When the collodion or wet-plate process was introduced in 1851, lantern slides were printed using the new method. The popularity of glass slides increased after the introduction of the easier to use dry-plate process in the early 1870s.

Well before the novelty of the cinematographé had become established as cinema in the very early 20th century, lantern slides were widely used for public presentations. The 1900 production Soldiers of the Cross³ by the Limelight Department of the Salvation Army, incorporated approximately 200 glass cinema slides to heighten the impact of the silent moving image footage. The slides were used as freeze frames or to provide lengthy intertitles.

Glass slides were still in routine use by amateur photographers and public speakers until the mid-20th century, when they were eclipsed by the more economical and practical 35mm colour slides popularised by Kodachrome.

The manufacture of glass cinema slides

A cinema slide consists of an image supported on a glass substrate or base (82mm, 3½ inches square), with a cover glass to protect the surface of the image. The two pieces of glass are held together with gummed paper or cloth tape. Often an aperture mask, cut from high quality black cardboard, is positioned between the pieces of glass. The mask may be cut in a variety of shapes, including square, rectangular, circular or oval or, more rarely, complex silhouettes such as hearts.

Cinema slides with photographic images were produced as duplicates from original or composite negatives.⁴ To print the image onto the slide, two methods were commonly employed – contact and optical.

Contact printing uses a frame to hold the emulsions of the negative and slide together in close contact. A light is shone through the negative exposing the emulsion of the slide. The slide is then processed using photographic developer, stop bath and fixer and then washed and dried. Once dry, the slide is mounted with the optional aperture mask and cover glass and then bound together with gummed tape.

The optical printing method exposes the image by projecting the negative, or negatives,⁵ onto a slide coated with emulsion. The processing procedure is the same as that used in contact printing.

From the 1850s to the 1960s, most photographic slides were produced in monochrome, with colour added later in the form of chemical toning or dyes.⁶ The dyes were applied in thin coats so as not to obscure the fine photographic detail. Multiple colours could be used to create the impression of a full-colour image. The skill with which the colours were applied ranged greatly from large blobs of colour to fine applications with delicate brushstrokes.

During the 1960s and 1970s cinema slides incorporated cut sheets of processed photographic film mounted between two sheets of plain glass, a technique that took advantage of advances in colour photography and was cheaper than using layers of glass. The slide dimensions were kept the same to ensure compatibility with existing projection equipment.

While many cinema slides were photographic in origin, others were screen printed, handwritten or typed in ink or paint directly onto the glass substrate or onto materials such as cellophane. The finished product ranged from text or simple line illustrations to sophisticated hand-coloured graphic designs.

Analysing the stability of cinema slides

Well processed photographic silver images are quite stable because the image is buffered from the external environment by the glass and tape binding. Although the actual dyes used for an individual slide may be from almost any source, they have generally proven to be stable, especially compared with early colour photographic processes. As indicated in the graph below, research by the NFSA in 1993 on commonly used hand-colouring dyes found that, under accelerated aging tests, the dyes had little if any effect on the silver image.

Pictured above: Accelerated ageing graph.

Glass, while fragile, is a stable substrate for the slide and for the image it supports. Apart from physical breakage, the point at which most slides fail is the tape binding the two sheets of glass. The tapes used were either cut from bookbinder’s cloth or a tough paper. Black tape was commonly used, but other colours were favoured for a range of purposes, for example, to identify a particular series of slides. There is very little in the historical literature about the nature of the gum used to adhere the tape to the glass. Using an FT-IR spectrophotometer we analysed samples of the adhesive taken from expendable slides. The results below indicate that the adhesive was probably a vegetable gum such as gum arabic.

Pictured above: Spectrum results 

References

¹ The most widely accepted theory is that Christiaan Huygens developed a lantern slide system in the late 1650s.

² Niépce de Saint-Victor was related to Nicéphore Niépce, who is credited with taking the first photograph in 1827.

³ Although Soldiers of the Cross has been referred to as the world’s first feature film, it may be better described as an ambitious, pioneering multi-media event comprising cinema slides, short reels of moving images, live actors and an orchestra.

⁴ Composite negatives are created by combining two or more photographic images, often by optical printing.

⁵ Titles or additional images may have been composite printed onto the slide.

⁶ Chemical toning changes the colour of a silver photographic image by reacting to the silver in situ with other chemicals such as sulphites or iron compounds. This gives an image that has clear highlights and coloured shadows, as opposed to adding tints where the resultant image has coloured highlights and black shadows.

This is the first in a three-part series. Read Part two and Part three.