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How to make a thin section

Dave's methods at WWU Geology
Note: this document is online at www.davehirsch.com (under "Miscellany")

What is a thin section?

A thin section is a 30 µm (= 0.03 mm) thick slice of rock attached to a glass slide with epoxy.  Typical thin section slides are 26 mm x 46 mm, although larger ones can be produced.  They are generally covered by another glass slide, a cover slip also attached to the rock with epoxy.  The epoxy ideally has an index of refraction of 1.54, although our epoxy is slightly higher, perhaps 1.56.

The sections may be left uncovered for chemical analysis on the SEM or electron microprobe.  If so, temporary cover slips may be weakly attached with glycerin.

Thin section making equipment

There are five main tools you will use: The slab saw, the trim saw, the grinder, the cut-off saw, and the lap wheels.  You will want a Sharpie-type permanent marker, some liquid paper and a fine-tipped permanent marker.

Warnings: Things you can damage in the lab

  • Be sure when you use either the cut-off saw, or the thin section grinder, that the bottle is less than half full of water. If it gets too full, water can be sucked into the building's vacuum system. This would be bad. Don't do this.
  • Be sure to clean and dry the grinding wheels promptly after you finish using each one, or they will rust rapidly. Instructions for this are given below.
  • Be sure to use the correct grit for each grinding wheel. From left to right, they get finer and the grit numbers get larger: 120, 240, 400, 600.

Making your thin section

This guide is described in steps, with photos of a sample rock shown alongside as it undergoes the various steps.

(Note: you can click any image for a greatly-enlarged version)

Step One*: Prepare the glass slide

*You don't actually have to do this first. In fact, I often do this for a whole bunch of slides at once, then use them as needed.

The glass slide you will glue the rock to must be flat in order for the rock section to end up with a constant thickness. In order to achieve this you need to "frost" the slide, which accomplishes two goals:

  1. Removes the thick spots on the slide
  2. Adjusts the slide face to be parallel to the grinding wheel's face.

To do this successfully, you need to be able to keep the slide in the same orientation both which you are frosting it and later when you are grinding it. I accomplish this by grinding off a corner of the slide using the 240 grit grinding wheel.

Here are the steps:

  1. Turn on the second wheel from the left.
  2. Spray some water from one of the bottles on the wheel until it's completely wet.
  3. Sprinkle some abrasive on the wheel - you don't need very much at all.
  4. Carefully let a corner of a glass slide drag on the left side of the wheel (if you use the right side, which is moving towards you, it may kick out of your hand).
  5. Once a small amount has ground away, you're done. Clean all grit off the slide in the sink.
  6. You must clean the wheel promptly, or it will rust. The next section describes how.
Figure 1. Grinding wheels. Wheels have particular abrasives: rightmost wheel is 120 grit, then 240 grit (open), then 400 grit, then 600 grit . Glass slide corners are ground using the 240 grit wheel. Figure 2. Putting water on spinning wheel.
Figure 3. Putting 240-grit abrasive on spinning, wet wheel. Figure 4. Grinding the corner of a glass slide on the wheel.
Figure 5. Glass slide with a corner ground off. Note reflection - it's not yet frosted.

Cleaning the grinding wheel

You must clean the grinding wheels promptly, or they will rust. Here's how:

  1. Fill a large half-cut jug with water. Clean off a sponge.
  2. Pour half of it on the spinning wheel.
  3. Clean the spinning wheel with the sponge. Pour some more water on the wheel.
  4. Repeat step C until the sponge stops picking up dirt from the wheel.
  5. Pour the remaining water around the outside of the plastic bucket to make the grit wash away.
  6. Dry the spinning wheel with a paper towel or two.
  7. Dry the spinning wheel with the hair dryer until it's totaly dry.
  8. Cover the wheel immediately.
  9. Clean the bench top that holds the wheels.
Figure 6. Pouring water on the grinding wheel for initial cleaning. Figure 7. Cleaning the wheel with the sponge until all dirt is gone.
Figure 8. Inital drying of wheel with paper towel. Figure 9. Final drying of wheel with hair dryer.

Step Two: Frost the glass slide

You will now "frost" or grind the glass slide to flatten it out and roughen the surface so the epoxy can bind well. To accomplish this, you must always place the slide on the grinder in the same orientation. (I always put the slide on so that the cut-off corner points up and towards me).

