**3. Results**

The optical images of the "Little Girl, Pretty Purse" daguerreotype before cleaning and after cleaning, as well as the XRF image from collecting Hg L<sup>α</sup> fluorescence X-rays, are shown in Figure 2. It is apparent from Figure 2 that the chemical and electrochemical cleaning methods are generally effective. We will inspect several selected areas and discuss the effects below.

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mental distribution and better contrast were obtained.

**3. Results** 

cuss the effects below.

setting the energy window of interest, e.g., Hg Lα and Au Lα, in the X-ray fluorescence spectra collected by a four-element Vortex-Me4 solid state detector (~250 eV energy resolution) and stored in a multichannel analyser (MCA). The experimental set-up is shown in Figure S2, and a snapshot of the MCA display is shown in Figure S3, where the fitting of the overlapping fluorescence X-rays with which we could obtain more accurate ele-

The optical images of the "Little Girl, Pretty Purse" daguerreotype before cleaning and after cleaning, as well as the XRF image from collecting Hg L<sup>α</sup> fluorescence X-rays, are shown in Figure 2. It is apparent from Figure 2 that the chemical and electrochemical cleaning methods are generally effective. We will inspect several selected areas and dis-

**Figure 2.** (**a**) Optical image of the plate before cleaning, the oval marks are due to some preliminary cleaning test using similar solution cell prior to this experiment. (**b**) Optical image after a series of local cleaning attempts with chemical (two regions marked by three wells of the solution cell) and electrochemical methods using larger oval shape cells (see text). (**c**) XRF image obtained with Hg Lα fluorescence X-rays. *3.1. Chemical Cleaning*  **Figure 2.** (**a**) Optical image of the plate before cleaning, the oval marks are due to some preliminary cleaning test using similar solution cell prior to this experiment. (**b**) Optical image after a series of local cleaning attempts with chemical (two regions marked by three wells of the solution cell) and electrochemical methods using larger oval shape cells (see text). (**c**) XRF image obtained with Hg L<sup>α</sup> fluorescence X-rays.

### Let us examine the three different sites cleaned using the three-well cell assembly (small oval marks) on the left of Figure 2b, each with its own solution, namely: 3% sodium *3.1. Chemical Cleaning*

thiosulfate, 1% ammonium hydroxide, and 0.5% ammonium hydroxide. An ammonium hydroxide solution was chosen as it removes halides from the surface by forming soluble ammonia silver complexes, and sodium thiosulfate was historically used in the production of daguerreotypes to remove silver-halides from the material surface. As the haze on a daguerreotype surface is normally attributed to halide formation, sodium thiosulfate would also be a good chemical cleaning method in addition to ammonium hydroxide [2]. The results of the chemical cleaning are closely examined in Figure 3. From the left panel of Figure 3, we see that all cleaning solutions were successful in removing the foggy haze from the surface. The 3% Na2S2O3 solution produced a noticeable reduction in surface clouding (Figures 3A,B). A similar increase in sample clarity was observed with the application of 0.5% NH4OH (Figure 3C,D). The 1% NH4OH (Figure 3F) Let us examine the three different sites cleaned using the three-well cell assembly (small oval marks) on the left of Figure 2b, each with its own solution, namely: 3% sodium thiosulfate, 1% ammonium hydroxide, and 0.5% ammonium hydroxide. An ammonium hydroxide solution was chosen as it removes halides from the surface by forming soluble ammonia silver complexes, and sodium thiosulfate was historically used in the production of daguerreotypes to remove silver-halides from the material surface. As the haze on a daguerreotype surface is normally attributed to halide formation, sodium thiosulfate would also be a good chemical cleaning method in addition to ammonium hydroxide [2]. The results of the chemical cleaning are closely examined in Figure 3. *Heritage* **2021**, *4* FOR PEER REVIEW 7

uncovered the masked floral print with fine details and good contrast. EDX maps (Figure

**Figure 3.** Optical images of the plate before (**A**,**C**,**E**) and after local cleaning (**B**,**D**,**F**) with solutions, as marked. The locations were situated on the optical image of the plate before cleaning. **Figure 3.** Optical images of the plate before (**A**,**C**,**E**) and after local cleaning (**B**,**D**,**F**) with solutions, as marked. The locations were situated on the optical image of the plate before cleaning.

*3.2. Electrochemical Cleaning with Cathodic Method*  A preliminary testing with silver-coated copper wires was performed to ensure that the applied potentials would not damage the daguerreotype. Although previous studies employed much higher potentials in their electrochemical cleaning procedures, our initial testing indicated that a potential of −0.9 V would be sufficient to cathodically clean the From the left panel of Figure 3, we see that all cleaning solutions were successful in removing the foggy haze from the surface. The 3% Na2S2O<sup>3</sup> solution produced a noticeable reduction in surface clouding (Figure 3A,B). A similar increase in sample clarity was observed with the application of 0.5% NH4OH (Figure 3C,D). The 1% NH4OH (Figure 3F) uncovered the masked floral print with fine details and good contrast. EDX maps (Figure S4) revealed

the optical images before and after cleaning are shown. The Wei method involved the application of a constant negative potential (Figure 1) to the daguerreotype plate, therefore constantly reducing the surface. The electrolytes were selected such that they had a negligible chemical cleaning effect. Then, 0.01 M, 0.1 M KCl, and 0.01 M K2SO4 solutions were used. All of the electrochemically cleaned sites showed an optically improved image (Figure 4B,D,F, left panel). Most remarkably, the hands (Figure 4E,F, left panel) revealed

To test the effectiveness of the cathodic electrochemical cleaning methods, we used

surface (the Wei method) [11] without causing any noticeable surface damage.

