A very brief outline of the photogravure process shown through photographs, with descriptive text. featuring is the plate for the photogravure exercise 1 by william kentridge.
The photogravure process starts with unsensitised pigmented gelatin tissue. The gelatin is sensitised in potassium dichromate and ferrotyped.
The ferrotyped gelatin is exposed with UV light to a transparent continuous tone positive. The exposure has a hardening effect on the gelatin tissue.
After the exposure it’s vital that the exposed gelatin is attached to a copper-plate immediately; even after the exposure has ceased the hardening effect in the gelatin continuous, which results in thicker gelatin and a more contrasted image (a loss of light tones). The gelatin is squeegeed to a degreased copper-plate in order for it to bond to the copper – this step is referred to as ‘lay-down’.
In the photo the dichromate stained backing paper that hosts the gelatin is seen. The gelatin tissue is sandwiched between the copper and backing paper. The bonding period is for an average of 20 minutes, to allow for the moisture in the gelatin to evaporate and thereby the gelatin adheres by ‘gripping’ onto the copper because of the slight shrinkage.
The four photos above show the pigmented gelatin tissue being developed in 45 degrees Celsius water. The purpose of developing the gelatin tissue is to wash away the unhardened gelatin; cotton wool is gently passed over the surface to dislodge any residue of unhardened gelatin.
After developing the pigmented gelatin the plate is transferred to a tray of isopropyl alcohol. The purpose of the alcohol is to replace the water in the pigmented gelatin. This step allows for the quick and even drying of the pigmented gelatin.
After the plate is removed from the isopropyl alcohol the pigmented gelatin tissue is dried with a strong fan and high powered hair dryer.
The plate must be ‘staged’ – this is the covering of the open copper on the sides and back of the plate either with tape or asphaltum.
In photogravure the conditioning of the pigmented gelatin tissue prior to etching is crucial – it has to have a consistent moisture content. The WE studio environment is maintained at an average relative humidity of 60% and an average temperature of 21 degrees Celsius. The plate is kept in this environment for about 24 hours before etching. The light areas in the pigmented gelatin tissue will etch as dark. And, the dark areas in the pigmented gelatin will etch as the light tones and highlights.
Zhané Warren etches the plate in 4 to 5 different strengths of ferric chloride. The darks etch first; the etching progresses through the tones until only the spectral highlights in the image remain – to enable the etching of the full tonal range, which photogravure has come to be valued for.
After etching the photogravure, the plate is polished. Thereafter the plate is proofed, if needed corrections are made on the plate, the edges trimmed off, the plate beveled and steel faced. The plate is then ready to be editioned.