Adventures in Thin Sectioning

[BentonlWalters]

This ongoing journey begins as many do, with an accident. While trimming down matrix from my collection to reduce weight, in this instance a piece of the Aust bone bed,  I accidentally sliced right through an ichthyosaur vertebra that I didn’t realise was there hidden in the rock.

 

 

After the initial annoyance wore off, I realized that the internal texture of the bone was pretty nicely preserved and that gave me an idea, maybe I could make a histological section of the offcut piece of the vertebra. I have always wanted to learn how to make thin sections and this gave me the catalyst, so I emailed my the lab technician in my department (I study Paleobiology for my PhD) and next thing I know I was scheduled for training on how to use the departments thin sectioning and polishing lab.

 

I hunted through my collection for a few fossil pieces that I could use as practice and decided on two Yorkshire belemnite fragments, as I have more complete examples and I thought that the rings of the belemnite would make an excellent subject. I am going to wait until I have more practice with the technique, and also have an opportunity to CT scan the block for posterity before I go grinding away what remains of the vertebra.

 

The first test pieces:

 

The process for making thin sections is relatively simple though it requires at least two days for the resin to cure. First you cut a wafer off of the specimen using a specialist saw which produces a very smooth cut surface.

 

 

The resulting wafer from the smaller belemnite tip:

 

 

Then you can bond the cut surface of the wafer to a frosted glass microscope slide using epoxy resin.

 

 

These are then placed in a clamp to cure

 

 

Two days later, I was introduced the thin section machine, which contains both a cutting wheel and a grinding disk with the arm attached to a micrometer. The glass slides are mounted in a vacuum chuck on the arm and then passed through the blade which cuts off excess material leaving only about 500-600 microns of sample material bonded to the slide. Next the arm is maneuvered to the grinding wheel and with successive passes ~20 microns at a time are ground off the sample until the remaining layer is only about 100 microns thick. At this point the sample is translucent and polarized light gives an idea of the crystal structure of the belemnite!

 

 

 

Unfortunately, due to some issues with the resin mix these first two attempts didn’t turn out great but for the purposes of learning I was shown how to polish them anyway, first using a 9 micron diamond suspension which after repeat use reduces the sample to approximately 60 microns and removes the scratches from the grinding disk, then with a 3 micron suspension which does the majority of the actual polishing.

 

Polishing slides:

 

And here’s what the initial results look like under the microscope, I’m looking forward to trying the technique again and improving, as well as figuring out how to attach my camera to the microscope to take better pictures. The growth-rings in the belemnite are very apparent and in the one where I cut through the phragmocone end you can clearly see the siphuncle!

 

Beautiful belemnite growth-rings from the larger specimen:

 

The siphuncle on the edge of the phragmocone:

 

The rings preserved in the smaller specimen:

 

The rings at higher magnification:

 

I’m excited to get to improve my technique and I have a few ideas for other pieces to test with. I will update this soon with more as I keep working at it. Please feel free to ask any questions you have and I’ll try to answer them.

 

Thanks for reading,

Benton

 

Edited August 24, 2023 by BentonlWalters
removed accidental additional image

[Gramps]

That is really cool!

[Tidgy's Dad]

Thanks for the thread.

Very interesting! 

[FossilDAWG]

Excellent explaination of the process!  is that your equipment, or fo you have access to a research lab?  It's nice to see some proper equipment.

 

Don

[hadrosauridae]

Great post! Thanks for showing the process.

[Sjfriend]

Very nice! The photos look great. Would love access to that kind of equipment. 

[BentonlWalters]

On 8/26/2023 at 1:37 AM, FossilDAWG said:

Excellent explaination of the process!  is that your equipment, or fo you have access to a research lab?  It's nice to see some proper equipment.

 

Don

Thank you! This is the equipment in the lab at my university. I wish I had equipment like this though.

[BentonlWalters]

A progress update, I have spent a fair bit of time in the lab since the last post and chose two specimens to use for learning. The first is the larger of the two belemnites sliced previously, but I had the idea to try to cut lengthwise to create a larger section of the whole of the remaining phragmocone. The second is a piece of turtle carapace from the Cretaceous formations at Durlston Bay. Turtle is relatively common from this location and this piece was previously broken and re-glued so I though it would be a good candidate for testing out bone slices.

