The authors gratefully acknowledge Ms. Christine Stefani for performing the XRPD measurements. Marian Schu&#776;ch and Rebekka Kuiter (State Academy of Art and Design Stuttgart) are acknowledged for the pictures of the tile (Fig. 7).

1. Sample Preparation
<ol>
	<li>Collection of material
	<ol>
		<li>Carefully pick a small amount (less than 1 mg) of sample 1 under a digital microscope using a scalpel and tweezers from settings of opaque blue-green cabochons on a historic clasp, belonging to the collection of the Rosgartenmuseum Konstanz (RMK-1964.79) (Figure 6).</li>
		<li>Carefully scratch a few mg of sample 2 with a scalpel from the surface of a glazed ceramic tile, dating at early modern times, manufactured in So...

<ol>
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Chem. 54 (6), 2638-2642 (2015).</li><li>Wahlberg, N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Crystal+Structure+of+Thecotrichite,+an+Efflorescent+Salt+on+Calcareous+Objects+Stored+in+Wooden+Cabinets.">Crystal Structure of Thecotrichite, an Efflorescent Salt on Calcareous Objects Stored in Wooden Cabinets.</a> Cryst. Growth Des. 15 (6), 2795-2800 (2015).</li><li>Eggert, G., Fischer, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gef&#228;hrliche+Nachbarschaft:+Durch+Glas+induzierte+Metallkorrosion+an+Museums-Exponaten+-+Das+GIMME-Projekt.">Gef&#228;hrliche Nachbarschaft: Durch Glas induzierte Metallkorrosion an Museums-Exponaten - Das GIMME-Projekt.</a> Restauro. 1, 38-43 (2012).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> TOPAS (current version 5.0). , (2015).</li><li>Coelho, A. 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XRPD is a suitable technique for conservation research as it is non-destructive, fast and easy-to-use. XRPD data can be used in routine qualitative analysis, owing to the fact that the powder pattern is a fingerprint signature to the corresponding crystal structure. The biggest advantage of XRPD over other analytic techniques is the ability of performing simultaneous qualitative and quantitative analysis of crystalline constituents in mixtures by using the Rietveld refinement method5. Moreover, the presence of...

Conservation science applies scientific (often chemical) methods in the conservation of artifacts. This includes investigations of the production of artifacts (&#39;technical art history&#39;: How was it made at that time?) and their decay pathways as a prerequisite to develop proper conservation treatments. Oftentimes these studies deal with metal organic salts like carbonates, formates and acetates. Some of them have been deliberately manufactured using suitable compounds (e.g., vinegar), others derive from deterioration reactions with the atmosphere (carbon dioxide or carbonyl compounds from indoor air pollution)1. As a matter of fact, the crystal structures of many of these corrosion materials are still unknown. This is an unfortunate fact, since the knowledge of the geometrical structure helps conservation science to better understand production and decay reactions and to allow for full quantitative analysis in the case of mixtures.
Under the condition that the material of interest forms single crystals of sufficient size and quality, single crystal diffraction is the method of choice for the determination of the crystal structure. If these boundary conditions are not fulfilled, powder diffraction is the closest alternative. The biggest drawback of powder diffraction as compared to single crystal diffraction lies in the loss of the orientational information of the reciprocal d-vector d* (scattering vector). In other words, the intensity of a single diffraction spot is smeared over the surface of a sphere. This can be considered a projection of the three-dimensional diffraction (= reciprocal) space onto the one dimensional 2&#952;-axis of the powder pattern. As a consequence, scattering vectors of different direction but equal or similar length, overlap systematically or accidentally making it difficult or even impossible to separate these reflections2 (Figure 1). This is also the main reason why powder diffraction, despite its early invention just four years after the first single crystal experiment3,4, was mainly used for phase identification and quantification for more than half a century. Nevertheless, the information content of a powder pattern is huge as can be easily deduced from Figure 2. The real challenge, however, is to reveal as much information as possible in a routine way.
A crucial step towards this goal, without any doubt, was the idea from Hugo Rietveld in 19695 who invented a local optimization technique for crystal structure refinement from powder diffraction data. The method does not refine single intensities but the entire powder pattern against a model of increasing complexity, thus taking the peak overlap intrinsically into account. From that time on, scientists using powder diffraction techniques were no longer limited to data analysis by methods developed for single crystal investigation. Several years after the invention of the Rietveld method, the power of the powder diffraction method for ab-initio structure determinations was recognized. Nowadays, almost all branches of natural sciences and engineering use powder diffraction to determine more and more complex crystal structures, although the method can still not be regarded as routine. Within the last decade, a new generation of powder diffractometers in the laboratory was designed providing high resolution, high energy and high intensity. Better resolution immediately leads to better peak separation while higher energies fight absorption. The benefit of a better peak profile description based on fundamental physical parameters (Figure 3) are more accurate intensities of Bragg reflection allowing for more detailed structural investigations. With modern equipment and software even microstructural parameters like domain sizes and microstrain are routinely deduced from powder diffraction data.
All algorithms for crystal structure determination from powder diffraction data use single peak intensities, the entire powder pattern or a combination of both. The conventional single crystal reciprocal space techniques often fail due to an unfavorable ratio between available observations and structural parameters. This situation changed dramatically with the introduction of the &#34;charge flipping&#34; technique6 (Figure 4) and the development of global optimization methods in direct space, of which the simulated annealing technique7 (Figure 5) is the most prominent representative. In particular, the introduction of chemical knowledge into the structure determination process using rigid bodies or the known connectivity of molecular compounds concerning bond lengths and angles strongly reduces the number of necessary parameters. In other words, instead of three positional parameters for every single atom, only the external (and few internal) degrees of freedom of groups of atoms need to be determined. It is this reduction of structural complexity which makes the powder method a real alternative to single crystal analysis.
Two pioneering case studies of the authors8,9 proved that it is possible to solve complicated crystal structures of complex corrosion products using powder diffraction data. The superiority of the crystallographic studies compared to other approaches was demonstrated among others by the fact that in both cases the reported formulas had to be corrected after considering the solved crystal structures.
The occurrence of both materials under investigation in museums is related to their storage in wooden cabinets or exposed to other sources of carbonyl pollutants. The first material under investigation was sodium copper formate hydroxide oxide hydrate, Cu4Na4O(HCOO)8(OH)2&#8729;4H2O (sample 1), which forms on soda glass/copper alloy composite historic objects (e.g., enamels) in museum collections, exposed to formaldehyde and formic acid from wooden storage cabinets, adhesives, etc. This degradation phenomenon has recently been characterized as &#34;glass induced metal corrosion&#34;10. For the second case study, thecotrichite, Ca3(CH3COO)3Cl(NO3)2&#8729;6H2O (sample 2), was chosen. Thecotrichite is a frequently observed efflorescent salt forming needlelike crystallites on tiles and limestone museum objects, which are stored in oak cabinets and display cases. In this case, the wood acts as source for acetic acid which reacts with soluble chloride and nitrate salts from the artifact.
In the following part of the text, the individual steps of the structure determination process using powder diffraction data applied to corrosion products from conservation science are presented in detail.

