LINGUA FRANCA – Chapter 5: Technical Studies

Chapter 5: Technical Studies. Image of a computer with a graph on the screen beside a daguerreotype that has been placed on a stand with a probe hanging from the top.

Technical Studies

Microfade testing

Microfade testing measures the accelerated fading of photographs and related materials. It is a highly precise and minimally destructive technique used mainly to identify colourants and materials that have a high sensitivity to light.

A computer with a graph on the screen beside a daguerreotype that has been placed on a stand with a probe hanging from the top. A close-up of a daguerreotype on a stand with the probe from above. A small light from the probe is piercing a handwritten inscription on the daguerreotype case.

Polarizing viewer

This non-destructive test can be used to determine if a negative is either cellulose acetate or cellulose nitrate rather than polyester. This test cannot differentiate between cellulose acetate and nitrate bases.

It is important to identify if a negative is polyester, since polyester is more stable than acetate.

Place the negative in question in a polarizing viewer or between two polarizing filters/films. Make sure that you observe the thinnest possible density of the negative through the polarizing viewer. The clear outer edge is a good choice. With the viewer polarized (the film looks black), hold the viewer up to a light source, while tilting to observe any colour change. If no rainbow colours are visible, the negative is on an acetate support. If a pattern of pink, blue, green rainbow effect becomes visible, then the negative support is polyester. This occurs because polyester film is highly birefringent and easily identified by interference patterns (i.e. rainbow colours) that are produced when the film is viewed through polarizing filters.

A negative between two polarizing filters showing a pink, blue, green rainbow effect on a light table.
Runners in a race. Foreground runner looking back with arm pointed in the air.

Credit: Ted Grant

Raking light

Raking light is a light source that is positioned on one side of the photograph so that the light falls or rakes across the surface. This lighting technique accentuates textures and planar deformations of the photograph.

Here you can see that a cat has walked across the surface of this photograph, leaving its paw prints on the emulsion.

Image of paw prints are exhibited over the photograph.
An industrial image of silos.


Fluorescence is the emission of light by a substance that has absorbed light during an exposure of radiation of a different wavelength, such as an ultraviolet light or black light. In conservation examinations it is often used to identify coatings, optical brightening agents, tarnish on daguerreotypes, re-touching materials, mould, foxing, tape and adhesive stains, protein glues and oils, varnishes and certain pigments and dyes.

Image of a girl with her back resting on a tree. The image is blue due to the fluorescent lights.

Transmitted light

Transmitted light is a light source that is positioned beneath or behind the support so that the light shines through the fibre matrix and media, watermarks, chain lines, etc.

Standing in low water are three people throwing a beach ball. There is a man to the right, a woman in the centre, and a small girl to the left. All are wearing bathing suits.
A copper wire is heated up with a torch. A blue flame is generated.
A copper wire is heated up with a torch. A green flame is generated.

Plastic testing

Testing different types of plastic is essential to determine if the plastic is harmful for the photograph. This is the Beilstein test method.

When heating a copper wire, melt the plastic in question onto the copper wire. Then place the copper wire with the melted plastic into a flame. If the flame burns blue, the plastic contains bromide and is therefore safe for photographs. If the flame burns green, the plastic contains chlorine and should not be in contact with photographs.

An image of a photograph housed in a plastic envelope.  In the photograph a man stands in the centre of a room. He is surrounded by other seated men, looking at him. It is a court room scene.

Visual inspection under binocular microscope

A visual examination under a binocular microscope provides a more accurate means of examining photographic materials than is possible with the unaided eye. In some cases this technique is essential for identification purposes and can aid in the conservation assessment.

Woman wearing a laboratory coat and gloves, looking at a photograph through a microscope.
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