X-ray Powder Diffraction in Conservation Science: Towards Routine Crystal Structure Determination of Corrosion Products on Heritage Art Objects

The overall goal of this method is to gain knowledge of the crystal structure of micro and nano crystal and corrosion products to better understand production and decay reactions of heritage art objects and to allow for full quantitative analysis of mixtures. This method can help answer key questions in the field of conservation science such as how the art object has been produced and why is it corroded. The main advantage of this technique is that it does not require single crystals but only small amounts of micro crystal and powder.

Though this method can provide insight into conservation science it can also be applied to other fields such as material science, inorganic chemistry or metallogy. We found that the idea for this method when we investigated at a gradation phenomenon which recently has been characterized as glass induced metal corrosion. Demonstrating sample preparation and data collection will be Ms.Stefani, a chemical technical assistant from our laboratory.

Under a digital microscope carefully pick less than one milligram of sample one from opaque blue green cabochons on a historic clasp using a fine needle. Following this, grind the sample carefully with a pestle in a small agate mortar. Distribute the sample between two thin x-ray transparent polyamide foils and mount them on a transmission sample holder with an eight millimeter diameter central opening.

Fix the transmission sample holder on the theta circle of a diffractometer. Measure the sample for 20 hours from five to 85 degrees two theta, with a step width of 0.015 degrees two theta in transmission mode. Turn rotation on in order to achieve better particle statistics.

The next step is to perform an automatic peak search using the first and second derivatives of Sovietske Golle polynomials of low order. First start the software program by double clicking the icon and then click Load Scan Files. Using the pull down menu select xy data files and then double click on the appropriate file.

Now expand the range of the file. Click on the Emission Profile tab, select Load Emission Profile and choose the desired profile. Next click the Automatically Insert Peaks button and unclick Remove K Alpha 2 Peaks, after setting the Peak Width to 0.12 set the Noise Threshold to 1.74 and press the Add Peaks button.

Zoom in on the pattern, move to the desired section of peaks and then open the peak details window. Set the peaks by pressing the left mouse button. Next click on Peaks Phase.

Mark all peaks yellow by clicking on Position and then left click in the yellow marked column, copy the selection and select Create Indexing Range. Deselect the range and select Range Indexing. Then select all bravais lattices.

Following this press the Run button. And Yes to keep indexing solutions. Press the Solutions button and then highlight the first solution by left clicking button one.

Now right click on the highlighted solution and copy the selection. Deselect Range Indexing and then select the range. At this point press Add hkl Phase, expand the range and then expand hkl Phase.

Under Indexing Details, choose Paste Indexing Details. Under Background, change Order to eight and then select 1/X Bkg term. Next click Instrument.

After setting the primary and secondary radii to 217.5 mm, select Receiving Slit Width under Point detector. Under Full Axial Model, set the source, sample, and RS length to six mm. Now click Corrections and choose 0 error.

Choose LP factor and set the value to 27.3. Then click Miscellaneous and set Convolution Steps to 2. Select Start X and set the value to eight, then choose Finish X and set the value to 75.

Under Click Peaks Phase choose Delete Peaks Phase and click Yes. At this point click hkl Phase and choose Microstructure. After selecting Cry size L choose Cry size G, select Strain L, choose Strain G and then press the Run button.

Finally create a list of brag peaks suitable for charge flipping. For crystal structure determination use the method of charge flipping supported by the inclusion of the tangent formula to find the positions of most of the heavier atoms. First under File select Close All and click Yes.

Under Launch, select Launch Kernel. From the Launch pull down menu, select Set INP file and then select the prepared input file. Now press the Run button and after approximately 20, 000 cycles press the Stop button and click OK.Following this press the temporary output window displaying selected atoms button and then press the Cloud options dialog button.

Set N to pick to 45. Choose width symmetry and press the Pick button. Copy the temporary output, save it to a text file and close the charge flipping graphics window.

The next step is to apply the global optimization method of simulated annealing to find the positions of all missing non hydrogen atoms. From the launch pull down menu select Set INP file and then choose the prepared input file. Press the Run button and after several thousand cycles press the Stop button and click Yes.

The final step is to turn off simulated annealing and switch to the Reitfeld refinement mode by commenting out the appropriate command. Under Launch choose Set INP file and select the prepared input text file. Finally press the Run button and click Yes.

High resolution X-ray powder diffraction was used to determine the crystal structures of two long known corrosion products on historic objects. Standard measurements using a state of the art laboratory high resolution powder diffractometer in transmission and Debye Scherrer geometry using monochromatic x-rays were performed. Crystal structure determination of sample one was performed by iteratively combining reciprocal and direct space methods with Difference Fourier analysis.

The determination of the crystal structures of these compounds confirms the exact composition, improves our understanding of the decay mechanisms and allows full quantitative phase analysis of corrosion products. Once mastered this technique can be done within one day if it is performed properly. Following this procedure other methods like DTA, Raman Spectroscopy and EDX can be performed in order to get complimentary and additional information concerning composition, bonding, possible phase transitions and stability.

After watching this video you should have a good understanding of how to prepare samples, record appropriate powder diffraction data, perform data reduction and determine the crystal structure of micro crystalline material. After its development this technique will pave the way for researchers in the field of conservation science and it is non destructive, fast and easy to use.