Under ordinary room lighting, the thionin solution is put in a tray. The photopaper sheet is now going to be placed upside down on the surface of the thionin dye soln. A good technique here is to place the sheet on the surface with both hands so that the front of the sheet is somewhat convex so that the middle of the sheet first touches the liquid in the centre of the tray and the sheet is then allowed to flatten out and push any surface bubbles away. It is then left to float flat on the surface for about 2 minutes (we have not found that it matters if it is 10 minutes). It does not matter much if the dye also gets on the back of the sheet, but it makes the squeegeeing procedure easier and wastes less dye if the amount on the back is minimal. The sheet is then lifted off the surface by holding just a corner and liquid is allowed to drip off the opposite corner for about 30 seconds before being placed face up on a clean flat very smooth surface such as the acrylic “carrier sheet”.
A “handling edge” is chosen which will not be in the final image area. The handling edge is held firmly down using the gloved thumb and index finger spread as widely as possible so that the sheet will not slip, the squeegee is then pulled down the coated sheet fairly carefully and evenly with a firm downward pressure so that the excess dye is swept off the other end of the sheet. This excess dye can be squeegeed into a folded strip of water-proof film when one edge is tucked under the carrier sheet. The excess dye can then be poured back into the bath. (The back of a discarded wide strip of the type of photopaper being used here works fine for this).
The coated sheet should now look homogenous (except for the handling edge). Each coated sheet must be completely dry before the next step to photosensitize it. We find it best to use warm air from a hairdryer clamped about 1 metre above the sheet which is resting below on paper towelling.
Under ordinary room lighting, the dried thionin coated sheet is then totally immersed face up in the bath of potassium ferricyanide solution, where it is gently rocked for about a minute, and then left for another minute. (No difference has been detected if it is left in this bath for 20 minutes). This step is transforming the thionine acetate coating into a much less soluble thionin ferricyanide coating. The sheet must then be washed entirely free of the very soluble potassium ferricyanide soln. We do this by using a good flow of cold tap water, first rinsing the yellow solution off the back of the sheet and then using a soft sponge to help fully remove all traces of solution off the front. The tap water is now squeegeed off the sheet before it is dried again under a warm, but not hot airflow.
The photosensitization step must now be carried out under dim lighting. The dried coated sheet is placed upside down in the photosensitizing solution. As before, a good technique here is to place the sheet on the surface with both hands so that the front of the sheet is somewhat convex so that the middle of the sheet first touches the liquid in the middle of the tray and is then allowed to flatten out and push any surface bubbles away. It is then left to float flat on the surface for about a minute. It is then lifted out and held by a corner so that excess liquid streams and drips off into the bath before being placed face-up on the carrier sheet. A few drips of this concentrated liquid on the back of the coated sheet serves to effectively hold the photosheet in place by capillary action after the squeegee is used. The sheet is held down firmly along the handling edge using a widely spaced thumb and index finger and the excess liquid is removed with a firm swipe of the squeegee. This excess concentrated sensitizer is not wasted. It can be squeegeed onto a wide folded strip of water-proof film when one edge is tucked under the carrier sheet. The excess liquid can then be poured back into the bath. (The back of a discarded wide strip of the type of photopaper being used here works fine for this).
The carrier sheet and coated photosheet are now ready to be positioned in front of the projector set-up. This needs to have been pre-arranged so that it will all be in perfect focus when the carrier sheet is put in a marked out vertical position. It is now important to keep in mind that using a projector as a photographic enlarger creates a strict requirement that you never have to consider normally when using a screen. This is that during the exposure period, there must be not the slightest movement or displacement between the photosheet and the projector, or the image will finish up blurred. The projected light is blocked off with something that can act as a shutter that can be lifted off or slid sideways. If you initially need to adjust the exact position of the photosheet then it should be done in less than 5 seconds from the start of the exposure. ( We have found it particularly useful to use a piece of high optical density sheet as a shutter so that we could adjust the position of the very dimmed down image exactly where we want it on the photosheet).
To judge if there has been enough exposure after a period, we completely block the projector light briefly and examine the sheet under non-bright torch-light. If just the brightest highlights in the image can be faintly seen, then the exposure is probably enough. If on the other hand the whole picture can be seen, then it is probable that it is overexposed. With regard to the image size being projected, our latest findings are for example that if DEA/EDTA was used as the sensitizer then an A5 sized bright image might need less than a 1 minute exposure, an A4 about 2 minutes, and an A3 about 4 minutes, but if the photosensitizer used was the TRIS/EDTA then these exposure times needed to be doubled. But all this can of course vary greatly depending on picture and projector.
Under dim lighting, the exposed sheet now needs to be briskly washed entirely free of the sensitizing solution or it will not form that vital Prussian Blue (PB) pigment in the acidic ferrous sulphate developer bath. This sink washing process is carried out using a good flow of cold tap-water while gently rubbing all over the sheet with a soft sponge, (preferably a different sponge from the one that was used to prepare the sheet earlier), this procedure should preferably take less than a minute, and should include a brief rinse of the back of the sheet.
The developing bath: The washed sheet is then quickly placed in the acidic ferrous sulphate bath (so that the whole sheet gets simultaneously covered). Bright lighting is OK now. The bath then needs to be rocked for at least a minute. Although the image usually shows up strongly in about 10 seconds, it is necessary to continue agitating the sheet for at least a minute so that the thionin dye can be solubilised and removed (unless you would like a violet print). It is then briefly rinsed, front and back, under running tap water and then immersed in the final bath of sodium sulphite solution and well agitated for about half a minute. It is then rinsed under running tap water, front and back to remove the sulphite solution. The water on the surface is then squeegeed off with a clean squeegee and the sheet is dried in a warm air flow.
Blyth, J. & Richardson, M. De Montfort University, The Gateway, Leicester, LE1 9BH
Non-holographic imaging systems