<table border="0" cellpadding="0" cellspacing="0" width="573">
	<tbody>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Stadi-P&#160;</td>
			<td style="width:71px;">Stoe &#38; Cie GmbH</td>
			<td style="width:119px;"></td>
			<td style="width:167px;">Powder Diffractometer</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Mythen 1-K (450 &#956;m)</td>
			<td style="width:71px;">Dectris Ltd.</td>
			<td style="width:119px;"></td>
			<td>Position Sensitive Detector</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Mark tube borosilicate glass No. 50, 0.5 mm diameter</td>
			<td style="width:71px;">Hilgenberg GmbH</td>
			<td align="right">4007605</td>
			<td>Low absorbing capillaries</td>
		</tr>
		<tr height="126">
			<td height="126" style="height:126px;width:216px;">Topas 5.0</td>
			<td style="width:71px;">Bruker AXS Advanced X-ray Solutions GmbH</td>
			<td style="width:119px;"></td>
			<td>Powder Diffraction Evaluation Software</td>
		</tr>
	</tbody>
</table>

x-ray powder diffraction in conservation science: towards routine crystal structure determination of corrosion products on heritage art objects

The crystal structure determination and refinement process of corrosion products on historic art objects using laboratory high-resolution X-ray powder diffraction (XRPD) is presented in detail via two case studies.
The first material under investigation was sodium copper formate hydroxide oxide hydrate, Cu4Na4O(HCOO)8(OH)2&#8729;4H2O (sample 1) which forms on soda glass/copper alloy composite historic objects (e.g., enamels) in museum collections, exposed to formaldehyde and formic acid emitted from wooden storage cabinets, adhesives, etc. This degradation phenomenon has recently been characterized as &#34;glass induced metal corrosion&#34;.
For the second case study, thecotrichite, Ca3(CH3COO)3Cl(NO3)2&#8729;6H2O (sample 2), was chosen, which is an efflorescent salt forming needlelike crystallites on tiles and limestone objects which are stored in wooden cabinets and display cases. In this case, the wood acts as source for acetic acid which reacts with soluble chloride and nitrate salts from the artifact or its environment.
The knowledge of the geometrical structure helps conservation science to better understand production and decay reactions and to allow for full quantitative analysis in the frequent case of mixtures.

High resolution XRPD was used to determine the previously unknown crystal structures of two long-known corrosion products on historic objects. The samples were taken from two museum objects and carefully grinded before they were sealed in transmission and capillary sample holders (Figures 6, 7). Standard measurements using a state of the art laboratory high resolution powder diffractometer in transmission and Debye-Scherrer geometry using monochromatic X-rays were perform...

Watch this Scientific Journal Video about X-ray Powder Diffraction in Conservation Science: Towards Routine Crystal Structure Determination of Corrosion Products on Heritage Art Objects at JoVE.com

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

Modern high resolution X-ray powder diffraction (XRPD) in the laboratory is used as an efficient tool to determine crystal structures of long-known corrosion products on historic objects.

The crystal structure determination and refinement process of corrosion products on historic art objects using laboratory high-resolution X-ray powder ...