| Carbon Essay, Research Paper
CARBON process, The
Print fading was a common occurrence in the earliest days of photography, and several people sought to address themselves to this problem. In the mid 1850s some began to experiment with carbon, and in 1864 Joseph Wilson Swan perfected the process, which he also patented.
Prints made using this process came in any colour, and were permanent. The sensitising solution consisted of a mixture of carbon, gelatin, the colouring material, and potassium bichromate. Once the paper was exposed to light, the areas exposed became insoluble in water. Development consisted of washing the unexposed soluble material away in warm water.
The image being laterally reversed, it needed to be transferred to another base which was usually paper, but which could be leather or wood; the image was in relief.
A variation on the carbon process was the Woodburytype, introduced a year later.
Prints made by this process would come in any colour, and were permanent. Carbon prints became very popular, and the process is still used occasionally.
Please see article on Safe Dichromate Use and article on A Safe Pigment Process
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Carbon Print
A tissue is coated with a gelatine solution carrying a pigment sensitized with a dichromate. When it is exposed to light the gelatin hardens and becomes insoluble. The gelatin hardens only on the surface, to counteract this the exposed and washed tissue is placed firmly on a final support and then peeled away, transferring the image to the final support. This leaves the image flipped over on the final support and exposes the unhardened gelatin.
The transfer tissue is the temporary support that holds the gelatin pigment mixture during exposure. After exposure the gelatin and pigment are transferred to the final support. There are no current manufacturers of transfer tissue. The pigment transfer tissue can be made by hand. This makes the carbon process somewhat complicated.
A pigment tissue is made by coating a support with a mixture of gelatin and pigment and other a number of other ingredients. After the pigment tissue has been coated it is sensitized with a dichromate, and left to dry. The pigment tissue is the exposed to UV light. The dichromate causes the gelatin to harden where it is exposed to light.
After exposure the tissue is soaked for a few minutes to allow the excess dichromate to dissolve out and to allow the tissue to flatten. The tissue and final support are placed face to face under water and aligned. The two sheets are then carefully pulled out of the water, placed on a flat sheet (glass) and lightly squeegeed together.
The sheets are then weighed down for about 20 minutes. The two sheets are then placed in water at about 40 degrees C. The soluble gelatin and pigment will begin to ooze out. After another minute, carefully peel the pigment tissue off the final support. At this point there is no visible image on the final support, it consists of a mass of undissolved gelatin and pigment.
The final image is developed by agitating in all directions the final support in water in a number of baths, until the image is fully developed. The image develops by the unhardened unexposed gelatin dissolving in the water. The exposed gelatin is hardened by the dichromate and made insoluable in water. After development is complete the image is immersed in cold water to harden the gelatin. The resulting image is made up of only gelatin and pigment, making the carbon print one the most archival processes.
Please see article on Safe Dichromate Use
Hazards of Carbon Printing
The sensitizer contains pottasium or ammonium dichromate, which are both moderately irritating to skin and highly irritating to the respiratory system. Dichromates can cause skin and respiratory allergies and ulcerations. They are both suspected human carcinogens. Ammonium dichromate is flammable and unstable when in contact with many materials. The hardening bath conatins formalin which ismoderately irritating to the respiratory system, causing severe skin and respiratory allergies including asthma. Formalin is poisonous if ingested and is a suspected human carcinogen.
Precautions for Carbon Printing
+ Wear goggles and gloves when handling pottasium dichromate or sodium dichromate.
+ Mix powders in a fume hood or dust box, or used an approved toxic dust mask.
+ Keep ammonium dichromate away from sources of heat and store seperately from other chemicals.
+ Wear goggles and gloves when handling formalin baths. For large amounts use a local ventilation system.
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Carbro Print
In carbro printing a pigment tissue similar to that used in Carbon printing is brought in contact with a bromide print. The gelatin in the pigment tissue loses its solubility through the chemical reaction between the sensitizer on the pigment tissue and the silver in the bromide print. The tissue is then transferred to the final support and developed in the same manner as a carbon print.
The advantages of a carbro print is that the pigment tissue is never sensitive to light and a UV light source in not necessary since the bromide print is made using traditional silver-gelatin methods.
Most bromide prints made today are coated with a hard surface to protect the fragile gelatin underneath. It is not possible to use these papers in carbro printing. Common unsupercoated papers currently available are: Kodak Polycontrast Rapid RC matte surface, Luminos RD Matte Bromide, Agfa Portriga Rapid No. 118, SupreBrome Royal Portrait Matte Paper, Ilfobrome Semi-Matte paper.
Hazards of Carbro Printing
The main hazardsof carbro printing result from exposure to the bromide developer and to the sensitizer for the carbon tissue. The developer can be a skin irritant causing allergies. The sensitizer for carbon tissue contains potassium dichromate which is moderately irritating to the skin and highly irritating by inhalation. It can causeskin allergies and ulceration. It is also a suspected human carcinogen.
Precautions for Carbro Printing
+ Use the least toxic bromide developer.
+ Do not heat or add acid to sodium thiosulphate
+ Wear gloves when handling dichromate sensitizer. Mix in a fume hood, glove box or wear an approved toxic dust mask.
+ Avoid inhalation of pottasium bromide powders.
+ Wear gloves when handling solutions during transfer and development of the carbon tissue to avoid skin contact with pottasium dichromate and pigment suspended in gelatin.
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