Editing Daguerreotype (section)

== Process ==
{{More citations needed|section|date=January 2019}}
The chemistry of the daguerreotype resembles the modern [[Gelatin silver print|gelatin silver]] process, beginning with [[Silver halide|silver halides]] which are formed in darkness before being exposed to light, forming a [[latent image]] which is then [[Photographic developer|developed]] into a visible form, before being [[Photographic fixer|fixed]] using [[sodium thiosulfate]] ("hypo"). The notable distinguishing features of the daguerreotype process are the formation of silver halides directly on a surface of [[Silver|metallic silver]], and the development by exposure to [[Mercury (element)|mercury]] vapour. [[File:Earliest known photograph of a living animal (cropped).jpg|thumb|The earliest known photograph of a living animal (excluding humans). This daguerreotype was taken by French photographer Joseph-Philibert Girault de Prangey while visiting Rome between April and July 1842.<ref>{{Cite web|url=https://petapixel.com/2019/10/23/this-may-be-the-oldest-known-photo-of-a-living-animal/|title=This May Be the Oldest Known Photo of a Living Animal|website=petapixel.com|date=23 October 2019 |access-date=2019-10-24}}</ref>]]
[[File:Turku 1842 - Henrik Cajander.jpg|thumb|A daguerreotype photograph of the Nobel House in [[Turku]] from 1842]]
[[File:Daguerreotype process.jpg|thumb|Graphic representation of the steps involved in making a daguerreotype]]

=== Plate manufacture ===
The daguerreotype image is formed on a highly polished [[silver]] surface. Usually the silver is a thin layer on a copper substrate, but other metals such as brass can be used for the substrate and daguerreotypes can also be made on solid silver sheets. A surface of very pure silver is preferable, but sterling (92.5% pure) or US coin (90% pure) or even lower grades of silver are functional. In 19th century practice, the usual stock material, [[Sheffield plate]], was produced by a process sometimes called plating by fusion. A sheet of sterling silver was heat-fused onto the top of a thick copper ingot. When the ingot was repeatedly rolled under pressure to produce thin sheets, the relative thicknesses of the two layers of metal remained constant. The alternative was to [[electroplating|electroplate]] a layer of pure silver onto a bare copper sheet. The two technologies were sometimes combined, the Sheffield plate being given a finishing coat of pure silver by electroplating.

In order that the corners of the plate would not tear the buffing material when the plate was polished, the edges of the plate were bent back using patented devices that could also serve as plate holders to avoid touching the surface of the plate during processing.<ref>{{cite web |last=Isenburg |first=Matthew R. |year=2001 |title=The Making of a Daguerreotype |url=http://www.daguerre.org/resource/exhibit/brochure.htm |url-status=live |archive-url=https://web.archive.org/web/20141030100307/http://www.daguerre.org/resource/exhibit/brochure.htm |archive-date=2014-10-30 |access-date=2014-11-12 |publisher=daguerre.org}}</ref><ref>[https://patents.google.com/patent/US10508 Reuben, Knecht. Improved Daguerreotype-plate Holder US 10508 A. Reuben Knecht, assignee. Patent 10,508. 7 February 1854. Print.]</ref>

==== Polishing ====
To optimize the image quality of the end product, the silver side of the plate had to be polished to as nearly perfect a mirror finish as possible. The silver had to be completely free of tarnish or other contamination when it was sensitized, so the daguerreotypist had to perform at least the final portion of the polishing and cleaning operation not too long before use. 

In the 19th century, the polishing was done with a buff covered with hide or velvet, first using [[rotten stone]], then [[jeweler's rouge]], then [[lampblack]]. Originally, the work was entirely manual, but buffing machinery was soon devised to assist. Finally, the surface was swabbed with [[nitric acid]] to burn off any residual organic matter.

==== Sensitization ====
In darkness, or by the light of a [[safelight]], the silver surface was exposed to [[halogen]] fumes to form a silver halide. Originally, only [[iodine]] fumes (from iodine crystals at room temperature) were used, producing a surface coating of [[silver iodide]], but it was soon found that a subsequent exposure to [[bromine]] fumes greatly increased the sensitivity of the [[silver halide]] coating. Exposure to [[chlorine]] fumes, or a combination of bromine and chlorine fumes, could also be used. A final re-fuming with iodine was typical.

