A thin section is a thin slice of rock only 30 microns thick that is mounted to a glass slide. The rock is glued using epoxy or Canadian balsam and preserved/protected by adding a glass coverslip.
1) Does the cover glass of thin section influence on the optical properties of minerals or on the image in general?
2) Do you use thin section with or without the coverslip?
Hello,
cover glasses obviously have an effect on the image they have, as images without cover are much worse than from the covered specimen. Especially at higher magnifications, this is quite obvious. It is due to not a completely flat surface, differences in refractive indices and then also how the objective lens is constructed and corrected. It is written on the lens for what it is intended for.
Uncovered and surface-polished thin sections are used for SEM, otherwise the petrographic samples always overlap. The (metallographic) sections are also not covered.
Hello, Dalibor Matýsek ! I agree. The cover glass actually improves the quality of the digital photo, as the gap between the rock slice itself and the cover glass is filled with Canadian balsam, which balances out the roughness of the profile of rock surface.
This is really interesting. I use thin section with cover glass (only petrographic analysis). I am also working on automatic identification of minerals in thin sections (software). And I wonder: how the missing of glass will influence on the quality of the automatic analysis?
I can't imagine anyone using uncovered thin sections for routine petrographic analysis. To control the quality before covering, it is always necessary to at least soak the thin sections in water.
Dalibor Matýsek , I thought so too, until I found out about several cases of the use of thin sections for both purposes (petrographic analysis and SEM).
It's a little different with SEM. They are made a little rougher for SEM and must be polished. In the end, I still look at it the same way, through the underlying glass :). But more often I prefer to have a separate covered thin section and a polished cross section for SEM. Thin sections are usually poorly polished and polishing quality is important to SEM.
Dear Alena,
The big problem of the covered thin section is air bubbles entrance during the preparation, but actually well prepared thin section with glass cover is absolutely better.
The most important thing is the polishing process in non-covered thin sections, it must be very well done to avoid scratches, pits and others unsightly features. The real difficulty is to find a very skillfull technician who does a superb job. The price for polished thin section is higher but is well worth it, because it ends up saving time and money.
I mainly use polished thin section in my work on both petrographic and SEM photography. I find that a polished thin section that is well done (with a mirror-like surface) is just as good as a covered thin section for microscopic photography and under the SEM the minerals are flat and rarely scratched.
Sadly I had some glass covered thin section that had bubbles or were dirty with a dusting of polishing compound and even had some old thin sections were the canadian balsam had started to crystallize (!!!). All these features get microscopic work sometimes hard to nearly impossible to do. When I used glass covered thin sections, I often had a deep regret when one rare interesting mineral or physical feature is found that I could not analyze it further via SEM because of the glass cover. That mineral/feature may not show up in the next polished thin section made from the same stone slab.
Thin-polished section is favorable to study opaques and/or metallic ores under reflected (ore ) microscope as well as is used for mineral chemistry carrying out by micro-probe technique.
Francine Robert , I've never removed the cover glass, but I've read about several methods. If cover glass is mounted with Canadian balsam:
1) you could warm them gently and possibly slide it off
or
2) by freezing cycles (ca. 8 cycles of freezing-unfreezing and the cover glass completely removed).
Mohamed Th. S. Heikal , Thank you for the information. Yes, it is very important, but depends on the skills of the technician.
*"The big problem of the covered thin section is air bubbles entrance..."
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