  1. Turn on the water to the grinder.
  2. Find a fairly clean slide (not frosted) and place it on the slide holder of the cut-off saw (wet it if it doesn't stick). This is because the two devices share a vacuum system, and the one not in use must have a slide to create a seal.
  3. Wipe clean the slide holder of the grinder. Any grit will produce scratches on the finished section.
  4. Place the section on the holder of the grinder.
  5. Turn on the vacuum (the valve is between the two instruments; it's on when it points away from the wall). Check to see that the slide is firmly held.
  6. Turn the control towards you until it reads about 70. (This is counter-clockwise; you should see the left part of the grinder move away from the right part which holds the grinding wheel).
  7. Turn on the motor.
  8. While moving the slide back and forth, slowly turn the control clockwise until the slide begins to contact the grinding wheel.
  9. Note the number (on the control) at which contact is made.
  10. While still moving the slide back and forth, advance the control wheel gradually about three ticks from the contact value. At this point, grind for a while without advancing the wheel while moving the slide back and forth, to ensure complete and even grinding.
  11. Turn off the vacuum and press the vacuum release valve (the black tube from the slide holder connacts to it; it is pressed towards you). When the hissing stops, remove your slide from the holder. (Try not to drag it, or it may get scratched).
  12. Dry it off and see if it's all frosted evenly. If not, repeat the above steps until it is. You should not need more than four ticks or so.
  13. Once it's all frosted, make a note of the position at which you stopped; you'll need it later. This value is the zero-thickness position, the position at which the grinding wheel contacts the glass slide.
Figure 10. Cut-off saw (left) and thin section grinder (right). Note that water controls are between sink and cut-off saw, and vacuum controls are between the two. Figure 11. Slide on grinder showing position including ground corner.
Figure 12. Placing slide on grinder for frosting. Be sure slide rests against the pins, and place it on the holder with the vacuum turned off. Figure 13. Grinder control with numerical gauge.
Figure 14. Slide mostly frosted, but with unfrosted portion at top. Figure 15. Slide completely frosted.

Step Three: Mark your rock

You need to decide where you will cut your rock.  This matters, especially with rocks that have a fabric.  You generally will want to cut a thin section on a plane perpendicular to any planar fabric, but for particular purposes you may want thin sections in other orientations.  Mark a line on the rock with the Sharpie.

Figure 16. Rock prior to cutting. Pencil for scale. Note that this rock was not marked before cutting, but normally it would be.

Step Four: Cut the slab

Using the slab saw in the back of the room, you will cut a slab from your rock along the line you marked.

  1. First clamp the sample in the holder so that the line is parallel to the saw blade, sticking out of the holder.  You do not make your first cut along this line!
  2. Find crank handle and attach to square protrusion on tray. Wind the tray about 11 turns away from the saw blade. There is some hysteresis, or "slop", so don't count crank turns where the tray isn't moving. Remove the crank handle.
  3. Close the cover, making sure the sides and bottom are tucked inside the lip.
  4. Make the cut. First turn the switch on to get the blade moving, then turn the knob until the weight begins to fall.
  5. Once the cut is done (you can hear it), turn the blade off and stop the weight moving by turning the knob away from you.
  6. Pull the tray back to the beginning, and use the crank to wind the blade back to your line, 11 real turns (where the tray is moving).
  7. Remove the crank handle, and close the cover.
  8. Make the next cut as you did the first. These two cuts should be parallel planes about 8-10 mm apart. The slab should fall onto the catch tray behind the blade.
Figure 17. Slab saw. Note location for crank in bottom center. Figure 18. Slab saw controls. Turn knob to move rock into/through blade, and switch start blade turning (only turn on with cover completely closed!).
Figure 19. Rock ready for first cut. Note that it sticks out extra far to allow for second cut without removing from clamp. Figure 20. After first cut. Next, close knob, pull tray back from saw, then move tray towards blade about 11 turns of the crank handle.
Figure 21. Ready for second cut. Figure 22. After second cut: slab complete (piece lying on catch tray at left).

Clean up the slab

  1. First wash the slab, the leftover rock, and the first cut piece (if there's enough to save) to remove any oil and grit from the slab saw process.
  2. Next (while the rocks are drying) set the leftover rock and the first cut piece on a paper towel to dry, but mark the sample number on the towel. It's very easy to mix up specimens at this point, because you may have needed to cut through the number previously marked on the rock. Also, the first cut piece likely has no number. Once these are dry, re-mark them with the sample number. I use liquid paper and an india ink pen, followed by nail polish once it's dry.
Figure 23. Washing oil and dirt off the rocks after slab cutting. Figure 24. Drying speicmens with sample number written on the paper towel .

Step Five: Cut the chip

You need to reduce the size of the slab to slightly smaller than a thin section. For this you will use the trim saw against the back wall of the room. You need to carefully decide from what part of the slab you want to cut the section.

