Historical photographic techniques at the National Technical Museum, Prague

In the previous parts of our series we described how to work with historical photographic techniques. Now we would like to let you look into the collection of the National Technical Museum at some original period photographs taken with the described techniques.

Calotype

Calotype and negatives are among the most important exhibits of our collection of photography. Calotype is the original process with which Talbot created his first photographs. Let’s not confuse calotype and kallitype. Kallitype is a direct (visible) copying process based on the sensitivity of ferrous salts to light (the Van Dyke process is considered as one of the kallitype processes). We have not directly described the calotype technique, so here’s a brief description:

A calotype negative is prepared as follows:

• Paper made of cloth sized by gelatine is treated with a solution of silver nitrate, then it is soaked in a solution of potassium iodide. In the process the paper yellowed, so calotype negatives are yellow.
• Several hours washing out and drying followed. This semifinished product was durable and not yet sensitive to light.
• Sensitisation was achieved with a solution of silver nitrate with gallic and acetic acid under red light.
• The sensitised paper was exposed while still damp in special cassettes, sometimes inserted between two pieces of thin glass. The exposure time was several minutes in sunshine with an aperture of 8. After exposure the paper was either completely clear or had a slight shadow.
• Developing occurred by application of silver nitrate with gallic and acetic acid, that is the same solution used for sensitisation. The picture appeared immediately and was further developed to the required coverage with only gallic and acetic acid (all under red light).
• After rinsing it was fixed with a solution of sodium thiosulphate (originally in a solution of kitchen salt or potassium iodide). Then it was washed out for about an hour.
• For copying the paper negative was made translucent most commonly with beeswax and ironed with a hot iron.
• Copying was achieved either by a salt paper or albumin process. Calotype negatives had great contrast, salt and albumin papers are much softer, so it was possible to acquire normally contrasting positives by the combined process.

The calotype negatives in the NTM collection come from the journeys of Wilhelm Herford to Palestine in 1856. The negatives are on yellow paper of relatively large dimension – about 27 x 34 cm. The backs of the papers are covered with black paint so that the paper structure does not affect the pictures in the white sky. In 1980 contract copies were made from them on bromide paper (Fig. 1 to 4).

Probably from the same author is the smaller negative with about 18 x 24 cm format showing the ruins of a temple with high columns (Fig. 5). Paper negatives are susceptible to damage, they easily rip and the negatives in question have already been repaired variously.

An example of a calotype negative of a portrait and the positive made from it are Figures 6 and 7. You can see that the handling of the tone scale is quite good, though the structure of the paper negative is quite clearly visible in the positive.

Salt paper photograph

The first calotype photographs were copied on salt paper. Photographs on salt paper are specific because the image is not in the binder, the photographs have a matt finish and do not curl up. The picture is usually very pale because the fine particles of silver are susceptible to degradation. Shown is a group photograph from an unknown photographer – Fig. 8.

Portraits on salt paper were often coloured. Literature was only copied weakly and then coloured with watercolours. Fig. 9 shows one portrait from our collection.

Wet collodion process

Negatives created with calotype had advantage that their paper structure was visible in the positive. Photographers therefore search through a way to fix the sensitive layer to glass. Collodion was found to be suitable for this, a material which was used from 1847 for medical purposes (for preparation of wounds). It is nitrated cotton dissolved in a mixture of alcohol and ether.

The process was developed by Englishman F. S. Archer in 1851 and replaced daguerreotype in the 1850.

• Glass plates were glazed with collodion with dissolved potassium iodide (later with potassium bromide which increased sensitivity).
• After drying the plates were sensitised by dipping in a silver nitrate solution.
• Exposure was necessary while wet. If the collodion layer dried completely, it would harden and would not allow the developer inside.
• The exposed plate was developed in pyrogallic acid or ferrous sulphate.
• It was fixed in a solution of sodium thiosulphate or potassium cyanide.
• After drying the plate was dried under a naked burner and while still warm varnished with juniper gum and lavender oil. It was necessary to varnish it, the collodion layer was very soft and got scratched easily even through careful handling.

Using a wet collodion process, it was possible to acquire a photograph with an unusually rich tonal scale and high definition. Plates were several times more sensitive than plates for daguerreotype. Despite its impracticability the process continued for more than 30 years. At the start of the introduction of the dry gelatine process it competed well with gelatine plates and up to the 1940s it was used in reproductive photography for the requirements of some print techniques.

An example of an original collodion negative from our collection is the portrait of a lady in a hat – Fig.10. There is a new copy from that negative in Fig. 11.

Ambrotype

Photographers who used the wet collodion process soon discovered that poorly covered collodion negatives appear as positives against a dark background. They also found that this effect is possible to emphasise when the plate is developed in ferrous sulphate instead of pyrogallic acid and when it is fixed in potassium cyanide instead of sodium thiosulphate (the highly poisonous nature of that material was obviously not such a problem in those days).

The collodion negative was exposed for a half or third of the exposure time of a normal process and was developed so that it had no fog. During adjustment the glass plate was backed with black paper or black velvet, sometimes it was backed with black asphalt. In the main that was how portraits which were adjusted in a frame or cassette with an oval – Union Cases type, format 3 x 4 inches, used originally for daguerreotype, were made.

Many ambrotypes were coloured (chiefly red in the faces and gold in the jewels). Examples of such portraits in our collection are the ambrotypes in Fig.12 and 13.

Pictures on black canvas were called pannotypes. The name vitrotype was sometimes used for ambrotype on glass. In our collection Fig. 14 is labelled as such. The glass on which the emulsion layer of the photograph is applied is broken; however, it is apparently the only known portrait of the painter Josef Navrátil.

Ferrotype

The principle is the same as for ambrotype, though instead of a glass plate, tin lacquered with black paint or covered with asphalt was used as a backing. An example from our collection is Fig. 15.

After introduction of dry gelatine emulsion the principle of ferrotype was used in photomatons (e.g. Bosco, around 1895). The photomaton developed the plate, fixed it and bleached it with mercury bichloride, so the picture had a white-yellowish shade, similarly as with ferrotype made with the true wet collodion process. The process was fast, in 3.5 – 4.5 minutes a still damp picture came out of the photomaton (P. Scheufler: Historické fotografické techniky).

The easy process meant that ferrotypes became a favourite attraction of “quick photographers”.

Materials for ferrotype were manufactured commercially for special cameras for direct positives or slot machines for instant photographs.