Sensitising The Tissue

Having made my first carbon transfer tissue, I will now prepare to sensitise it before I expose it

Ammonium-dichromate-2D.png

Ammonium Dichromate

The picture is of the two ions that make up Ammonium Dichromate - they look a bit like male and female tarantula spiders to me

Spirit Sensitising

There are two ways of sensitising the tissue that I know of. One involves a lot of wasted chemicals and uses a bath in which the tissue is washed - called, unsurprisingly, the 'Bath or Tray Sensitising'. I prefer the other technique that mixes a volatile agent with the dichromate and is referred to with the wonderful term 'Spirit Sensitising. The two volatile agents are acetone or isopropyl alcohol. I use this alcohol for a variety of things (including cloud chambers to watch cosmic rays, but that is a different blog) so I prefer to use it.  

The Workflow

Because the process of sensitising must be followed immediately afterwards by exposure and development, it is important to have a clear idea of the workflow before you begin. Here they are in summary

  1. Prepare and set aside your negative
  2. Select and set aside the final support
  3. Sensitise the tissue and set a timer for the drying time.
  4. As the end of the drying time approaches, get the mating bath ready with squeegee or roller, gloves, paper towels and the supporting glass and weights.
  5. Expose the tissue
  6. Using the bath, mate the tissue to the support
  7. Using weights to press the two together – while waiting get the development bath ready and to temperature
  8. Start the development process with the support and tissue mated – when ready separate the two and continue the development of the photograph
  9. Use cold water to stop development
  10. Hang out to dry. Any clearing and finishing can be performed after the print has dried

In this blog I am only preparing steps 3 and 4 - the other steps are covered in other blogs

Before the Sensitising Process

Photographic Items

  • Tissue, clean and ready

On hand

  • Negative, printed and ready
  • Final support, cut and ready

After the Sensitising Process

Photographic Items

  • Negative, printed and ready
  • Tissue, sensitised and ready to be exposed
  • Final support, cut and ready

The Negative

King & Lockhart suggest that: "There is no exact rule for what dichromate concentration is best for a give negative. You will need to rely on experience as a guide. However, as a starting point consider using a 1 percent concentration for low contrast negatives, 3 percent for medium contrast negatives, and 5 percent for high contrast negatives."

I prefer to print out my negatives at the contrast I desire for the particular image I am about to print and use only one concentration for consistency. So I am going to use 5% v/v Ammonium Dichromate

I describe the technique I use in another blog "Printing a Negative (or Positive) for a Single Colour" and I will use the same technique here. Though, with gum printing I use a calibration curve in Photoshop to adjust for the measured non-linear sensitivity response curve of the gum bichromate technique I use. Initially with the carbon transfer I use the highest contrast that I feel comfortable with for the image

Instructions

Ingredients

  • Ammonium Dichromate solution 5% w/v - volume measured to the tissue
  • 100% Isopropyl Alcohol - keep in the freezer (mark 'Poison')

These are mixed 1:1 by volume at 6,500 ml/mm2 for total volume

Equipment

  • Brush for sensitiser
  • Means of holding tissue while spreading sensitiser - pins, tape or silicone pad
  • Sealed container for sensitiser - sealed between the two coats
  • Measuring jug
  • Photo-safe light
  • Fan or hairdryer set on cool

Applying the Sensitisers

  1. Fix the tissue so it does not move during sensitising - push pins work well
  2. Pour the isopropyl alcohol into measuring jug from the freezer - add the dichromate, which is at room temperature (don't get it anywhere near the fridge)
  3. Apply two coats in the same way
  4. Pour our half of the mix onto the tissue
  5. Using horizontal, vertical and diagonal strokes, keep brushing until you can feel slight friction on the brush, which indicates that the sensitiser has been absorbed. The second coat can take slightly longer
  6. Move the sensitised tissue to the drying area
  7. Clean the brushes, measuring jug and pinned area

Once the sensitised tissue is fully dry, should be 30 minutes, move straight to exposing the tissue

Lessons Learnt

  1. I enjoyed this step, it is quite different to gum printing
  2. Despite other difficulties with the tissue the sensitising seems to have been quite uniform