=== Exposure ===
The plate was then carried to the camera in a light-tight plate holder. Withdrawing a protective [[Dark slide (photography)|dark slide]] or opening a pair of doors in the holder exposed the sensitized surface within the dark camera and removing a cap from the camera lens began the exposure, creating an invisible [[latent image]] on the plate. Depending on the sensitization chemistry used, the brightness of the lighting, and the light-concentrating power of the lens, the required exposure time ranged from a few seconds to many minutes.<ref name="daguerre_spectat_claudet">{{cite journal |date=11 September 1841 |title=Photographic Miniature. To the Editor of ''The Spectator'' |journal=The Spectator |location=London |issue=689 |pages=877–878 |quote=In a letter to the editor of ''The Spectator'', Claudet explained that he gave his exposures as in June 10 to 20 seconds; in July, 20 to 40 seconds and in September, 60 to 90 seconds. |url=http://www.daguerre.org/resource/texts/spectat-claudet.html |access-date=14 September 2014 |archive-url=https://web.archive.org/web/20150923212319/http://www.daguerre.org/resource/texts/spectat-claudet.html |archive-date=23 September 2015 |url-status=live |df=dmy-all}}</ref><ref>{{cite journal |last=Burgess |first=N.G. |date=June 1855 |title=Amusing Incidents in the Life of a Daguerrean Artist |journal=The Photographic and Fine Art Journal |volume=8 |issue=6 |page=190 |quote=On a cloudy day, the exposure was given as three or four minutes |url=http://www.daguerre.org/resource/texts/burgess2.html |access-date=2014-09-14 |archive-url=https://web.archive.org/web/20150923212316/http://www.daguerre.org/resource/texts/burgess2.html |archive-date=2015-09-23 |url-status=live }}</ref> After the exposure was judged to be complete, the lens was capped and the holder was again made light-tight and removed from the camera.

=== Development ===
The latent image was [[photographic processing|developed]] to visibility by several minutes of exposure to the fumes given off by heated [[Mercury (element)|mercury]] in a purpose-made developing box. 

The toxicity of mercury was well known in the 19th century, but precautionary measures were rarely taken.<ref>{{cite journal |last=Nelson |first=Kenneth E. |title=The Cutting Edge of Yesterday |journal=The Daguerreian Annual 1990 |publisher=The Daguerreian Society |page=35 |url=http://www.cdags.org/wp-content/uploads/cuttingedge.pdf |access-date=2014-01-04 |archive-url=https://web.archive.org/web/20140104213303/http://www.cdags.org/wp-content/uploads/cuttingedge.pdf |archive-date=2014-01-04 |url-status=live }}</ref> Today, however, the hazards of contact with mercury and other chemicals traditionally used in the daguerreotype process are taken more seriously, as is the risk of release of those chemicals into the environment.<ref>{{cite web |url=http://cnx.org/contents/fb2244d4-3210-4b3e-baa1-5957b557ed95@5/The_Myth,_Reality,_and_History |title=The Myth, Reality, and History of Mercury Toxicity |last=Barron |first=Andrew R. |publisher=cnx.org |access-date=2015-02-06 |archive-url=https://web.archive.org/web/20150206092911/http://cnx.org/contents/fb2244d4-3210-4b3e-baa1-5957b557ed95@5/The_Myth,_Reality,_and_History |archive-date=2015-02-06 |url-status=live }}</ref><ref>{{EMedicine|article|1175560|Mercury Toxicity}}</ref><ref>{{cite book |last1=Berg |first1=JM |last2=Tymoczko |first2=JL |last3=Stryer |first3=L. |title=Biochemistry |url=https://www.ncbi.nlm.nih.gov/books/NBK22340/ |edition=5 |year=2002 |publisher=W. H. Freeman and Company |location=New York |chapter=17.3 |quote=The proverbial phrase "mad as a hatter" refers to the strange behavior of poisoned hat makers who used mercury nitrate to soften and shape animal furs. This form of mercury is absorbed through the skin. Similar problems afflicted the early photographers, who used vaporized mercury to create daguerreotypes. |access-date=2017-08-30 |archive-url=https://web.archive.org/web/20180509171903/https://www.ncbi.nlm.nih.gov/books/NBK22340/ |archive-date=2018-05-09 |url-status=live }}</ref>

==== Becquerel variation ====
In the [[Edmond Becquerel|Becquerel]] variation of the process, published in 1840 but very rarely used in the 19th century, the plate, sensitized by fuming with iodine alone,<!-- the process was also used as a latensification step before mercurial development by some practitioners (Southworth & Hawes?) who surely did not limit themselves to iodine sensitization, so the modern claim of incompatibility with other sensitizers may be erroneous --> was developed by overall exposure to sunlight passing through yellow, amber or red glass. The silver iodide in its unexposed condition was insensitive to the red end of the [[visible spectrum]] of light and was unaffected, but the latent image created in the camera by the blue, violet and ultraviolet rays color-sensitized each point on the plate proportionally, so that this color-filtered "sunbath" intensified it to full visibility, as if the plate had been exposed in the camera for hours or days to produce a visible image without development. Becquerel daguerreotypes, when fully developed and fixed, typically take on a somewhat bluish hue. The image quality may not be as magnificently sharp as a daguerreotype developed using mercury vapor, although modern photographers pursuing daguerreotypy tend to favor the Becquerel process due to the hazards and expense of working with mercury. 

=== Fixing ===
After development, the light sensitivity of the plate was arrested by removing the unexposed silver halide with a mild solution of [[sodium thiosulfate]]; Daguerre's initial method was to use a hot saturated solution of common salt.

Gilding, also called gold toning, was an addition to Daguerre's process introduced by [[Hippolyte Fizeau]] in 1840. It soon became part of the standard procedure. To give the steely gray image a slightly warmer tone and physically reinforce the powder-like silver particles of which it was composed, a [[gold chloride]]{{which?|date=December 2021}} solution was pooled onto the surface and the plate was briefly heated over a flame, then drained, rinsed and dried. Without this treatment, the image was as delicate as the "dust" on a butterfly's wing.