Isolating Contaminants in Glass

Step 1.  A contaminant is located no farther than 100 µm below one of the glass surfaces. If one is not found, then the glass has to be fractured, exposing fresh surface, until one just below the surface has been located.  Its location is carefully marked with the custom-made diamond scribe. One or two drops of the reconstituted collodion solution are put on top, allowed to dry, and form ~10 µm film.

Step 2.  The glass directly above the contaminant is crushed with the custom-made diamond as illustrated to the left.  The collodion film will keep the fragments from scattering (see Figure 6).

Step 3.  The crushed fragments and the cut collodion film are fused together with a 10 nL drop of amyl acetate, followed by a drop of the reconstituted collodion solution. After the film dries, it is peeled off the glass surface and placed on a microscope slide (see Figure 7).

Step 4.  The film is “cleared” with a drop of amyl acetate and then examined with PLM at 500X magnification and strong illumination. Frequently, the contaminant has a much higher refractive index than the glass and is easy to relocate (see Figure 8).

If the contaminant is not found in or among the glass fragments, it most likely has not been dislodged (see Figure 6). Repeat until the contaminant is located (see Figure 8). This may occur when the contaminant is >50 µm below the surface of the glass.

Step 5.  The glass fragment with the contaminant is separated from the rest as follows: the collodion film can be made viscous with amyl acetate and the fragment can be picked out with a tungsten needle. Or the glass fragments can be spread over a larger area in the viscous collodion and the fragment with the contaminant can be cut out when the amyl acetate evaporates.

Step 6.  If the contaminant in the glass fragment is not exposed sufficiently to obtain elemental and compound data, it is prepared for crushing on a custom made diamond press as follows: the ~0.2 mm replica with the <50 µm glass fragment is placed on the bottom diamond and most of the collodion is dissolved.

Step 7.  The diamond cell is made from a deep well slide that has been coated with rubber cement. Note: A rubber cement coating keeps the lower diamond in place and prevents it from damaging the bottom of the well slide. There is also less chance of losing the upper diamond when the well slide surfaces are coated with rubber cement. The sample to be pressed is placed between two diamond plates resting on the bottom of the well. See diagram to the left. More pressure can be applied by hand with a tungsten carbide scribe than can be applied with a commercially available diamond cell.

The microscopist can observe the fragment and the collodion film spreading between the two diamond surfaces. The thickness of the preparation is estimated from the interference colors that form between the two diamond surfaces when they are being pressed. Films ~300 nm (orange) are easy to obtain and are suitable for analysis on the analytical electron microscope (AEM) (see Figure 9).

Step 8.  The pressed sample can be removed from the diamond surface using the extraction replication technique described on page 2 of the article. Approximately 0.2 mm square of the replica that contains the pressed sample is placed either on a polished substrate for elemental data on the SEM/EDS or on a carbon-coated copper grid for compound identification by electron diffraction on the AEM. The collodion is dissolved from the two preparations as illustrated in Figure 5. Part of the AEM preparation is illustrated in Figure 10.

This procedure works well for submicrometer-to-micrometer size inclusions in glass matrices.