archaeometallurgy

Today is all about sample polishing. Welcome to part six of the slag microscopy series.

We need to polish our samples for polarising reflective light microscopy. A perfectly polished section is a joy to work with. More importantly it allows you to gather the optimum amount of information from your sample. This is true for optical microscopy, and even more so for electron microscopy with energy or wavelength dispersive spectrometry.

For this we need:

• polishing cloth
• polishing agents
• polishing lubricant
• sample polisher/grinder

Polishing cloth

Polishing cloth holds the polishing agents which are being used for polishing. There is a wide variety of different polishing cloths around: They are woven or napped textiles and differ in hardness. they also differ in their material. This does have significant influence on the sample polishing results: E.g. if the cloth is too soft, it is likely to create a strong relief. For slag I have had very good results with silk cloths for the final polishing step. For metals and ceramics I had  good polishing results with short napped discs.

Polishing agents and lubrication

There is no definite answer as to where polishing begins and where lapping ends, but generally we speak of polishing, when the polishing agents are smaller than 10 microns. Sample polishing is done on polishing cloth, by a polishing agent and polishing lubricant. Most common polishing agents are corundum and diamonds

Sample polishing

For manual sample polishing move the sample in figure eight fashion across the polishing cloth. You can also use a star like motion, however this is difficult  with polishing discs smaller than 300 mm diameter.

Polishing needs to remove all previous traces of grinding or lapping. Make regular checks with your stereo microscope to minimise your preparation time. As with lapping the ratio of diamonds to lubricant is essential for the polishing success. Use too much lubrication and the samples will only skid across the polishing disc. Use too little and you may end up damaging your polishing cloth.

If you start with a new polishing cloth, make sure to note the grain size on the metal platen. If you prepare a lot of different materials, it might also be a good idea to note the materials you wish to polish on the platen.In this series I am using diamonds of three and one microns grain size. Very seldom I do use a quarter micron as well.

Make sure to inspect the section before and after polishing. Put three to four 1 cm long strips of 3 micron diamond paste on the polishing cloth, rub them into the cloth, so that no diamond paste protrudes from the cloth. Add a few squirts of polishing lubricant. There is enough lubrication on your finger becomes slightly glossy when you touch the disc. Start the polisher, move the sample in a figure eight fashion across the polishing disc. There is no need to check the progress within the first three to five minutes.

For checking up on the polishing progress, place the cleaned sample under a reflected light microscope as well. I have an old one for this purpose in the sectioning lab. Sections should be clean, because abrasive diamond residues and optical instruments should not be mixed.

Before you continue to polish the section with the next finer diamond abrasive, make extra sure to have cleaned the sample in a fresh bath of IMS in the ultra-sonic cleaner. Rinse samples with IMS before putting them onto the new polishing disc. A single 3 micron diamond will leave your sample badly scratched. Repeat all polishing steps with the finder abrasive.

Contamination is a problem

A word of caution: Utmost care must be taken to keep polishing cloths and polishing agents of different grain sizes  separate! Cleaning the polishing machines is essential. Wash your hands and wash them often! I cannot stress this point enough. Careless work with polishing agents of different grain sizes will only lead to frustration.

Under no circumstances may the cloths be stored in a way that they touch each other. Do not leave them to dry in a manner that water drops from one cloth onto an other. If you use storing cabinet for polishing discs, make sure that the larger grain sizes are below the finer grain sizes.

If your disc is contaminated, there are three possibilities:

1. exchange the cloth
2. rinse the cloth with a lot of water and hope the contamination is removed
3. Polish the section with the contamination, take images form non-scratched areas

Recap

• clean your hands, sections and tools before and after use, especially if you work in a bigger lab: you never know how thoroughly the previous user left the place
• Clean the sample before you place it anywhere, otherwise you are spreading diamonds around the lab
• three to four 1 cm strips of diamond paste are enough for polishing a section
• massage diamond paste and lubricant into the polishing cloth
• polish by moving the section in a figure eight fashion across the cloth
• inspect polishing progress with a microscope
• clean all tools, machines and sections with IMS

That’s it for now. Be sure to be back for the next instalment, where we are going to explore the microscope.

This article is about managing your micrograph collection without the need of a dedicated data base structure. You can keep your filing structure and will not be dependent on a proprietary management system. The article is centred around embedding meta information within the digital image. Micrograph, microscopical observations and related notes are always kept together. Because of the standardised way meta data is stored in digital image files, it may be easily exported.

Ddigikam enables you to save your microscopical observation notes directly into the meatadata of your micrographs.

Continue reading

There are a number of Linux software solutions that do tethered shooting, aka remote capture and they do it well. In this article I am describing the difficulties of setting up darktable for tethered shooting with a Canon EOS 600D digital camera on Ubuntu 12.04. Plenty of people managed to do this and I meant to do the same. I was quite surprised when it did not work the way it should have worked.

I am using a Canon EOS 600D digital camera as a microscope camera, because I can put its liveview functionality to very good use. Either to teach or to produce good quality micrographs. If you are using Mac OS X or Microsoft Windows you can use the Canon EOS utility to use tethering to control a digital camera with your PC. However if  you like to – or have to – use a brand of Linux you will find that the Canon software does not run with Wine. Of course you could run Windows or OS X on a virtual machine, such as Virtualbox, but that became too cumbersome for me.

The image shows the darktable interface for tethered shooting. With Canon Liveview cameras the computer may be used to control the camera and take pictures. The image shows a section of an etched copper roof from the 18th century.

My enthusiam for tethered shooting on a linux machine was beginning to wane when I connected my camera to the USB port. All I got was a generic error message:

Unable to mount Canon Digital Camera Error initializing camera: -1: Unspecified error

The camera was not  mounted. I could not even get it to work with digiKam to download images. I did check for SD card corruption, updates for all sorts of related software libraries, tried to access the camera with  gphoto2. …and nothing did work. Eventually it turned out that the USB 3.0 ports did not work well with the camera. After figuring this out I did find this post which seems to confirm my findings. Plugging the camera into the USB 2.0 port  the cameras was mounted as a storage device, however now Ubuntu issued the following error message:

Error (-60: 'Could not lock the device')

After checking for this error I found this post, which solved the problem:

• make sure your camera is connected to a USB 2.0 port
• unmount the camera with your file browser
• start darktable, switch to the “Tethering” tab
• the software should be able to find your camera

Addendum 9.11.2014

If you remove the SD card from your camera darktable will work out of the box as long as you start after you have plugged in your camera. Where there is no storage device the gvfs cannot mount it. Hence the camera is not blocked. Give it a try. 1. If If you are looking for more permanent solutions check this post: Delete or rather rename the following files, so you can undo things if they go wrong:

/usr/share/dbus-1/services/org.gtk.Private.GPhoto2VolumeMonitor.service
/usr/share/gvfs/mounts/gphoto2.mount
/usr/share/gvfs/remote-volume-monitors/gphoto2.monitor

and optionally:

/usr/lib/gvfs/gvfs-gphoto2-volume-monitor

2. Try this solution: remove the execute rights of the following file:

1. open terminal
2.  command: sudo killall gvfs-gphoto2-volume-monitor
3. command: sudo chmod -x /usr/lib/gvfs/gvfs-gphoto2-volume-monitor
4. close terminal and restart.

These two solutions have the same effect, because they disable the same feature: It prevents gvfs from mounting your cameras SD card as a storage device. So unless you are happy with the fact the camera will not appear in your file browser any more, I would recommend the other solutions. Photo managing software should still be able to find your camera, though.

Well, I hope this helps. Let me know if you found any other solutions.