Time needed 290 million years and a working day

The granite we are using - from De Lank Quarry on Bodmin Moor - has been dated at around 290 Ma (million years)*.    It takes about a day to make a slide, though much of that time is waiting for adhesive to set, or for machines to finish grinding and polishing.

collect fresh ‘silver-grey’ granite, slice very thin with a diamond blade,

This granite is very strong and large blocks can be worked, so it is used as 'dimension stone' - i.e. it is cut to shape, rather than just crushed.  It is marketed as 'Silver-grey', and is cut with large diamond saws.  Geological labs also use diamond saws (the diamonds are set in the copper 'teeth' around the edge of a circular steel disc).

A thin slice parallel-side block is cut off a piece of 'fresh' (i.e. unweathered) granite.

glaze, bake for 1 hour 10 minutes at 65°C, thin until transparent, glaze again

Adhesive is put on a glass slide, and the slice is put on this.  The set-up is then put in an oven and baked.  Adhesives vary in the required temperature and time to cure, but Epotec 301 (an epoxy resin) needs 1 hour 10 minutes at  65°C.  (Some labs use  epoxy resins which are cured in a mere 10 seconds by UV, in a similar way to those used by dentists).  After baking it is allowed to cool down and the rock slice is ground down to 10 microns.  At this stage it is transparent.  The exact thickness can be decided by looking at the thin section through a microscope.

view through microscope with one polaroid

Geologists look at thin sections of rock using comparatively low power microscopes.  The main difference from a biological microscope is the presence of two polaroid sheets which can be slid into the light path, one above and one below the thin section.  The sheets are orientated so that one polarises at 90 degrees to the other, so that if both are 'in' (i.e. in the light path) no light can be seen through the eyepiece.  You get the same effect if you look  with one eye through  two pairs of polaroid sunglasses, one worn normally and the other held at right angles. 

Normally one polaroid is 'in' all the time and when you look at a thin section everything is transparent except for some ore minerals (especially the sulphides like chalcopyrite, pyrite, etc and most oxides).   The transparent minerals often are almost colourless, but biotite mica appears light brown - which is what you can see on the bookmark.  The other minerals - quartz, feldspar and muscovite (white mica) - are colourless.

add a second polaroid and view again

When you put the second polaroid 'in' and there is a thin section on the microscope stage you will usually see a mosaic of colours, as well as black.  if you rotate the stage the colours change, and each fragment of the mosaic goes from its colour to black, then back to the original colour.  Each fragment of the mosaic is a separate crystal of mineral.  Geologists can use these colours, and other features, to identify the minerals present.  The colours depend on the mineral, how it is oriented in the thin section, and its thickness.  But the thickness is standard - when making the thin section the technician checks the colours, and if the colour of a particular known mineral shows the rock slice is too thick then more grinding is done.

If you want to see how different minerals look through a microscope there are many websites which have pictures. For example the Green Bay campus at Wisconsin has images and explanations. 

serve with tongue in cheek

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