Manipulation of contrast
There is much discussion of ways to increase or decrease contrast in traditionally made Cyanotypes in refs  Some of these ways may also work on the Blythtype.
Discussion of possible chemical mechanisms
- The coated thionin dye in the form of its acetate or chloride salt is readily converted to a less soluble salt by immersing it in concentrated potassium ferricyanide solution. The resulting thionin ferricyanide coating is then made light sensitive by coating it with the concentrated solution of diethanolamine EDTA salt.
The relatively viscous and concentrated photosensitizing layer on the photosheet needs to be squeeged off after a minute for two reasons.
1) A thick layer causes dye to dissolve into it and this can act as an uneven light filter.
2) A thick layer has a lensing effect which slightly defocusses the image.
However, if the sensitizing solution is made more dilute and less viscous in the first place, it reduces its photosensitizing ability very substantially. The pH is neutral and the solution is not harmful if it accidentally gets on unbroken skin. It is very water soluble.
It is thought that in the imaging process, light causes the dyed photosheet to bleach and momentarily to produce a colourless compound known as leuco-thionin. This reduced form of the dye then seeks to return to its non-reduced coloured form by reducing its ionically attached ferricyanide anion to form thionin ferrocyanide. The proportion of ferricyanide ion converted to ferrocyanide ion is dependent on the light intensity on that area and also of course, the exposure time. In the highlight areas of the image, the conversion to ferrocyanide can approach 100% and in the darkest areas the conversion can be almost zero. When the exposed sheet is immersed in the acidic ferrous sulphate solution, the famous Prussian Blue (PB) pigment forms only where thionin ferricyanide still exists, and this allows the thionin dye to get washed into solution as a sulphate salt. But PB does not form when it meets thionine ferrocyanide in the light-struck areas, (which makes this a positive-working system). Therefore in the highlight areas very little Prussian Blue is formed and it may be predominantly “Prussian White” or “Williamson’s White” (WW) and this is a form of an insoluble ferrous ferrocyanide salt. [ref.2, p293]. WW formation also allows the thionin to form a sulphate salt. However this WW salt can quite rapidly absorb oxygen when wet , or even apparently dry and this then forms PB and can start to create a blue veil over the image, particularly in the highlight areas. We have spent some time working on how to deal with this effect easily and cheaply. We were pleased to discover that a final bath of sodium sulphite stops the “blueing” effect permanently. Its effectiveness is not simply caused by a chemical reduction process, because more powerful reducing agents such as sodium bisulphite do not work.
We made the important discovery some years ago that neutralizing EDTA with certain amines had a strong synergistic effect when it came to photobleaching the dye methylene blue (tetramethyl thionin) . After testing a number of aliphatic amines for our new discovery, the best amine found that worked, and was also not volatile, smelly or inflammable and could be bought on line by home users was diethanolamine. We had originally been disappointed to find that triethanolamine and other tertiary aliphatic amines which had long been known to photobleach MB  ]did not work well in our system, but thanks to an inspired suggestion from Alan Meredith-Jones of Magnacol Ltd, we found that diethanolamine worked very well indeed, much to our amazement, because it was a secondary amine. He also suggested trying the primary amine “TRIS Base” tris(hydroxymethyl)aminomethane which is a very convenient crystalline powder commonly used in biotechnology. Astonishingly, this one also worked although the exposure times needed were twice as long, this was still very good and a viable alternative to liquid DEA.
Unfortunately the cheaper methylene blue (MB) dye failed to work in this Blythtype method. It appeared to be due to the colourless leuco MB ferricyanide failing to form the sufficiently stable ferrocyanide salt that was hoped for. But that then led us to the important discovery that unlike MB, thionin did actually work as was hoped.
Blyth, J. & Richardson, M. De Montfort University, The Gateway, Leicester, LE1 9BH
Non-holographic imaging systems