Defining Standard Recipes For Carbon Transfer Tissue Glop Using Das (Diazidostilbene)

After some time using Das as a sensitiser for the carbon transfer printing process, and now more recently for collotype I thought it might be useful to share my experiences with others, with the idea of eventually producing a complete video tutorial that would allow a complete newcomer to the process to be able to establish a workflow, and create prints that would at least serve as a satisfactory starting point from which alterations could be made according to the specificities of personal equipment, such as negatives and light source for example.

I began with a recipe that I had more or less settled on for standard black and white prints, which I used to make prints for my painting-realistic photography series. At the time I was using a large, 100w 375nm LED UV floodlight, which I eventually replaced with a smaller and cheaper, but more practical light of 365nm.

A small and economical UV lamp for alternative photography processes

I removed both the reflector and the glass screen which gives much more even coverage while giving a slightly greater intensity of light. The reflector creates noticeable hotspots so I didn’t even bother trying printing with it.

The version I bought was advertised as being 50w, but then later I discovered that what they meant was that the 6 LEDs have an equivalence to a standard 50w bulb, and that the actual wattage is around 10w. With this in mind it made me wonder whether my 100w floodlight was actually 100w, but in any case it’s interesting to note that such a small lamp is capable of functioning for our purposes, without a huge difference in exposure times as would be expected if my old floodlight really was 100w.

What seems crucial in practice though, is that my new light is of a different wavelength, that being 365nm, compared to the previous 375nm. Assuming the manufacturers are honest and correct (shorter wavelengths are noticeably more expensive than longer ones), what I experienced with the 365nm light, which is supposed to be the ideal sensitivity for Das if I remember correctly, is that my previous tissue that printed well with the old light was no longer able to produce a useable black. According to the theory, the longer wavelengths are capable of penetrating deeper into the tissue, and result in longer exposure times, while a light matched to the sensitivity of Das would print faster with shallower penetration. This certainly reflected my experience, so I went about adjusting my recipe in order to find a new baseline.

Rationale

With the knowledge that my old tissue recipe in theory contained enough pigment to create a full black, and that I was using an extra thick layer in order to produce as much relief in the print as possible, the aim was to gradually reduce the thickness of the tissue, while maintaining the same pigment content, until I reached a point at which the light could penetrate sufficiently to once again obtain a full black. What I was looking for was the maximum tissue thickness combined with the minimum amount of pigment to get black. This would represent the highest possible relief obtainable from a single layer, given my light source, the thickest highlights (which is useful for retaining detail), and the longest tonal scale. Once the max gelatine: min pigment ratio was found, I would then simply adjust the pigment concentration in order to achieve greater levels of contrast.

Tests and more tests made with a Stouffer Transmission Step Wedge

What happened was that in order to get black from the same pigment content, I had to reduce the amount of gelatine to what is considered standard levels, effectively meaning that I would have to print multiple layers in order to get significant relief.

Here’s the recipe that I settled on as my new baseline:

  • 60ml total volume to cover an A4 sheet (alternatively add extra water to facilitate even spreading)
  • 6g gelatine – I was originally using photographic beef gelatine, but switched to 240 bloom pork gelatine for making sweets which comes in a powder, gels quickly and doesn’t seem noticeably different in use.
  • 3g sugar
  • 0.84g pigment – Winsor & Newton Lamp Black watercolour
  • 5ml Das – I mix 3g of powder with 100ml water, so that each ml of solution contains 0.03g Das, this means that 5ml contains 0.15g Das for 6g gelatine OR 0.025g Das : 1g gelatine OR 1:40
  • This recipe has a gelatine to pigment ratio of 1:0.14
5ml of Das solution is the standard dose for an A4 tissue

With this standard recipe, a tissue exposed for 60 minutes at 14cm distance yields a full 21 steps from black to pure white. Now, in order to determine the ideal exposure time, it’s entirely up to the individual to determine what they are happy with accepting as black. The same is also true when initially determining pigment concentration, but for the sake of finality I chose 30 minutes and 19 steps as my baseline exposure time and tonal scale length. To give you an idea of what those 19 steps roughly correspond to, I have measured the density of some of my large format negatives made on Foma Pan 100, developed in HC110-B, but also negatives made on x-ray film and developed in paper developer. Out of the ones that I have deliberately made with a high density, they average between 13 and 14 steps, meaning that they would print quite flat using this recipe.

The above print was made using this recipe and printing time of 30 minutes, and the negative has a maximum density of 17 steps.

From this point I began increasing pigment concentration in order to see what kind of increases would be necessary to get the desired shifts in contrast. The next recipe contained 1.26g grams of pigment, while all other parameters remained the same. This time a comparable black was achieved after just 8 minutes exposure, which yielded a total of 14 steps from black to pure white – 5 steps less than the baseline recipe. This new recipe would be suitable for printing the average negative developed to a high contrast, however there seemed to be side effects associated with such high pigment content. Not only were the highlights very fragile and prone to washing away during development, but the excess pigment seemed to have a tendency to scum up the paper in the areas of the print that are supposed to be clean. With this in mind I decided that these effects would only get worse by increasing pigment further, so instead I would gradually decrease it in order to find recipes that would correspond to 15,16,17 and 18 steps. This very small working range highlights the difficulty of working with Das, which doesn’t benefit from being able to produce significantly greater contrast by altering the solution concentration. No longer being able to get the same level of penetration in the tissue is also a negative factor, so in the future it may be useful to test out something such as a 405nm light, which are easier to obtain and cheaper still. The increase in exposure time may be worth the gains in final relief height and greater flexibility in tissue design.

Once I have finalised the formulae for the remaining 4 tissue types I will make test prints to demonstrate the same negative made with different contrast levels, and will make some more prints for demonstration before beginning work on filming a comprehensive tutorial.

[The cover image is a multi-layered, single-transfer print made using earlier versions of the standard tissue – see more here on Instagram]

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