasive and less pressure. If it still sticks you may want to wash it off
and check the glass blank to see that it is still flat. If the rock is concave and the blank a flatter curve the
work will kick and stick badly. It is unlikely that this will be a problem with a first piece done alone but
may if you do a few at a time. In order to have a problem with this you would have to be a few
hundredths of a millimeter out of flatness so it is advisable to only do a couple pieces at a time. If you do
have to re-true the glass plate tool, be sure to do a minute or two of grinding with the previous abrasive
to make the specimen match the new curve of the tool.
Slide Mounting.
There are a number of different micro-slides available (medical, petrographic etc.) plus you can make
your own like the ones I make to fit a 35mm slide projector. It does not matter what you use but be sure
to cut your specimens to fit the format of the slide before grinding.
All slides must be cleaned thoroughly. This is a most important step in making a successful slide. No
slides are clean as delivered by the factory, in fact, they can't be. If the glass slides were perfectly clean
cohesion of the glass surfaces would make it impossible for you to pry them apart. To avoid this
manufacturers use a light dusting of powders like talc or thin films of silicone oils on the glass to keep
them from sticking. This must be completely cleaned off for bonding to be strong enough to resist the
tremendous shearing forces of polishing. I have successfully used Windex (ammonia) as the water
soluble cleaner, or Isopropyl alcohol. You should check each slide for any smears or films in reflected
light and take every effort to get the slides scrupulously clean.
Lay the slides on a layer of paper several sheets thick on a solid flat surface. Be sure the surfaces are
clean. For bonding agents like epoxy can be used. Epoxies tend to be tough but plastic, which slows the
grinding (actually the rock will grind much faster than the glue) and slows the polishing, plus the glue
itself rarely polishes to clear. When choosing epoxy you should avoid the fastest setting glues (5 min)
and go for the longer (20 min. or longer) as these have different chemical formulae and bond to the
glass better.
You have some time to arrange the specimen. You may want to use popsicle stick or some such soft
prod to push the specimen around as the glue will bond the slide or specimen to your fingers much
faster than the specimen to the slide. It will take at least 24 and maybe 48 hours to get this glue to
completely harden. After 24 hours it is advisable to go back and put a bead of the glue around the edge
of the specimen to seal it in well. It also helps if the slide can be very mildly warmed. A screen over a
lamp shade with slides resting on it in the warm flow of air works very well. They should not get hot,
only warm.
If epoxy is used mix enough to do four or five slides at a time. Do not try to do too many at once. For
your first attempt just do one or two. Take a Popsicle stick and wipe the glue on the specimen first to fill
any pores. Then put some on the slide and push the sample into it working it back and forth a bit to seat
it and get a good bead of glue around the outside. Do not use too much glue. It will become a messy job
and a wide glue margin around the work will slow grinding and polishing. These will need 24 – 48 hours
to bond completely before any grinding.
In both cases, after the glue has set but not bonded you can look at the backside to inspect for bubbles.
It is unlikely that there will be any but wherever there are will be a weak spot in the slide that may cause
problems later. There is virtually nothing that can be done to fix this so you will have to be aware of it as
we go on and treat this slide with greater care.
Thinning.
Now that the little rock slab is glued to the glass microscope slide on it's polished side it is necessary to
grind it down to the required thickness for transparency so you can literally see through rock. The most
important thing now is to make sure the front surface being ground becomes parallel to the glued down
polished side, or the surface of the microscope slide itself. The latter is much easier.
Go slow with the grinding in these stages, don't rush. You've put a lot of work into this and you don't
want to wreck it now. Inspect the work frequently. When you can clearly see details in the minerals
through the sample, go to the last grinding grit. If a grain pops out of the sample or disintegrates in
grinding, as can happen, then stop and go to the final grind stage. If you are in the final grind when this
happens then go straight to polish. This will not produce a high quality slide but it will save the slide to
that point. To go on further with the grind would result in the disintegration of all or most of the slide.
Better to have something than nothing!
Use the last grinding grit until the work has a shine to it when a light is reflected off of it at a low angle.
Try to remove wedge but in a first effort this may not be successful. This is not a disaster. The last wet
with the finest grinding grade should be with very light pressure and should just skid about on the glass
tool. This will smooth out the pits of this final grade and make the polish go even easier.
Final Polish.
Once you have a good grind where you can see a reflection of a light bulb off the work when viewed at a
glancing angle (see diagram at end of paper), and the specimen is quite transparent, then it is time to
polish. Again, the piece of denim or twill is wrapped around the well cleaned grinding tool and a charge
of cerium is applied to the cloth. Polish until the adhesive is as clear as possible. The details in the fossils
will become more and clearer as the specimen takes on a polish.
Occasionally the glue will begin to peel at this stage, especially the cyano-acrylic glues like PaleoBond. If
this happens trim away some of the excess that is peeling with a razor blade or very sharp knife but
don't overdo it. Leave the best margin around the specimen that you can. This peeling is a warning that
the glue-glass bond is not as good as it could be. It will be necessary to put a cover glass over the
specimen after the polishing.
Cover glass.
If it is necessary to put a cover glass on then wash the specimen and slide well making sure all the
polishing residue is gone. Then let it dry for a day. Obtain your microscope cover glasses or cover-slips
from a scientific supply house. Also try and get a low viscosity epoxy resin and its hardener. The larger
cover glasses are best to ensure the entire sample is covered. After the cover glass is well affixed, trim
off excess epoxy with a razor knife (x acto hobby knife).
Fixing broken slides.
It will happen that you will break a slide. As Murphy would have it, this will be one of your better works.
All is not lost as long as the break does not occur through the specimen, but the remedy is tricky. Take
another microscope slide the same size as the broken one and using the cover slip mounting glue, glue
the broken one to the new slide aligning it as best you can. Put a large drop or two in the middle of the
slide and push the other squarely into the work. Work the two slides against each other well to force out
any bubbles and don't use too much glue or the job will get quite messy.
Labeling.
This is done by running the slide up and down along the straight side with the portion to be frosted
grinding on the surface of the tool. Finding a permanent but still small enough font labels was difficult
and is still an ongoing process. The best solution so far has been the old tried and true method of hand
lettering with pen, nib and India ink on a frosted end of the slide. We know this will work and last for at
least a century. If your slides do not already have a frosted or fine ground end on them it will be
necessary to make one. This is simply done by using a flat piece of glass with one straight side. , some
400 or 600 abrasive and grinding the section you want (see diagram at end of paper).
Reference:
Allman, Michael, and Lawrence, David F., (1972) Geological Laboratory Techniques, ARCO Pub. Co., Inc,
New
York.