The Polishing Rush

Anna Funke

The last weeks of term are always a challenge in getting everything finished, but this term one task in particular – polishing SEM samples – took its toll on everybody’s patience.

The SEM (Scanning Electron Microscope) is a powerful analytical tool for our work. However, the sample preparation process is rather tedious. Just a quick reminder, a scanning electron microscope uses an electron beam (rather than light with standard optical microscopy) to produce images up to 500,000 x magnification.

Although there are several different methods for preparing these samples, our technique involved mounting our samples in resin (Clear Casting Resin POLYLITE 32032-00 from Alex Tiranti Ltd., to be specific), using ice-cube trays as moulds.

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Resin-cast samples of glass beads prepared by Emily Williams.

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An SEM image of the above sample, as prepared by Emily Williams. The glass appears grey in this image, and the swirls show that the glass was poorly mixed. The white specks in the sample are unmelted metal colorants in the glass.

Once cast, the resin block has to be prepared and sawn into the correct shape. This often requires some elbow grease and several sacrificial saw blades. The sample is then polished to the smoothest possible surface. This required four grades of abrasive paper, followed by 4 grades of micro mesh that added up to a good 5 hours per sample. One of the challenges is to make sure you are not left with scratches all over your sample.

The polishing work station of Megan Narvey.

The polishing work station of Megan Narvey.

The author and Megan Narvey discovering a significant amount of scratches on their samples.

The author and Megan Narvey discovering a significant amount of scratches on their samples.

At the end this long process you should have a shiny smooth resin surface, and are likely to have repetitive strain disorder. But if, against all odds, you do succeed in preparing a beautifully smooth surface, the images you will get in return for your labour are really quite amazing!

Sample of a cross-section of a painting by Kristen Gillette. Layers 1, 2, and 3 are surface pigments, probably acrylic paints; Layer 4 is a pigment ground layer; Layer 5 is the ground; and Layer 6 is animal skin.

Vanessa Applebaum’s cross section of painted plaster along with elemental maps of the sample.  Explain what can be seen here

Vanessa Applebaum’s cross section of painted plaster along with elemental maps of the sample. The cross section shows the plaster, applied pigment, and a carbonation layer. The elemental maps identify where specific elements are located in the sample. They are, starting from the top and moving left to right: silica, iron, chlorine, calcium, titanium, aluminium, potassium, and sodium.

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Discover What is in Your Lab: Unlabelled Blue Pigment

Julie Flynn

As part of an assigned research project in the conservation materials science course, I was given an unknown bright blue crystalline material. The original context of the sample was unknown, but it was assumed that it was a pigment, possibly Egyptian Blue. Visually comparing my sample to a known sample of Egyptian Blue, it was obvious that they were not the same. In order to demonstrate this, I began investigating this unknown blue using various techniques available to me at the Institute of Archaeology at UCL.

The bottle of unknown blue pigment

The bottle of unknown blue pigment

Polarised Light Microscopy (PLM) was the first technique that I used to examine the sample. In this technique a prepared sample is illuminated under magnification with polarised light.

For comparison, here are the PLM images of the unknown sample and the known Egyptian Blue. Out of all the known samples I analysed, Egyptian Blue pigment had the largest grain size. However, as you can see in these two images at the same magnification, the unknown sample still has much larger grains!

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PLM image of the unknown sample

 

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PLM image of Egyptian Blue

The unknown blue has grains at least twice as large as the Egyptian Blue. The other known samples of blue pigments in the lab had even smaller grains – smaller than 10 µm in length.

I was able to carry out elemental analysis using both portable X-Ray Fluorescence (pXRF) and Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM-EDS). In pXRF, secondary electrons in a sample are excited by bombarding the sample with high energy x-rays. Each element fluoresces differently, making this a good elemental analysis technique. IN SEM-EDS, an electron beam is scanned over the sample’s surface, and the electrons striking the sample produce signals. These specific signals can identify specific elements. Here are the results for identifying the blue portion of the samples.

Major Elements Found Within Pigments With Two Methods of Elemental Analysis

Pigment SEM XRF in Mining Mode
Unknown Pigment O, Na, Cu Cu
Egyptian Blue O, Cu, Si, Ca Si, Ca, Cu

The elemental composition of the two samples are different therefore, at this point, I believed that the unknown blue was not Egyptian Blue.

Now that I had established that the unknown sample was definitely not Egyptian Blue, I needed to find out what it was. For this, I used Energy Dispersive X-Ray Diffraction (EDXRD) to determine the unknown blue sample’s chemical composition. EDXRD uses X-Rays to reveal information about the crystal structure of the sample, which in turn reveals information about the sample’s chemical composition.

When I analysed the sample using EDXRD, the ICDD (International Centre for Diffraction Data) database of minerals found a very close match to the composition and crystal structure of the unknown blue pigment: sodium copper carbonate (Na2Cu(CO3)2).

 

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EDXRD diffractogram comparing the unknown blue sample to sodium copper carbonate (Na2Cu(CO3)2). This mineral matched closely to the chemical composition of the unknown blue sample. It also matches the results from the prior elemental analyses.

There is a rare mineral of sodium copper carbonate known as Juangodoyite. This mineral was discovered in situ in a Chilean mine in the Atacama Desert in 2003 by Arturo Molina, Iquique. It is formed by the dehydration of chalconatronite, which has the chemical composition of Na2Cu(CO3)2•3H20, and is a corrosion product of bronze. There is no evidence to suggest that this mineral has ever been used as a pigment.

Besides elemental and chemical composition analyses, more evidence that the unknown blue sample is juangodoyite is that they both form in crystals and have a bright ultramarine colour. Evidence against the unknown blue sample being Juangodoyite is that the minerals found in the Atacama Desert were 5 µm in grain size, which is much smaller than the unknown blue sample.

It’s curious to consider how this rare mineral came to UCL in the first place. Its origin at the Institute of Archaeology is shrouded by age, having passed from one professor to another professor many years ago under the name “Egyptian Blue”. It could have come from one of the Institute’s many international expeditions, or from a previous experiment to create a historic type of blue pigment. There are many possibilities – what do you think?