Understanding the object: how a slide is built and in what way does this concern its digital reproduction, including some remarks on identification

Lantern slides have special characteristics which distinguish them from other forms of creative works on glass-plates. The taxonomy for magic lantern slides, established by María Carmen López San Segundo, Francisco Javier Frutos and Beatriz González de Garay from Universidad de Salamanca, shows: slides differ not only in depicted subjects and aesthetics, manufacturer, production technique, intended use, but also in practical parameters such as format (e.g. rectangular, square, round), size / dimension (e.g. 8 x 8 x 0,5cm, 18 x 5 x 0,1cm, 17,7 x 10,5 x 0,8cm), shape (e.g. flat, with mechanism extending the slide’s frame in width or length), materiality (e.g. glass, metal, wood, paper), production method (e.g. photographed, printed, creatively hand-painted or coloured following a master drawing, in colour or black and white), structural form (single plated without protecting cover-glass nor thick frame, sandwiched) etc.

To impose a rigidly standardized method to digitise these heterogeneous items is not productive. A standard formula could only be applicable to a collection of harmonious nature, with all parts equal in their specific qualities. In archives and museums this is rarely the case. Having existed for over 350 years, slides are too variable in nature. Nevertheless, these transparent images have several elements in common which make them slides: 1. as the word says: they are transparent; 2. they are created to be projected; 3. they are positives; 4. they are made of glass; 5. they are flat.

Other objects are also made of glass (e.g. crystoleums, autochromes), are flat (e.g. photographic glass negatives, glass “sheets” for early colour procedures around 1900), were projected (e.g. a Kromskop with a triple lantern to reunite three colour separations), but they cannot be considered slides, as they were made for different purposes (e.g. to replace pricey paintings and etchings when a person wanted a portrait, to experiment with colour techniques). If flat transparent glass plates are in the collection, it is therefore advisable to identify each object and to distinguish between “projection images” and “photographic images”.

The terminology used to write and speak about transparent images is not always rigorously handled. In both historic and contemporary photographic, media archaeological and film books terms such as e.g. slides, (film / positive) transparencies, diapositives and others are often used as synonyms. This text will use the following terms and concepts: a lantern slide has a carrier made of glass, it shows a (mainly) transparent positive image and was created for projection with a magic or optical lantern. Its size was now and then recognized by authorities which issued regulations: e.g. the British Standard Institution differentiated in 1952 between “miniature lantern slides” in 2 x 2 inch (5,08 x 5,08cm) for educational purposes and those in 3¼ x 3¼ inch (called “3¼ in. square”, 8,26 x 8,26cm) for use in optical lanterns; but “intermediate lantern slides” in 2¾ x 2¾ inch (6,99 x 6,99cm) were also common (Horder 1958, p. 604, 692).

If a transparent positive image has a plastic frame which can be opened, is put between two protection glasses, is mounted in a fixed card board or plastic frame without covering (e.g. 3 x 3 cm, 5 x 5 cm, 7 x 7 cm according to Heyse 1981, p. 161), contains a single acetate image – commonly in “24 x 36mm” (strictly speaking 23 x 35mm, often on colour reversal film stock) produced by a photographic camera using 35mm film –, it is considered by us as a diapositive. Other formats were 30 x 30mm, 40 x 40mm, 50 x 50mm, 70 x 70mm, all not thinner than 1,3mm (0,05 inches) and not thicker than 3,2mm (0,125 inches) (Focal Press 1973, p. 1408); also 6 x 6 cm were mounted. The diapositive was the more practical successor of the lantern slide as it was smaller, more light-weight, much less breakable and easy to produce with a roll film camera on a special diapositive film material (colour reversal film) which, once developed by a photo studio, could be mounted at home by the lecturer and projected with a still projector.

All lantern slides carry an image on one thin glass plate, the picture being often protected by a second glass plate of the same size. Slides made purely of glass are normally framed on all four sides with (coloured) paper to protect the fingers from being cut, and also the edges from splitting; for a double glass slide a solid gummed binding strip keeps the plates together. Glass plate negatives and sometimes home-made slides do not have this (paper) protection. Others have frames made of metal or wood to protect the carrier along and around the edges; many wooden slides have a circular hole in the centre with one round “glass window” fixed by a metal ring (and sometimes little nails when the ring is missing) covering c. one fourth or one third of the surface.