 

The turtle, an isolated carapace fragment, trimmed to size:

 

In order to slice the belemnite a slightly different method was necessary, first I used the large, high speed, saw to shave a flat face into the side of the belemnite. This face was then bonded to a plate with thermoplastic resin to provide a stable mount for cutting on the wafering saw.

 

 

I took a slice off the belemnite with the blade positioned just off center so that the resulting thin section should preserve both the tip of the belemnite and the siphuncle. The cut is beautiful but I need to learn how to change the chuck on the thin section machine itself in order to accept different sized slide glass before I can move forward, so focus switched to the turtle block.

 

 

Since the surface of the bone is on the outside of the block I was concerned about chipping and decided to cap the piece with resin, this protected the bone from damage in cutting and made it easy to produce clean cuts with no tear out.

 

 

With the resin hardened, I first cut off the excess and then took a series of 3mm thick wafers through the block, leaving 12 decent pieces to practice with, plus an end that I might try polishing at some point.

 

 

 

I started with 6 wafers and the mounting behaved much better this time than previous. Specimens were bonded, sliced and the ground to approximately 60 microns with only one slide causing issue, likely from uneven pressure during curing. I am super pleased with the results and excited to see how they polish up. Interestingly, even this thin there are still opaque regions, which I suspect are pyritic.

 

The slides after cutting and grinding:

 

Piece 6, showing opacity:

 

Here are some initial microscope images to show the bone texture before I get to the polishing step. magnification 100x, unpolarised light. 

 

 

Edited August 30, 2023 by BentonlWalters

[grandpa]

What saw are you using?

[BentonlWalters]

12 hours ago, grandpa said:

What saw are you using?

The wafering saw is a Buehler IsoMet 1000, which is a gorgeous machine. I'm not sure what the model of the high speed saw is. 

[BentonlWalters]

The first round of turtle bone sections and the long slices through the belemnite are now polished. I've been learning how to use the departments' photomicroscope so here are some better images of the sections, first in transmitted light and then the same area with the polariser on. 

 

The end of the belemnite phragmocone:

 

 

 

Various turtle bone close-ups: 

 

 

 

 

 

 

 

 

 

Higher magnification:

 

And lastly, the end of one belemnite slice under polarised light: 

 

 

Thanks for looking. I am excited to continue improving.

[caterpillar]

Very impressive

[BentonlWalters]

A quick update:

 

I ground one more turtle carapace slice with the intention of getting closer to the ideal 30micron thickness. Here is the result.

 

The whole slice reconstructed as a rough mosaic:

 

Some highlights under crossed polarised light. The thinner section does result in much better clarity in the ring structures: 

 

Under higher magnification:

 

Lastly, here's a sneak preview of what I'm working on a the moment. All that's left to do is the final polish and then photography:

 

[BentonlWalters]

At long last I have finished the thin sections of the ichthyosaur vertebra from the Aust Bone Bed that kicked off this exploration. 

 

Here is the first of four slides I produced and the best preserved one:

 

On average this slice is ground to bout 60 microns thick and you can clearly see at least three main constructions of the bone across the vertebra as well as a much higher density of dark regions within the bone (osteocytes) when compared with the previous turtle carapace sections. Black regions outside the bone but within the lattice are pyrite which is opaque even when ground this thin.

 

Ordered perpendicular bone filaments from the righthand side of the slice: 

 

A less-ordered boney sponge making up much of the internal area:

 

Much denser, osteocyte rich, bone making up the outer wall of the vertebra

 

 

In addition to the vertebra there are a few other points of interest on this slide which illustrates some of the fossil density of the Aust Bone Bed. 

 

Another small laminated bony structure, potentially a fish scale:

 

transverse sections through bryozoans:

 

 

Lastly, here is a mosaic look at one of the other slices taken from the very edge of the vertebra, imaged in crossed polarised light.

 

I hope you enjoyed, I am excited to start another set of sections in the next few weeks.