Transmission of light through a specimen is necessary to examine the structure of some materials, including petrographic materials. The sample needs to be exceptionally thin for this to work effectively on rocks and minerals – typically 5-30 microns. In order to be able to analyze it most effectively, many applications need the thickness of the specimen to be extremely consistent.
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In order to prepare materials to such a small dimension, they are supported by bonding the sample to a glass slide prior to thinning. The attachment to the slide is very important to ensuring an accurate and effective result. The specimen may not be of consistent thickness or it may even come off the slide during preparation if there is not an effective bond between the material and the slide.
A procedure known as ‘frosting’ is used to elevate bond strength and ensure the most uniform thickness. Frosting maintains uniformity across the full glass slide by ensuring that the surface is parallel to the diamond grinding wheel, and creating microgrooves on the bonding side of the glass surface.
The operation is straightforward – attach the clean slifde to the vacuum chuck and grind using the diamond grinding wheel in ~10 micron increments, until the surface of the slide is evenly marked – it will look milky white at this point.
For optimal results, keep the glass slide in the same orientation throughout processing. This is most easily done by grinding briefly with 600 grit SiC paper on one corner of the glass slide.
Low Viscosity Epoxy
Using a low viscosity epoxy such as EpoThin 2, the sample is bonded to the frosted surface. The bond thickness is kept to a minimum between the specimen and the glass slide as a result of the low viscosity epoxy, in addition to maximizing the penetration of the epoxy into the sample, which supports it during later polishing stages.
Using the PetroThin, thin sections from samples that have been bonded to glass slides are constructed. Fixed to the glass slide is the material of interest, and prior to polishing the PetroThin, it is used to grind and cut it to less than 100 microns thick. The specimen may not be placed without error or may even come off the slide during preparation if there is not an effective bond between the material and the slide.
Thin sections from samples that have been bonded to glass slides are prepared to under 100microns using the PetroThin. The sample is held in the Petrothin and cut using a diamond blade to ~500microns. Without any re-clamping or other adjustment, the sample is transferred to a grinding wheel where it can be reduced in thickness to under 100 microns with exceptional precision. The sample is then ready for polishing to its final thickness as defined by the material being examined.
The PetroVue thin section viewer is used to check specimen condition and thickness during preparation. It is the fastest and most effective way of controlling material removal during final stages of preparation, and prevents the need to examine under a high powered polarized microscope during the preparation stages.
This information has been sourced, reviewed and adapted from materials provided by Buehler.
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