From Quinn Jacobson's website:

What Is The Wet Plate Collodion Process?


In 1846, Christian Frederick Schönbein (1799-1868) of Basel, Switzerland, discovered nitrated cotton (guncotton) by combining cotton fibers in a mixture of sulfuric and nitric acids. Two years later, in 1848, a young medical student in Boston by the name of John Parker Maynard formulated a durable, skin like, medical dressing (like liquid bandage) from the guncotton called “collodion” (from Greek koll?d?s, glutinous, glue-like) that could be used to treat wounds.
In 1850, Gustave Le Gray (1820-1884) proposed the idea that Dr. Parker’s collodion solution could be applied to photographic purposes because it was an excellent vehicle for holding a light-sensitive solution on glass.

In March 1851, Frederick Scott Archer (1813-1857) described an application of salted collodion on sheets of glass for the purpose of making glass plate negatives. Archer detailed a process where potassium iodide was combined with a solution of diluted collodion (diluted with alcohol and ether), applied to a glass plate, which was then immersed in a silver nitrate bath resulting in a light-sensitive layer of silver iodide. Unlike the handful of processes before collodion, Archer did not patent the process and died penniless a few years after its invention (1857).

Collodion is nitrocellulose (a.k.a. guncotton) dissolved in sulfuric and nitric acid with ethyl alcohol and ether added. Its generic name is pyroxylin solution. It is toxic and highly flammable. As the solvent evaporates, it dries to a clear, celluloid-like film. Today it’s used in the medical field to attach devices to skin and in “Compound W” (a wart remover). In the Astronomy field, it’s used to clean telescope lenses, and it’s used in theatrical makeup for various effects, such as simulating old-age wrinkles or scars. However, we all know its most important use!

Defining the Collodion Processes

There are three (3) variants of collodion photography:

1. The Collodion Negative
The wet collodion negative was the first negative-positive process that gave sharp (near grain-less), detailed prints. The collodion negative was most commonly printed on albumen paper. Talbot’s Calotypes (paper negatives) preceded collodion. For over thirty years, from the 1850s to the 1880s, the wet plate collodion process was the most commonly practiced photographic method around the world. It’s important to note that the only way you could be called a “photographer” was to make negatives and beautiful prints. People working with positives, especially Tintypes, were not called photographers. They were thought of in more simple, commercial terms.

2. The Ambrotype or Collodion Positive on Glass
The Ambrotype process (from Greek “ambrotos”, “immortal”) or amphitype was invented by James Ambrose Cutting (1814-1867) in 1854. Cutting was a 19th century American photographer and inventor. The Ambrotype resembled a daguerreotype (and is often mistaken for one) except glass, instead of a silver plate, was used for the base of the image. The Ambrotype is a thin or underexposed collodion negative on a glass plate. When backed with black varnish, paper or cloth the thin negative turned into a positive photograph. This technique was popular until the 1860s. Today, black glass and other stained glass are used in addition to clear glass for Ambrotypes. The Ambrotypes on black glass are called “BGAs” (Black Glass Ambrotypes) and the Ambrotypes on other colors of stained glass are sometimes called “Rubytypes”. There is a new substrate that I recently tried, black Plexiglas. I think it’s a viable alternative for positives. I call them, “Acrylotypes”.

3. Ferrotype/Tintype or Collodion Positive on Metal
The Tintype, also “Melainotype” and Ferrotype, is a photographic process invented in the United States in 1856 by Hamilton Smith, professor at Kenyon College, in Ohio, who patented the process on February 19, 1856. These photographs became the most common form for inexpensive images made during the Civil War. The Tintype was very similar to the Ambrotype, except a blackened piece of sheet iron was used, instead of glass, as the base of the photograph. The word “tin” is kind of a misnomer; it refers to the image as an inexpensive photograph, rather than the material it is made on. Today, black anodized aluminum is used to make “Alumitypes”.

What is a Carte de Visite or CDV?

The carte de visite (French for “visiting card”) was a small paper print (Albumen) made from a glass negative. This new ability to mass-produce prints from a collodion negative fascinated the nation.

What about Plate Sizes?

There are different sizes of Whole Plates and Half Plates. These are generally accepted as the norm today:
Whole Plate or Full Plate: 6.5” x 8.5”
Half Plate: 4.25” x 5.5”
Quarter Plate: 3.25” x 4.25”
Sixth Plate: 2.5” x 3.5”
Ninth Plate: 2.0” x 2.5”