Caring for photographic materials – part 2

Once you know how to handle photographs the next key step is being able to identify them.  To preserve an object you need to know what it is!  Although this isn’t necessarily easy when there are over 1500 types of photographic processes.  It sounds obvious but you must look a the object, not the image. Sarah advised us to go through a process of elimination to help us identify the process that happened to produce the photograph or negative:

• Is it a true photograph or photomechanical?
• Is it a positive or negative?
• Is it paper-based or otherwise?

Slide copyright of Sarah Allen

Are you now wondering what photomechanical means?  It isn’t a photograph but can look like one.  The best way to identify that isn’t is using a magnifying glass.

It is ink on paper, made up of either a series of dots (letterpress halftone), gridlines (collotype) or squiggles (photo gravure).

So what are some of the photographic processes…?

> Negatives:

– Paper negatives: were in use from 1840 to 1865, though can be dated to later.  The glue is in the title, they will unmistakably be on paper.  Sometimes they are waxed for to make the details of the image clearer, this can make the negative more translucent.  They were often used by armature photographs and pioneers of photography like William Fox-Talbot.

– Glass plate negatives:
Wet collodion glass plate negatives: used between 1850 and 1900. There appearance would be a creamy brown colour and not uniformly coated with emulsion. They provided high quality prints as collodion emulsion has very small silver salts in it allowing prints to be blown up without losing definition. They could only be processed by professionals with a dark room on site. You can very often see a corner thumb mark showing previous handling.

Wet Collodian Negative

Wet Collodian Negative

Wet Collodian negative and print. Images copyright of Sarah Allen

– Gelatine dry plate negatives (1870-1920) look very black and white and the emulsion is applied by machine.  This had been developed by George Eastman of Kodak.  His work led to a rise in amateur  photography as ready made negatives could be bought in advance. Gelatine dry plate negatives are still sometimes used in astronomical photography today.

– Film negatives:
Cellulose nitrate negatives (1885 -1950s). Cine film was also made using the same process.  These negatives / or cine film can be highly unstable if allowed to deteriorate or are in poor storage.  Signs of deterioration include an acrid smell, a sticky orange/brown substrate and powder being formed. Deteriorated cellulose negatives can self ignite or explode. Eventually because of the risks that nitrate posed cellulose negatives were banned.

Deteriorated cellulose nitrate negative.

Deteriorated cellulose nitrate negative.   Image copyright of Sarah Allen.

– Cellulose acetates in use from 1920 to the present day. A more stable negative in that it is not a fire risk.  Signs of deterioration include a strong vinegar smell, wrinkling and channelling of the different layers. It is considered a health and safety risk however if in poor condition as they can give off acetic acid.

– Polyester negative films are the most recent negatives to have been developed.  PET from 1955 and from 1996 PEN.  Both still in use today and extremely stable meaning that it does not deteriorate, as far as we know!

> Positives

– Non- paper based positives are unique and no copies could be made.

– .Daguerreotypes 1840-1860.  Named after its inventor Louis Daguerre.  They are made from silver plated copper sheet, which is highly polished. They look distinctively mirror like. They can be both positive and negative depending on the angle it is viewed from. It was a high end product most popular in USA but not England or elsewhere. The patent for the use of the Dauerrotype was free for the entire world except England, making them costly to use for British photographers and a rare object today.

– Wet collodion positive (UK) or ambrotype (USA, named after American inventor and photographer, James Ambrose) 1852-1865.   One side of a very clean glass plate is covered with a thin layer of collodion, then dipped in a silver nitrate solution. The plate is exposed to the subject while still wet. (Exposure times vary from five to sixty seconds or more depending on the amount of available light.) The plate is then developed and fixed. The resulting negative, when viewed by reflected light against a black background, appears to be a positive image: the clear areas look black, and the exposed, opaque areas appear light.  There is often 3D effect from different layers.

Image side of wet collodion positive.

Reverse, blacked out.

– Tintype (or ferrotype).   Made on sheet of blackened iron which is then varnished, but often has an uneven coating.  They were cheap and cheerful.  Using a magnet is the easy way to identify if it is a tintype.  An obvous clue to look out for signs of decay is iron oxide (rust).  The dates at which tintypes were used can be a bit ambiguous. They were used initially during the 1860s and 1870s, but lesser use of the medium persisted into the early 20th century.

A magnifying glass with a light is a very useful tool when trying to identify photographic processes.

> Paper based positives:

> One layer positives = one layer of paper with photographic emulsion sat in paper fibres. The fibres can be clearly seen and it will have a very matt surface.

– Salted paper prints: 1840-1865. Warm brown colour in appearance, with a matt surface.  They did not produce crisp images.

– Platinum prints: 1880-1930
Platinum salts was used instead of silver salts as they were more stable. The image would have a cool slate grey colour with a matt surface.  This processed produced very stable prints with little or no image degradation.

Platinum print. Image copyright of Sarah Allen.

– Cyanotype prints: 1880-1920.  As the name suggests these prints have a distinctive blue colour, again with a matt surface.  They were easy to produce and therefore used by amateurs quite often.  They are sensitive to alkaline environments.

> Two layer positives = the paper fibres are only partially visible. The surface has some gloss and a surface coating is discernible.

– Albumen prints: 1855 -1920s The binder used is egg white from hens eggs instead gelatine. The prints will  have a semi-gloss surface and tend to fade to a warm colour. Very often there will be cracking across the surface.

Albumen print. Image copyright of Sarah Allen.

– Carbon prints: 1860 -1940s. A pigmented process with gelatine instead of silver salts. The print will have a semi gloss surface with a relief visible.  This is caused where the darker colours in the print have more pigment than in the lighter areas.  The use of raking light will show this. No image degradation occurs.

> Three layer positives = the paper fibres are completely obscured. Print surface can be anywhere from very matte to very glossy and more than one layer is discernible.

– Gelatine P.O.P (printing out paper):  1880s-1920s.  Prints would have warm tones, monochrome not cool black and white. Usually a glossy finish and they come in many different formats.  They are very sensitive and cannot be put on permanent display, best stored at cold temperatures.

Gelatine P.O.P. Image copyright of Sarah Allen.

– Collodion P.O.P: 1880s -1920s. The prints produced had warm tones, possibly with a purple tinge, and a glossy surface.  They can look like the gelatine P.O.P prints, the use of raking light to show ‘Rainbow’ interference colours helps to distinguish them from Gelatine P.O.Ps.  The prints can become brittle and scratched.

– Gelatine D.O.P (developed out prints): 1880s – present.  They come in many different surface finishes and are very stable. It is the most common black and white processed used in the 20thC. Produced in a developing bath and resulting in very crisp images.

In part 3 we’ll explore remedial and preventive conservation practices for caring for photographic materials.



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