- Safety glasses are required at all times in the sample prep labs (M&M 714 and 715)
- Nitrile gloves
- Smock or apron
- Some work must be performed in a fume hood
As most thin sections prepared in this lab are cut from concrete and other portland cement-related products, nearly all of our thin sections begin life as a billet that will fit on a 27mm x 46mm petrographic slide. The polishing equipment in the lab is designed for that size slide so these procedures will be geared for thin sections of that size.
Another consequence of working with portland cement products is that kerosene and mineral oil are used as a lubricants, not water. Water will hasten the carbonation of the specimen and will also damage any partially soluble phases in the specimen.
- Cut the specimen to size (use a slide as a template if necessary). Make every attempt to keep the specimen prismatic with parallel and relatively smooth sides.
- A working glass will be epoxied to the side opposite the side to be sectioned. Setup your work area with the appropriate number of clear glass slides and place the specimen(s) with the side of interest down. If you have labels, organize them as well. A two part liquid epoxy is used to affix the working glass:
- Place equal-sized puddles of epoxy on a piece of cardboard and thoroughly mix with a stick (1/2 a clothes pin) and then smear a thin layer on the side of the specimen facing up.
- If you have a label, place it face up in the middle of the epoxied surface of the specimen.
- Line up the slide with the specimen and press into the epoxy with as little subsequent motion as possible (want to avoid smearing epoxy over the label).
- Let the epoxy cure for the period recommended by the manufacturer.
- The surface opposite the working glass will be ground and polished prior to affixing the final glass:
- Depending upon the geometry of the billet, the thin section grinder may spray oil onto you and/or the floor and surrounding work surfaces. Should that occur STOP WORK and take measures to contain the spray so that the oil stays in the sump.
- Clean the working glass with a razor blade and paper towel - it will need to be pristine to get the best possible seal on the vacuum chuck.
- Put nitrile gloves on.
- Plug in the thin section grinder lubricant pump.
- Plug in the thin section vacuum pump.
- Clean the vacuum chuck with a paper towel.
- Slide the surface of the working glass onto the chuck making sure that the slide is seated all way down against the chuck stops.
- Apply vacuum to the chuck by opening the vacuum valve and watch the vacuum gauge and listen for any sucking noise indicating that the glass is not seated properly on the chuck. Reseat the billet in the chuck and/or clean the surface of the working glass. Do not proceed until you have a good seal and vacuum.
- Adjust the thin section grinder platen such that the surface of the specimen just barely comes into contact with the face of the cup wheel (portion of the grinding surface closest to the rocking arm handle).
- Start the grinder motor and slowly advance the billet into the cup wheel. If there is a lot of resistance, the billet needs to be backed away from the cup wheel - make the appropriate adjustment before continuing.
- Patience pays big dividends here. Slowly grind away the surface of the specimen, making relatively small advances between several passes of the rocking arm. Grind until the surface is parallel to the working glass or you have reached the part of the specimen you wish to section.
- Turn off the grinder motor and wait for the wheel to come to a stop.
- Turn off vacuum with the valve and remove the billet.
- Thoroughly wipe the ground face of the specimen and the billet to remove oil.
- Wipe off any oil that may have been deposited on the exterior of the thin section grinder or surrounding work surfaces.
- Setup a #600 grit hand lapping glass on a piece of cardboard on the work table in the hand lapping/thin section cut off saw hood. NOTE that glasses are marked with the grit used upon them - do not use a #400 or #1000 glass!
- Sprinkle a small amount (2 tsp) of #600 SiC grit onto the glass and make a thick slurry with kerosene.
- Place the billet, working glass up, into the slurry and with significant downward pressure, move the billet in figure eights, making several laps for 2-3 minutes.
- Rinse the billet thoroughly with clean kerosene over a beaker and wipe thoroughly with a paper towel.
- Place the billet in a oven at 38 to 50 degrees C until dry. Do not leave in ambient atmosphere if the specimen is concrete - the surface that will be sectioned will carbonate.
Affixing final glass
This procedure can only be completed with thoroughly dry specimens. Acetone will be used so locate your workspace near a hood or another source of ventilation. Acetone will soften epoxy, so use extreme care when working with it around specimens.
- Put nitrile gloves on.
- Setup your work space by getting the appropriate number of frosted petrographic slides, setting up the epoxy burets and gathering a suitable number of twill jeans and paper towels.
- Spray a small amount of acetone on a clean twill jean and quickly and thoroughly wipe down all of the frosted slides to remove any oil.
- Spray a small amount o acetone on a clean twill jean and very quickly wipe off the ground face of the billet.