By A work in progress by Aidan Leetz
*Useful in case either Logitech LP50 and/or PM5 machines are out of order
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- This can be done using the MetaServ Grinder Polisher.
- The hardness of the material one is working with governs which grit sandpaper to start with. For very hard material such as some igneous and metamorphic lithologies it is best to begin with the 180 grit sandpaper disks.
- Simply apply the sandpaper disk to a magnetic disk and place the magnetic disk on the wheel of the MetaServ. (Insert Figures)
- It is important to turn on the water feed while flattening billets. This inhibits any dust particles from rising to the air.
- Keep RPMs between 300 to 400. This is manageable, faster and it will become slightly hazardous and any slower and it would simply take too long to flatten.
- Keep polishing until surface is visibly flat and free from any cut marks caused by the saw.
- Place a 240 grit disk and repeat the process, do this for 400 and 600 grit as well.
- After the 600 grit polish the billet should be completely flat and highly reflective (material dependent), a method to visibly check for flatness is to hold the billet under a light source and it should reflect the light evenly across the entire surface. (see figure below).
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- This is not necessity for this method but it can be used.
- Instead of simply placing the bonded billet and glass slide on the hot plate, you could place the billet under one of the fingers of the jig. Place a Teflon puck above the glass slide slowly release the finger of the jig so that it meets the indentation of the puck. (insert figure). Do this very slowly as a quick release will most likely shatter the glass.
- Is this step necessary? Not really, choosing to this step vs not doing this step ultimately does not affect the quality of the end product.
- Potential issues with step: Because of the pressure which the slide and billet is under, often times, with certain material, epoxy is either forced into pores, divots and cracks etc. as well as out along the side. This could potentially result in spaces on the surface where the is no epoxy and thus no bond between slide and billet. This potential issue could have severely negative effects later in the process. Another potential issue is that releasing the pressure from the jig slightly too early whilst the epoxy is not yet hardened, could cause the bond between the slide and billet to fail as the pressure release causes the elastic epoxy to expand and possibly lift off either the glass or billet.
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- While holding the slide against the chuck, turn on the vacuum pump at the wall plug (Do not touch the “vacuum” button on the actual PetroThin).
- Close the VS2 port. Turn the knob anti-clockwise for this. The dial on the VS2 should move towards the green zone and settle around 30 in/Hg. The slide should be difficult to move from the chuck. This indicates a good vacuum.
- With the slide secure to the chuck, turn on the PetroThin at the switch that says “motor”.
- Turn on the water feed slightly by gently opening the “water” dial. Avoid opening the water feed to wide. Just enough water to wet the blade is good.
- The numbers/scale on the dial above the switches indicates how close or far the blade will be from the slide during cutting. This ultimately determines the thickness of the section before grinding.
- It is best left at the set measurement.
- Move the arm as far left as possible keeping pressure on it whilst lowering it down gently towards the cutoff blade.
- The arm is spring loaded and thus pressure must always be kept on it to ensure it doesn’t violently jump back to its original and damage the system.
- Do not however apply any pressure when the blade is cutting, the weight of the arm itself is sufficient.
- When the blade is almost through the billet, hold the arm so that it does not swing down when the cut is complete.
- Lift the arm and slowly move it over to the diamond wheel, apply slight tension in the opposite direction of motion as the arm is spring loaded and will jump to the right.
- Set the scale on the right so that the wheel and section do not touch when you lower the arm.
- Find the values on the digital gauge where the section just about touches the wheel. Use this as your starting point.
- Decrease the values on the scale slowly, no greater than increments of 20 at a time.
- How to know when to take the sample off the PetroThin? Ideally once the section starts becoming quite thin you would proceed with caution on the PetroThin. It is entirely possible to grind away the entire section. It is best to continually check the thickness of the slide. How to do this is, using the micrometer, measure the thickness of the section (sample and glass) and note this value. Then subtract the thickness of the glass slide only (which you measured earlier) from the thickness of the section. This should give you the thickness of the sample. A completed thin section should be between 30 and 40 microns. Ideally, you would like to remove the sample from the PetroThin at around ±60 micron.
- Why not complete the thin section on the PetroThin? It is advisable to not take the thin section all the way down to ±35 micron on the PetroThin as even though the section will be the correct thickness it will have a very poor finish and very visible scratch marks from the diamond wheel, which is usually removed by hand polishing the section after the PetroThin. In this case the sample would be too thin for further polishing/grinding, any additional grinding/polishing after this would begin to remove material from the slide. Thus, keep the section thick enough to remove the scratches from the diamond wheel but not so thick that you are left spending too much time on the next step.
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- This step is done as a “finishing” step.
- Here you will remove the last few microns from the section, taking away any damage from the diamond wheel and getting the material to the correct thickness.
- By doing this step you are also ensuring that the section will take a good polish later on.
- Ready a glass plate on a flat surface.
- Mix water and 600 grit Silicon Carbide such that it has a muddy consistency, done earlier.
- Spread the Silicon Carbide and water slurry evenly across the glass plate.
- Take the thin section and place it face down on the glass plate.
- Apply direct downward pressure with your fingers on to the section and move the section in circular motions.
- To check the thickness of the section take it off the plate, rinse and dry it and then make use of the micrometer as outlined earlier.