**Figure 4.** Left panel: Optical images (**A**,**C**,**E**) before and (**B**,**D**,**F**) after electrochemical cleaning using the Wei method with an electrolyte solution: B, 0.1 M KCl; D, 0.01 M KCl; and F, 0.01 M K2SO4. Mid panel: Optical image after all of the cleaning

greater details than before any cleaning attempts.

the presence of Ag, Au, Hg, S, and Cl, as well as Hg-coated image particles that were slightly less than a micrometre. After cleaning, they remained intact, while the Cl and S signals were reduced.

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### *3.2. Electrochemical Cleaning with Cathodic Method* **Figure 3.** Optical images of the plate before (**A**,**C**,**E**) and after local cleaning (**B**,**D**,**F**) with solutions,

A preliminary testing with silver-coated copper wires was performed to ensure that the applied potentials would not damage the daguerreotype. Although previous studies employed much higher potentials in their electrochemical cleaning procedures, our initial testing indicated that a potential of −0.9 V would be sufficient to cathodically clean the surface (the Wei method) [11] without causing any noticeable surface damage. as marked. The locations were situated on the optical image of the plate before cleaning. *3.2. Electrochemical Cleaning with Cathodic Method*  A preliminary testing with silver-coated copper wires was performed to ensure that the applied potentials would not damage the daguerreotype. Although previous studies employed much higher potentials in their electrochemical cleaning procedures, our initial

To test the effectiveness of the cathodic electrochemical cleaning methods, we used different electrolytes that were applied to selected sites, as illustrated in Figure 4, where the optical images before and after cleaning are shown. The Wei method involved the application of a constant negative potential (Figure 1) to the daguerreotype plate, therefore constantly reducing the surface. The electrolytes were selected such that they had a negligible chemical cleaning effect. Then, 0.01 M, 0.1 M KCl, and 0.01 M K2SO<sup>4</sup> solutions were used. All of the electrochemically cleaned sites showed an optically improved image (Figure 4B,D,F, left panel). Most remarkably, the hands (Figure 4E,F, left panel) revealed greater details than before any cleaning attempts. testing indicated that a potential of −0.9 V would be sufficient to cathodically clean the surface (the Wei method) [11] without causing any noticeable surface damage. To test the effectiveness of the cathodic electrochemical cleaning methods, we used different electrolytes that were applied to selected sites, as illustrated in Figure 4, where the optical images before and after cleaning are shown. The Wei method involved the application of a constant negative potential (Figure 1) to the daguerreotype plate, therefore constantly reducing the surface. The electrolytes were selected such that they had a negligible chemical cleaning effect. Then, 0.01 M, 0.1 M KCl, and 0.01 M K2SO4 solutions were used. All of the electrochemically cleaned sites showed an optically improved image (Figure 4B,D,F, left panel). Most remarkably, the hands (Figure 4E,F, left panel) revealed greater details than before any cleaning attempts.

**Figure 4.** Left panel: Optical images (**A**,**C**,**E**) before and (**B**,**D**,**F**) after electrochemical cleaning using the Wei method with an electrolyte solution: B, 0.1 M KC **Figure 4.** Left panel: Optical images ( l; D, 0.01 M KCl; and F, 0.01 M K **A** 2SO4. Mid panel: Optical image after all of the cleaning ,**C**,**E**) before and (**B**,**D**,**F**) after electrochemical cleaning using the Wei method with an electrolyte solution: B, 0.1 M KCl; D, 0.01 M KCl; and F, 0.01 M K2SO<sup>4</sup> . Mid panel: Optical image after all of the cleaning processes. The regions of interest are marked with squares. Right panel: Optical images before (**A**,**C**) and after (**B**,**D**) electrochemical cleaning with 0.01 M KCl as the electrolyte. Image B, Barger method (−0.9 V to 0.9 V), and image D, Barger method (−1.2 V to 1.2 V).

It should be noted, however, that cleaning with a KCl solution (0.1 M), introduced Clions into the solution. This site showed a greater amount of residual white haze (formation of AgCl) following cleaning than the site using the lower concentration KCl solution (0.01 M, Figure 4D, left panel). This was the result of the common ion effect reducing the solubility of AgCl. An improved image is observed for Figure 4B, left panel, where 0.01 M K2SO<sup>4</sup> solution was used. All three sites showed great improvement optically after only 90 s of cleaning. EDX maps were also obtained before and after cleaning, showing results like those obtained from the chemical cleaning experiment noted above.
