Photography has been around for more than 150 years. Over that time the process of making pictures has been continually refined. Recognizing which of the various techniques has been used to print an image will help you date and value it.
The art of photography, literally 'drawing with light', was pioneered in the 1820s and 1830s
by independent inventors in France and England. They coated a variety of surfaces - various metals, glass or paper - with light-sensitive materials, then exposed the coated plates to light. The resulting images were fixed with a chemical wash that stopped the reaction.
The first person to perfect a commercially viable method of taking photographs was Louis Daguerre. It involved coating copper sheets with silver halide, and fixing the exposed image with mercury fumes. The result was a positive image; light areas showed white and dark ones black.
The means of making daguerreotypes, as they were known, became widely available from 1839. The new technology was enthusiastically taken up as a method of making portraits; most towns and cities in Europe and the USA had their own professional daguerreotypist by 1850.
The majority of daguerreotypes were made on plates 16.5cm / 6 1/2
inches by 21.6cm / 8 1/2 inches. As many as 16 different exposures were made on a single plate, though the more usual number
was six, and the daguerreotypes mostly commonly seen today are sixth-plate examples,
7cm / 2 3/4 inches by 8 .3cm / 3 ¼ inches.
Because they were fragile, they were protected by decorative hinged cases, usually covered with embossed leather or
papier mache, which tend to add to their appeal. They should never be removed from these cases. Because they are printed on polished metal, rather than paper, daguerreotypes have an unmistakable sheen. If you tilt them in the light, the image will sometimes appear to switch from
negative to positive and back again, and may disappear altogether.
The first affordable way of making a portrait, they were tremendously popular, and have survived in great numbers. Their major drawback, in retrospect, was that the sheet of copper on which the picture was exposed was the finished article; no copies could be made.
The first widely-used method for producing negative images, invented in 1851, was to coat a glass plate with collodion, a light-sensitive mix of cellulose, ether and alcohol, and expose it while it was still wet. From the glass negative produced, prints were made on paper ready-coated
with salt and albumen - derived from eggs - and prepared with silver nitrate immediately before printing. The addition of gold chloride at this point produced a sepia-toned print. Albumen prints resisted fading and are usually fairly sharp.
Wet collodion was adapted to commercial photography in 1854, using a special camera with several lenses to make eight or 10 images on one plate. The resulting prints, known as
cartes de visite portraits, were 9cm / 3
1/2 in by 5.7cm / 2¼ in and mounted on a 10cm / 4 in by 6.3cm / 2 1/4 in card, roughly the size of a postcard. Promoted as personalized calling cards, their standard size meant they could be kept in albums along with portraits of the famous sold by stationers.
In the 1860s, larger cabinet prints, mounted on a card 16.5cm / 6 ½ in by
11.4cm / 4 1/2 in suitable for framing, became popular, and larger and larger formats were used as the 19th century wore on.
It was possible to combine the relative cheapness of glass negatives with the attractive casing of daguerreotypes. Ambrotypes, invented in 1851, were glass negatives backed with black paper or metal to reverse the image, and set in a daguerreotype case. The easiest way to tell them from daguerreotypes is to tilt them in the light; they look the same from any angle.
Tin-types or ferrotypes were made by coating sheets of tin or iron with collodion to make a direct positive image. The process was first used in the USA in 1856. Tin-types were tough enough to be left unmounted, but many were cased like ambrotypes, and can resemble them closely; they have a slightly duller surface that may ripple.
A revolutionary new process was introduced in 1871, using dry plates ready-coated with a mixture of silver bromide and gelatin. This mix was adapted by George Eastman in the 1880s to produce celluloid roll film,
and from the 1890s silver took over from albumen as the basis for developing paper. Where albumen prints usually have sepia tones, silver prints are generally in shades of grey.
Platinum prints were soon developed as a refinement of the new silver-based technology. The resulting prints have a matt finish with a very wide range of tones of grey. Platinum paper was expensive, and became prohibitively so after World War 1, and platinum prints are generally only seen in the work of art photographers.
All these processes, of course, make black and white prints. One type of early colour photography known as autochrome was patented by the Lumiere Brothers, pioneers of moving pictures, in 1904, producing a positive image on glass.
However, these lovely, luminous images are very rare, and hand-tinting was the only way to get a coloured image before colour film began to be marketed in the 1930s and 1940s. It's unusual, though, to find colour prints more than 20 years old or so that have not faded, unless they have been kept out of the light all their lives.