Glass Positives
1880s–1930s
For the second half of the 19th century and into the 20th, glass was used as a support for the creation of photographic negatives. But glass supports could also be used to create positive images, too. The earliest positive images on glass were ambrotypes, but by 1879, as the wet plate process was abandoned in favor of pre-sensitized dry plates, other forms of glass positives came on the scene.
Lantern slides (usually about 3.25 by 4.25 inches) were positive images on glass made to be viewed in a projector or “magic lantern.” The processed image was monochromatic, but many lantern slides were colored by hand in order to give their subjects a more naturalistic appearance. Larger glass positives, viewed by transmitted daylight or lamp light employed the same process. These larger objects were usually fitted with a frame or other mounting system that permitted them to be placed near the appropriate light source. Orotones were another form of glass positive, easily identified by the gold coloring of the plate that replaced the white or clear highlights of the image.
IV-A. Lantern Slides (1850s–1930s)
Before the invention of photography, magic lantern slides were made from drawings or paintings that were mounted in a device, backlit by candles and mirrors, and projected on a screen or wall with a lens. Audiences were fascinated by the projections, which were often animated and accompanied by music. The magic lanterns were also used for scientific purposes.
Photography was a natural fit for the magic lantern technology. Lantern slides were made first with albumen (ca. 1850), then collodion (ca. 1850–1860), and finally gelatin (ca. 1870) on glass plates. Their standard size is typically 4 by 3.25 inches. Two methods or techniques were possible: using a plate that would produce a direct positive, or using a negative as an intermediate step to create a positive.
In the first method, a plate capable of producing a direct-positive image would be placed in a camera and, when exposed and processed, produced a single unique image. When using a negative to make a positive, two options were available. The first was to use a camera loaded with a negative to photograph another negative lit from behind. When developed, the negative of the negative would be a positive. The second way was to place a processed negative directly against an unprocessed one (contact printing) and expose both to light, creating a positive. Collodion and gelatin positives differed from their negative counterparts (see ambrotype) in length of exposure time and processing chemistry.
Opalotype Process (1830s–1930s)
The opalotype process, while not widely used, also evolved throughout the history of photography and could be made with either collodion or gelatin as the light-sensitive emulsion coating. Opalotypes were named for the substrate, opal glass, a translucent white glass. They were also called milk-glass positives. Opalotypes evoked the look of portrait miniatures painted on ivory. Two basic techniques were used to make an opalotype. One involved the transfer of a print to a milk-glass substrate, and the other used a negative to print directly on light-sensitive emulsion applied to the milk glass. Some examples were hand-tinted to enhance their resemblance to painted miniatures.
Orotone Process (1880s–1930s)
The orotone process, though it did not enjoy widespread use, came into practice around 1880 and consists of a gelatin positive on a clear glass plate. The name derives from the gold color of the photograph, applied to the back of the processed plate. The gold tone was achieved through various methods, commonly particular to each practitioner. One of the most stable gold-tone applications was developed by Edward S. Curtis in 1916. After producing a glass positive, he coated the reverse in a mixture of gold and bronze metallic powders in a banana-oil binder.
