*1.1. The Daguerreotype*

Daguerreotypes are the earliest form of photography, produced on a silver-coated copper plate, which was used predominantly in the mid to late 1800s [1,2]. The process of creating a daguerreotype was developed by Louis-Jacques-Mande Daguerre and perfected in 1839. As creating daguerreotypes was a lengthy and costly practice, only people of high status could afford to have their photo taken this way [3,4]. Nevertheless, daguerreotypes offer snapshots, literally, for the very first time, into this era of human activities, and are of artistic, cultural, and historical significance. Efforts to preserve and restore these images have aroused considerable interest in the art preservation community [5].

The production of a daguerreotype image requires several steps. The result is a high contrast, one-of-a-kind photograph [1–4]. The process begins with making a finely polished silver-coated copper plate. This is followed by the exposure of the surface to iodine, making the plate photosensitive upon the formation of silver iodide. Later variations utilized alternative halogens, chlorine, bromine, or a combination of these, in order to increase the sensitivity of the surface to light. The photosensitive plate is mounted in the lightproof interior of a camera. When the photo to be taken is appropriately framed, the lens cap is removed, exposing the plate to light. This step causes the formation of silver image particles that result from the photolysis of the silver halide on the silver surface, creating the image. Areas with dense distributions of image particles of a rather consistent shape and size correspond to a high light intensity, whereas areas exposed to a low light intensity

**Citation:** Stark, A.; Filice, F.; Noël, J.J.; Martin, R.R.; Sham, T.-K.; Finfrock, Y.Z.; Heald, S.M. Retrieving Tarnished Daguerreotype Content Using X-ray Fluorescence Imaging—Recent Observations on the Effect of Chemical and Electrochemical Cleaning Methods. *Heritage* **2021**, *4*, 1605–1615. https:// doi.org/10.3390/heritage4030089

Academic Editors: Antonio Alvarez Fernandez-Balbuena and Daniel Vazquez-Molini

Received: 6 July 2021 Accepted: 31 July 2021 Published: 5 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

display thin and nonuniform image particles. Bright regions are the result of the particles scattering light, and where there are few image particles, only light from specular reflection can be seen. This is why the light and dark areas can change if the daguerreotype is tilted [1]. After the image has formed on the surface, the plate is then exposed to mercury vapor, which fixes the image on the silver-coated copper plate. This is the crucial step in the entire process and, as we shall show below, the presence of Hg on the image's particles allows us to retrieve the fine image details from a tarnished daguerreotype. Excess halides are removed with a salt solution, such as sodium thiosulfate. This makes the surface insensitive to light, and halts the creation of additional image particles, which could cloud the image [1,2]. Next, the silver-coated copper plate is washed with distilled water, and a gold chloride solution is poured on the daguerreotype to ensure the longevity and durability of the image [1]. Note, the addition of gold chloride, known as the gilding step, was later introduced into the final stages of the daguerreotype procedure. Finally, the plate is heated to dry the surface. This process produces a completed daguerreotype image of the subject.