As they were produced over multiple centuries and under varying conditions the slide glass may have different properties. Ingeborg Eggink, responsible for the photography collection at the Nationaal Museum van Wereldculturen in Leiden, notes that glass was blown until the First World War when it was replaced by “vertically pulled glass”, so it can contain little air bubbles, and it was of different thickness until the pulled glass became the standard after 1900 (Eggink 2010, p. 19). Glass is “[…] made by the fusion of different silicates. Physically it is a supercooled liquid, with no regular solid structure […]. Different mixtures of various metallic silicates produce glasses of different optical properties.” (Focal Press 1973, p. 711) Thus they can be brilliant (highly reflective) or rather dull (low reflectivity) and have a surface which is totally flat or with minimal irregularities. Other little flaws may be present (which can be signs of the production process). Before the industrialisation of the glass production each slide plate had its own refraction index which counts for its transmission qualities. Not much research has been done into this subject. Soda glass seems often to have been used for slide-cover plates (Waddington 1995, p. 6). In a photographic book published towards the end of the (amateur) slide production the author gives the advice that for photographically produced slides “no absolute clear glass” should be used (Horder 1958, p. 604). When did this rule come up, and did it apply to photographically taken slides as well?

As the projection of transparent objects follows optical rules such as reflection, refraction and absorbency, the glass plate has a certain influence on the screening results: this means that a glass plate with its partially irregular structure, its thickness of c. one millimetre, and the fact that a light beam is bent twice while it travels through the slide makes that a portion of light is reflected, absorbed and shattered. (Optical rule: every switch from air (medium 1) to glass (medium 2) and from glass to air, makes the light bent as every medium has its own optical properties.) Besides, dyes also have absorption power, especially black which is used to form the background of a scene or to blacken the areas e.g. around the four images of a German toy slides to create “lunettes”. Slides for toy lanterns mostly have just one glass, but most of the others have two – one carrying and one protecting the image – which results in four air-glass surfaces bending the ray four times. It would be interesting to measure the amount of light falling onto slides and the transmitted quantity and how it changes according to the age of the artefact and the composition of the glass.

Additionally, it would be of great interest to do special research on glass plates themselves. Identification is now mainly done by picture analysis in combination with particular signs found on, and characteristics of the frame, which then leads to reading a manufacturer’s lists of produced series, if the indicators have revealed potential names. Without such clues, slides stay unidentified. To support the identification effort, it would be important to know which chemical mixture was used for glass when and by which company, this being completed by knowledge about the “raw material” suppliers from whom different slide manufacturers purchased. Also the fact that “almost all kinds of glass are sensitive to light” (Vogel 1883, p. 261) and change their original aspect could be studied to understand not only the extent to which the dyes that were used are altered, but also how the projector’s light beam affected the glass. A series of experiments by the American glass-maker Thomas Gaffield (Experiments in Photography and the Action of Sunlight on Glass, 1863-1894) (https://libraries.mit.edu/archives/research/collections/collections-mc/mc139.html)  revealed that “the colour of the glass became deeper on exposure to light” or lighter, due to chemical reactions among the compounds (Vogel 1883, p. 262). To sum up this digression into the art of glass making: as it is the tradition in art restoration to use tools developed in natural sciences to better understand the material composition of a painting, it would certainly be highly rewarding to apply those to slides, too.

A note on glass plates with a negative image:

If a transparent image on a glass plate is a negative it could be the original of a photographed (positive) slide, and as many negatives are found in collections it should be mentioned in this context.

Negatives are not made for (regular) projection as this would ruin them: scratches, stains, pinholes etc. would prevent their use for making positives. Exceptions were made for an enlargement on photographic paper or on a canvas to become the basis for a painting: “Sometimes, the negative was projected onto paper in the dark room while the artist sketched in the details with a graphite pencil or charcoal, then painted over to produce the oil painting. These portraits were popular because they were less costly than free-hand paintings which required long sittings. They also had the detail and realism that often were not captured in conventional oil portraits.” (Ruggles 1985) (http://cool.conservation-us.org/jaic/articles/jaic24-02-004.html)

Due to their greater sharpness and wider range of nuances in light and shadow (compared to negatives on paper) professional photographers used glass plate negatives when they came up in the middle of the 19^th century and used them until the 1960s. It is not easy to distinguish between a negative made to produce positives on sensitive paper and a negative to create a photographic glass plate for projection. When a corresponding positive is missing and the content is not identifiable by its aesthetic characteristics (e.g. fictional narrative situation taken in a studio), then the collection history, and background of the individuals who created a collection may be informative: was its “author” the owner of a studio or an amateur but definitely a devoted photographer, or was s/he a lanternist, slide manufacturer, member of the teaching corpus of an academic institution or an explorer and researcher who had the habit of publishing the discoveries by showing images when giving talks? What could have been the intentions in making the negative: to produce an artwork, to create a fine portrait or to produce an object for daily use in teaching or amusement?

