Sample Preparation for Archaeological Materials – COLD MOUNTING
Notes for Different Material Types
Polishing and grinding procedures are highly dependent on the type of material to be prepared. This section provides a brief overview of the advantages and disadvantages of preparing different types of material using the methods described in the document attached above.
Glass is the easiest type of material to prepare. Its uniformity means there is little risk of fragments or inclusions scratching the sample as you prepare it, and its hardness allows it to be polished effectively without producing more scratches. There are no special techniques for preparing glass other than making sure any bubbles, pits, or vesicles do not fill up with diamond paste during the polishing stage. However, cold-mounted samples must be carbon-coated (*See below) to ensure electrical conductivity when using the SEM.
Slag is typically an easy material to prepare. However, the heterogeneity of slag means that they can be the source of their own contaminants (particularly for non-ferrous slags). In addition, vesicles are common and can easily be filled with silicon carbide grains and diamond paste.
Metals are often the hardest material with which to work (as the softness of the material relates directly to the difficulty of polishing). Iron rarely scratches and polishes quickly. Because tin, lead, zinc, and copper are softer metals, they may have to be polished minimally as any contaminants will scratch the surface (be sure to thoroughly clean or change cloths before polishing these metals). If the metal is corroded (copper particularly, but also iron), the corrosion products may also contaminate the sample. Beware of pores in your sample, as these are oftentimes diamonds that have become embedded in the sample (and they will interfere with analysis); increase the amount of lubricant you are using to prevent such an occurrence.
Pottery is also a potentially difficult material to prepare because its porosity and inclusions affect the polishing process. Overpolishing is a common occurrence because the matrix and inclusions may behave differently (i.e. the matrix may sink away from the inclusion); to prevent this, you may not be able to polish as thoroughly as you would wish. Porosity is another problem, but may be solved with vacuum impregnation or by using different resins (see you lab supervisor).
NOTE: a strip of copper tape should be affixed to the finished cold mount, touching the sample, to ensure electrical conductivity; apply the tape before using the SEM and be sure to remove the tape immediately afterwards.
* Carbon Coating:
In the Electron Microscope Facility (room 014, basement level Barus and Holley), there is a carbon evaporator that can be used to carbon coat the surface of a cold mounted glass sample AFTER it has been ground and polished. The following procedure may be used (please ask Tony McCormick to supervise your first time doing this):
- Remove the lid and rest it in the recess so that it does not roll.
- Cut a 40 mm long piece of carbon-coated thread and mount it beneath the rectangular metal blocks that are attached to the lid; pull the thread taught and clamp it cross-wise by pushing the clamping springs down. Position the shutter so that it is directly in front of the thread.
- Remove the glass cylinder and adjust the working distance of the metal stage: for a 100 nm carbon coating, set the stage height so that the top of your sample will be at 40 (using the graduated markings on the cylinder).
- Place the resin block on the metal stage and replace the glass cylinder; place the lid back onto the glass cylinder.
- Turn the unit on by flipping the green switch; wait for the chamber to evacuate until the light indicates the vacuum is at approximately 0.02 mbar, and then tap the UP button until the carbon-coated string glows red; wait 15-20 seconds for the degassing process to finish and then press RESET to end degassing.
- To evaporate the carbon, turn the yellow knob on the top of the lid to open the shutter; *DO NOT LOOK DIRECTLY AT THE BELL JAR* and press the HIGH CURRENT button once (the string will flash brightly and go out). Turn the power switch to the off position.
- Remove the lid and glass cylinder and retrieve the sample block (be careful to not touch the surface, and allow it to dry thoroughly); throw any remnants of the carbon thread away and clean the metal parts of the unit with a lint-free cloth.