  1. You will use a special rock-cutting blade which is made of metal with diamonds embedded in it. Essentially, such blades are narrow grinding tools. You can even safely put your finger on it as it moves (if it is lubricated with water), however you should not put your fingernail on it.
  2. You should wear eye- and ear-protection gear when using this saw.
  3. Start the water using the valve above and to the right of the saw, and start the blade using the switch to the right.
  4. You just put the slab on the platform and move it into the blade. You can use a glass slide to get the size correct (the chip should be slightly smaller than the slide).
  5. After cutting the chip, the side from which the thin section will be cut must be polished to remove marks from the saw blade. For best results you can grind on each of the four wheels, but a useable section will result from just grinding on the seccond (240 grit) wheel.
  6. As above, clean the grinding wheel when you are done with it.
Figure 25. These blades are safe to touch as long as they are lubricated, but don't put your fingernail on it. Figure 26. The platform moves, so you just push the slab through the blade.
Figure 27. Final cut for the chip. Figure 28. Grinding the chip will remove any saw marks from the cutting.
Figure 29. Washing the chip.

Step Six: Glue the slide to the chip

Next, you attach the frosted side of the slide to the side of the chip you just ground down. You need to ensure a constant thickness of epoxy across the section, or when you grind the section down, one end will be thicker than the other.

  1. Heat up the chip by placing it on one of the hot plates with the polished side up (so it doesn't get dirty). This allows the exopxy to flow more easily and cure faster.
  2. Write the sample number in pencil on the frosted side of the slide. This number will get covered by epoxy shortly.
  3. You now need to mix a batch of epoxy.
    • For a single thin section, this will be too much, but it is difficult to accurately mix a small amount.
    • You need to use two parts epoxy (the large bottle) with one part hardener (the small bottle). It's best to use the balance to measure by weight.
    • A small batch would have about 4 grams epoxy and 2 grams of hardener. (Be sure to take into account the weight of the tray!)
    • These materials now must be well-mixed, but while creating as few bubbles as possible. In order to achieve this, I drag the coffee stirrer in a sinuous pattern first in one direction, then in the same pattern rotated 90 degrees.
  4. Once the epoxy is well-mixed, spread a few drops across the top (polished side) of the warmed chip.
  5. Many rocks are porous (even some that you might not expect) and you must let the epoxy penetrate fully, or the thin section will not survive. So, spread epoxy, then wait for it to soak in (5 minutes) then spread more epoxy. Repeat until no more epoxy soaks into the chip.
  6. Place the slide, frosted side down on the epoxy. It's best to put one side down, then let the other side fall to avoid trapping bubbles.
  7. Move the slide around with your finger or a pencil eraser. This will squeeze out extra epoxy to achieve a constant thickness, and if you work at it, you can remove most bubbles this way as well. While doing this, be sure that the sample number gets coated with epoxy.
  8. Let this sit to cure. Check it periodically for the first 5-10 minutes to be sure the slide has not slid off the chip.
  9. The epoxy will cure in about 20-30 minutes. Do not take the next step before that.
Figure 30. Cut and polished chip warming up on the hot plate.. Figure 31. Sample number marked in pencil on frosted side of slide.
Figure 32. Epoxy and hardener prior to mixing. Figure 33. Pattern for mixing. Should be rotated 90 degrees after each time through pattern.
Figure 34. Spreading epoxy on chip. Figure 35. Chip coated with epoxy after allowing it to penetrate.
Figure 36. Placing slide on chip, frosted side down. Figure 37. Moving slide around to expel bubbles and extra epoxy, and coat sample number.

Step Seven: Cut off the chip from the slide

Now you have a rock chip epoxied to a glass slide. You will next cut most of the chip off, leaving a thin slice attached.

  1. Place a blank glass slide on the grinder to block the vacuum.
  2. Turn on the water to the cut-off saw.
  3. Place the slide on the cut-off saw (note direction of ground corner) and turn on vacuum to hold it. Be sure edges of slide are lodged against pins in holder.
  4. Turn on saw motor.
  5. Use the handle to move the chip into the blade. Go very slowly, or you can break your slide and have to start over.
  6. Once the chip is cut off the slide, retrieve the chip from the water tray and set it aside (it will likely need to be labeled with the sample number).
  7. Turn off the vacuum, and saw motor.
  8. Using the button (as with the grinder - see above), break the vacuum. Once the hissing stops, remove your slide from the saw.
  9. Rinse your slide to remove any particles.
Figure 38 . Slide with chip mounted in cut-off saw. Figure 39 . Slowly cutting off the chip from the slide.
Figure 40 . Chip nearly cut from slide. Figure 41 . Chip and slide separated.

Step Eight: Grind your slide to the correct thickness

You must now grind away much of the rock that remains on your slide, but not all of it. This is the step in which most thin sections go bad. The key is: go slow, especially near the end.