Also size and form can be an indication: glass plate negatives had other measures such as e.g. 4 x 5, 5 x 5, 5 x 7 cm (Herskovitz 1999, p. 3) or 4 x 11, 6 x 12, 9 x 12, 10 x 15, 13 x 18 cm and bigger (Eggink 2010, p. 14 in annex) which are not common in the lantern business. Negatives for photographic slides normally correspond to the format of the positive which were standardized as the glass plates had to fit the slide holder made to fit the slot between condenser and lens of the lantern. Amateur photographers, especially in the US, had a great choice: glass plates in e.g. 4 x 5, 4½ x 6½, 5 x 7, 5 x 8, 6½ x 8½, 8 x 10, 10 x 12, 14 x 14 and 14 x 17 inch were offered in January 1891 by E. & H.T. Anthony & Co. in New York in their Illustrated Catalogue of Photographic Equipment and Materials for Amateurs. As to the projection business, already in 1889 the international photography congress in Paris had agreed on standard dimensions for projecting images which were adapted afterwards by many countries: 3¼ x 3¼ inch (8,26 x 8,26 cm) in the UK, 3,35 x 3,95 inch (8,5 x 10 cm) on the continent, 3¼ x 4 inch (8,25 x 10,1 cm) in the US (Robinson et.al. 2001, p. 117).

Unfortunately, the two formats 8,5 x 8,5 cm and 8,5 x 10 cm were also typical for a negative in photography, but a close examination of the emulsion may help. (If the negatives are in an original cardboard packaging this may also indicate the intentional use of the material but one can never be sure that negatives were sold effectively in this wrapping.) Some photographic processes were common before slide manufacturers started to bet on photography: the wet collodion process (c. 1851-1880). It can be distinguished from its followers: “To isolate black-colored collodion negatives from gelatin dry plates, the plate edges and corners where there is image density (dark areas) should be examined. A gelatin plate will generally have a very straight, regular, and even surface along the edge due to its production by machine. Evenly fogged image silver along the edge may also be a clue that the plate was coated by machine, especially if the fogging is not related to the photographic image. […] The image along the edges of a collodion plate will only rarely be uniform, indicating that the image-bearing layer was coated by hand. Other physical characteristics, such as fingerprints in the image at corners or at edges, provide additional evidence that the plate was coated by hand.” (McCabe 1991). “Plate coated by hand; edges often ground; uneven coating at the edges; varnished” for the wet plate process and: “Plate is machine coated; cut edges, even coating at edges; occasionally varnished” for the gelatin dry plate, indicates the Northeast Document Conservation Center in its preservation leaflet on “Types of Photographs” (https://www.nedcc.org/free-resources/preservation-leaflets/5.-photographs/5.2-types-of-photographs).  Negatives that show an uneven thickness of the emulsion when looking at the edges are thus not part of the lantern field.

The dry plate process based on a gelatine coated glass plate became common practice after 1880 and was quickly applied to both areas. It is possible that the emulsion side looks slightly different: some manufacturers produced a special coating for positive plates, fitting in contrast, variety in grey tones and fine details the needs of light absorbing tinting in slide projection, thus the emulsion of the negative had to be special in speed and different in gamma compared to the negative-positive-process on paper. (In the 1950s e.g. Ilford promoted its black-and white positive “Special Lantern plates” used in projection printing which were much slower than negative plates, and the “Contact Lantern plates” for contact printing which had an even slower emulsion (c. 1/100 of the speed of the “Special Lantern Plate”) and “yield blue-black tones”. Here more research is needed to know e.g. when manufacturers started to produce emulsions specifically for those negatives to be used together with glass projection positives.

From what material should scans be made?

In her guide “Measuring Quality for Digital Masters” digital expert Franziska Frey (2000) asks the key question: if negative and positive are both available, which should be used for reproduction? She refers here to photographic negatives and paper positives, but her arguments are also valid for glass plate negatives and positives. She votes in favour of the positive although “every generation of photographic copying involves some quality loss” in sharpness, contrast, spatial resolution etc. which would suggest that it is preferable to scan the camera original as it shows the highest visual quality. But there can be, and generally are, “substantial differences” between paper prints as well as glass positives made from negatives: “Artists often spend a great deal of time in the darkroom creating their prints. The results of this work are lost if the negative, rather than the print, is scanned. The outcome of the digitisation will be disappointing. Moreover, the quality of negatives varies significantly: one might show extreme contrast and the next might be relatively flat. Care has to be taken to transfer this into the digital master. For example, in the case of flat negatives, the bit-depth of the scanner must be high enough to discriminate between the different levels.”   (https://www.oclc.org/research/publications/library/visguides/visguide4.html)

As for slides, there is not only the colourisation of the positive which accentuates certain features and creates an atmosphere, but more substantial manipulations can be observed. As lanternist Ludwig Vogl-Bienek (2016, p. 256-264) in his book Lichtspiele im Schatten der Armut has shown, slide manufacturers such as Bamforth in the English town of Holmfirth significantly reworked their negatives, on the one hand to eliminate flaws, but also for aesthetic reasons, when “special effects” were added, the format of the scenery “reshaped” or, with the help of chemicals, sections of the image eliminated to make room for inserting texts or images. One should not reject the scanning of negatives per se, but one must carefully analyse each slide to recognise its advantages and disadvantages.