- You should check that you are grinding down evenly by measuring each corner of the section as well as the center.
- This should all be the same value.
- If one side is thicker than the other, adjust where you apply pressure on the slide and also the position of the slide when hand grinding.
- You should not only make use of the micrometer to check the thickness, it should be used in conjunction with the microscope and a Michel – Levy colour chart.
- Practical way of using the Michel – Levy colour chart (see figure below) : This chart uses the observed interference colours of certain minerals to judge the thickness of the thin section. Using quartz as a reference mineral, if it is observed that quartz is displaying intereference colours (in cross-polarised light) that are first order orangeblue/purple, it means that the quartz grains are approximately 50 60 microns thick. They would need to be ground down further to around ±30 micron. As you grind down the sample the colour of quartz should change from a purple to bright orange to a bright yellow to dull yellow and eventually a dull grey to white. It must be noted that quartz cannot be always used as your indicator mineral, as it is not present in every rock. The principles still applies to other minerals though.
- Once your indicator mineral is the correct colour and the thin section is the correct thickness (which is check again using the micrometer) the grinding is done.
- There is a chart posted on the cork board in thin section, with common minerals listed.
- Michel-Levy colour chart (right) and quartz grain (left) that displays the change of interference colors as the thickness of the grain decreases. The purple is the thick part of the grain and the white is the correct color.
- Example of a thin section that is too thick (left)Example of a thin section that is too thick, and the resultant interference colours and one that is the correct thickness (right). Thin section is of granite.
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- Once the thin section is the desired thickness, clean off Silicon Carbide grit with water.
- Ensure all grit is removed and that there is no residue on the thin section. This is very important as to not contaminate the following steps.
- Place a Beuhler polishing cloth upside down on a flat surface and remove the paper back so that the adhesive side faces up (Insert fgure).
- Place a magnetic disk in the center of the polishing cloth and press down firmly so that a good bond forms.
- Trim of excess polishing cloth. (Cloth is bigger than the disk).
- Place the disk on the MetaServ and ensure it is securely in place.
- For the polishing compound, fill half of the PM2’s drum with Ethanediol and mix in one vile of 3 micron diamond powder.
- Mixing it in the drum allows you to make enough for an expedition and also affords a place to store the ready-mixed compound.
- Decant a small amount into a plastic measuring jug.
- Pour out a small amount of the mixture onto the polishing cloth. Just enough to moisten about a third of the surface area.
- Add a little water to moisten the rest of the polishing cloth.
- Turn on the machine, but with the water feed off, place the thin section face down on the polishing clothing and start the rotations.
- How fast do you need to turn up the machine? This depends entirely on the users comfortability with the machine. Polishing at 500RPM will not damage the slide by any means. This will in fact speed up the entire polishing process. Hence the reason it preferred (for time’s sake) to the PM5 (max 70RPM). If you are however not comfortable with this it is best to start slow and work up from there.
- Hold the section firmly down with as many fingers as you can and allow the cloth and diamond compound to do the work. It is advisable to move the section around in small circles on the rotating plate for an even polish.
- If you feel that the pad/cloth is becoming dry/hot, add a little of water to moisten the cloth, using the water feed system on the machine.
- Dry off the section with KimWipes and assess the polish. At around 200 – 300RPM, with firm pressure applied, check polish minute or so.
- How to assess a good polish visually? The surface of the thin section should have a mirror finish. It should reflect light evenly across the surface as well. This can be checked by placing the thin section underneath a light source and tilting it in various directions to check how the surface interacts with light. Again, it should be a mirror finish, meaning that you should be able to see reflections of the surrounding by looking into the section. Below is two images, the first disp;lays how an unpolished thin section reflects light. The second displays a mirror finish on a thin section. These two thin sections are made from the same billet. Note the difference in how the thin sections reflect light. They are directly under the same light source.
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- How to assess a good polish under the microscope? This is simple as you should now be able to view opaque minerals in reflected light. The clearer the mineral looks the better the polish (insert figure). In transmitted light the general field of view should also become much clearer, think of transitioning from 1080HD to 4K view on a television. Small pits and scratches in large grains should also become less visible and even disappear.
- Polishing different materials. The quality of polish you will achieve on a surface is not solely governed by how well you polish. The actual material itself affects the final polish. As an example, if one were to polish a hard rock and a sediment section together, the hard rock would achieve a better polish much faster than the sediment thin section. The sediment thin section might not even seem as if it is taking a polish no matter how long you polish it for. This has to do with the composition of the material. Think of trying to polish something made up lots of miniscule minuscule grains of soft material vs something made of large visible hard minerals, or try to visualize polishing metal vs polishing dirt.
- The images below illustrate the point of how different material take a polish. The image on the left details a polished thin section containing clasts which initially do not seem visible, everything apart from the large dark circles appear to be a matrix. The image on the right details how well the clasts reflects the light and how the matrix does not. This is because the clasts have a larger surface area to accept a polish whereas the matrix (composed of small individual grains) behaves adversely.
- Once the polish is satisfactory the thin section can now be used for reflected light microscopy and SEM analysis.
- The polish will deteriorate over time as it is being handled, this is fine and you can always re-polish.
- The polished surface can also be removed with silicon carbide slurry or on a silicon carbide/sandpaper disk. Be careful not too go too far here as you might start removing material again.
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