Making Slides

Two notes in this section.

  1. Suitable rocks for making thin sections
  1. Methods used at Geosec (more details to follow shortly)


I have been asked on several occasions if I will cut sections using samples supplied. This poses a few difficulties, in theory it should be no different from making slides in the normal way, but I find that what has become second nature to me when collecting, only picking up rocks that are suitable, requires explanation. Nearly any rock sample can be made into a thin section, but some take much work and time, and basically that makes it uneconomical.

Set out below is a list of what makes a good slide, but bear in mind that only about half of what I collect I find to be any good, and I am supposed to know what I am doing! As the princess discovered, you have to kiss a lot of frogs before you find a prince. What appear on the Geosec Shop are mostly princes, the frogs are in the waste bin.

If you have some special rock the most satisfactory way is for you to supply me a suitable sample, I will make one slide for you for free from the sample, and sell the rest on ebay in the normal manner. If the sample is unsuitable, I can return it, but it would be at your cost. Please send a list of samples before dispatching anything as I can often advise suitability. Also please send details of source, rock type if known, age and anything else I can use in the item description.

What makes a good sample?

  1. It must be big enough. Fist size is good, bigger is quite acceptable. The first stage is to cut a 20 to 24mm cube from the sample, so ideally it needs to be double that size in every dimension. I know it is going to cost more to ship, but trying to make sections out of small samples is expensive. Only exception is really rare stuff, but that would have to be dealt with on a piece by piece basis. It is difficult to overstress this, fist size is really the smallest acceptable, bigger is better.
  2. The right shape. Important for schists and rocks with layers or foliation. I usually cut these at right angles to the layering, so need to cut the cube with one side parallel to the layers. Again make sure it is thick enough. Schist naturally forms flat slabs, min thickness would be about 30mm as it is sometimes difficult to get thicker pieces. Please check that it is not going to cleave too readily along the layering.
  3. Not crumbly or porous. Often a problem with volcanic rocks. Can be overcome but makes more work. The rocks I offer from Stromboli and Montserrat are all impregnated with resin, otherwise they just fall to bits.
  4. Not cracked. If you can see a crack in the sample it may not be suitable. If you have used a hammer to break the rock you may have induced cracks that I will find out about later. Best to collect from scree, beaches, or prise bits out .
  5. Avoid rocks containing any quantity of oxidised/metallic element (unless that is not the reason for wanting the section). May look pretty in sample, but will be opaque in section.

Some examples

Most Plutonic rocks are good, (although Granite can sometimes be uninteresting) Most Volcanic rocks and dyke rocks, although Rhyolite can be just glass, some sedimentary rocks, but not chalk. Metamorphic, varies, schist can be good but difficult often splits along layers, granulites and gneiss ok, phyllites and slates, usually too small to be interesting. Hornfels ok, quartzite and cryptocrystalline quartz, uninteresting and usually very hard.

Finally please let me know when you have dispatched a parcel, so I know to expect it, and let you know if it does not arrive.

How to make a thin section slide

Before detailing the techniques used at Geosec it should be noted that there are many different techniques, and no one technique is correct, you just use the method that suits you. It is the finished item that is important, not how you get there.

I have several friends that make thin sections and we have an informal information exchange. We all use different methods, glues, machinery etc, and all produce good slides.

I was never taught how to make slides, but found by trial and error a method that suits my requirements. My machines have been constructed in my workshop, and though crude by commercial standards they work well enough, and cost a fraction of the “standard” ones. All my slides are finished by hand, so any inaccuracy that comes from the machines is eliminated anyway.

So, Starting from a piece of rough rock, a small block 2cm x 2cm x 3cm is sawn by hand on a diamond saw.

The top surface need to be absolutely flat, so is lapped on a glass plate with fine silicon carbide grit.

Glue is applied and a microscope slide placed on the block. Any air bubbles in the glue are carefully worked out to the edge.

Geosec has moved from the traditional epoxy resin glues to UV setting ones, it is now necessary to put the slide/block under a hi-intensity UVlamp. The glue sets in a few seconds and is now ready to cut.

The slide/block is sawn as close to the glass as possible using a thin bladed diamond saw. The section is now 1mm to 0.5mm (500 microns) thick.

The slide is now mounted on to the first grinding machine. The slide is retained by vacuum in a slide holder and is held against a rotating diamond flat wheel. 

The arm reciprocates to ensure even cutting. This machine reduces the thickness to 200 microns.

The slide is transfered to the second grinding, or flat lap machine. 

 Using a finer grade diamond disc this can cut to almost the finished thickness, but there is always some score marks left by the cutting process, so the slides are removed when they are about 50 microns. The slide is now ready for hand finishing.  

The final stages are done by hand on a glass plate with silicon carbide grit, and repeated checking of the thickness by judging interference colours with the microscope.