  1. Place a blank glass slide on the cut-off saw to block the vacuum.
  2. Turn on the water to the grinder.
  3. Turn the control towards you (counter-clockwise) a full rotation to zero.
  4. Be sure the slide holder and the slide are clean and free of any grit or particles.
  5. Place the slide on the grinder (maintaining the same orientation using the corner you notched).
  6. Turn on the vacuum and grinder motor.
  7. Moving the slide back and forth with the handle, gradually advance the control until the slide contacts the grinding wheel.
  8. While moving slide back and forth across the grinding wheel with the black handle, gradually advance the control until you reach a point about 15 ticks below your "zero value" (see step 2).
  9. Remove the slide (turn off the vacuum, break the vacuum with the button, then remove the slide) and see if you can identify any minerals in the section.
    • If you can, then proceed slowly until those minerals achieve the correct maximum interference color (e.g., quartz should show a maximum interference color of a pale straw yellow)
    • If you cannot, then advance one tick at a time until you can identify a mineral. If you are within 5 ticks of your zero value, and you still can't identify anything, stop and ask someone who knows more than you.
    • Go slow, especially when you are close to the correct thickness! It's quite easy to go from slightly too thick to slightly too thin (or ground completely away), even without advancing the position control at all!
  • Troubleshooting
    • Warning: Most sections fail at this stage because you are grinding too fast! Remember, any time the grinder makes noise against a slide, you are removing rock. You often don't have to advance the wheel at all to just grind a small thickness away.
    • If your section comes out with the minerals all cracked: you were grinding too fast.
    • If your section comes out with the edges thin and the center thick: you were grinding too fast.
    • If your section comes out thick on one side and thin on the other, you either didn't frost the slide well, or there was an uneven epoxy layer.
Figure 42 . Unground slide mounted in grinder holder. Figure 43 . Moving slide across grinding wheel while advancing control (with right hand).
Figure 44 . Checking the slide thickness using the microscope. Figure 45 . Quartz in a thin section, way too thick.
Figure 46 . Quartz in a thin section, still fairly thick. Figure 47 . Quartz in a thin section, still a bit thick.
Figure 48 . Quartz in a thin section, now too thin. At this point, much of the remaining section has been ground away.

Step Nine (version 1): Add a cover slip

If you do not expect to perform chemical analysis on your minerals in this thin section, you should add a cover slip to protect the section from damage, and increase the clarity observed in the microscope. This process is a simple one:

  1. Make sure the section is clean and free of grit or dirt.
  2. Place it on the hot plate.
  3. Mix up a small batch of epoxy and hardener.
  4. Place a small drop of epoxy on the section.
  5. Drop a cover slip on the drop.
  6. Move it around to expel bubbles and fully coat the section.
  7. Let it cure.
  8. After it has cured, there may be extra epoxy on the top, sides, or bottom. You can remove this, very carefully, with a razor blade.
Figure 49 . Putting a drop of epoxy on the ground slide. Figure 50 . Container for cover slips.
Figure 51 . Dropping a cover slip on the slide. Put one end down first in order to prevent bubbles. Figure 52 . Moving cover slip to spread epoxy and expel bubbles.

Step Nine (version 2): Polish for Electron microprobe / SEM-EDS analysis

If you plan to perform chemical analysis on your minerals, then you need to polish the section much better than the grinder polishes it. This is because these analysis tools require a flat, smooth surface at the micrometer scale. You perform this polishing step using different tools in a different room.

The key diffference for our purposes here is that you should stop grinding when the section is still a little bit thick, because you will be removing material during the polishing process.

The polishing apparatus is located on the ground floor, at the other end of the building (near the XRD).

(Full instructions for polishing may be added to this document in the future. For now, ask a knowledgeable user for help).

Step Ten: Clean up the place

You must clean the lab thoroughly when you are done using it. You may store samples in progress in one or more trays clearly labeled with your name and the date. These trays are placed in the lower cabinets in the thin section lab room, or (for grad students) in the cabinets in the center of the adjacent room.

  1. Make sure that the grading wheels are clean and dry (see instructions above).
  2. Completely dry off the dark metal section holders on both the cut-off saw and the grinder.
  3. For each of those two tools, spread some oil on a paper towel, and coat the dark metal portion thoroughly with oil, then place the oiled paper towel underneath the section holder.
  4. Empty out the water from the vacuum bottle (covered with tape).
  5. Place any epoxy stirrers crosswise on the epoxy holder so that when the epoxy dries, both can be reused.
  6. Mark your name in the log sheet.
  7. Turn off the lights and close the door when you leave.
Figure 53. Drying off the section holder. Figure 54. Oiling the paper towel.
Figure 55. Oiling the section holder. Figure 56. Tool dried, oiled, and paper towel placed for storage.
Figure 57. Removing tubing from vacuum bottle. Figure 58. Emptying the vacuum water bottle.
Figure 59. Epoxy stirrer and tray stored for curing and reuse. Figure 60. Filling out the log sheet.

-Dave Hirsch, 4/12/04



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