HAVING obtained a set of tri-color negatives it is obvious that one can as readily make lantern slides or transparencies as prints, and almost all the processes described for the latter are applicable to slide making.
The easiest method is by the use of celluloid film sensitized with bichromate and exposed through the back. While roll film is not ideal, because of the back non-curling coating, it is superior to the cut sheet film as it has only one-third the thickness. Those who like to make their own materials will find it possible to coat even the thinnest celluloid evenly by a little trick; the difficulty in doing this is that the celluloid instantly curls up on the application of a warm solution. But by the use of the following plan there will be found no difficulty from this. Clean some old negative glasses, of a goodly size, and level them carefully. Prepare the following mixture:
Gelatine 53 g.
Golden syrup 53 g.
Glycerine 65 ccm.
Chrome alum 1 g.
Water to 1000 ccm.
Soak the gelatine in 750 ccm of water with the syrup and the glycerine for half an hour, then melt by heat at 500 C. (1200 F.), add the chrome alum dissolved in 100 ccm water, and make up to the quantity given. It should be noted that the syrup is cane sugar syrup, not corn syrup, though probably the latter will serve the purpose. Coat 65 ccm of the mixture on every 100 square inches of glass and allow to set, and then dry; this may take twenty-four hours, and one must not be misled by the feel of the film, as it actually never dries because the syrup and the glycerine are hygroscopic. The result will be a fairly thick tacky film that will hold anything. If the thinnest celluloid be rolled down to this, it will stick for weeks unless pulled off. Thus it will remain perfectly flat while coated with the various solutions. As soon as these coatings have dried the celluloid may be stripped by lifting two corners and one edge; one of the best things to do this is to use a flat bone paper knife run carefully along the edge so as not to damage the tacky coat; then a straight steady pull will detach the celluloid. The tacky glass may be used over and over again; exactly how many times, will depend on the care with which it is handled, but at least a couple of dozen times is easily possible.
To prepare the gelatine for the sensitive coating there is nothing better than the following:
Carpenter's fine glue 55 g.
Water 500 ccm.
The glue is the best and palest carpenter's glue that can be obtained, and it should be allowed to soak in the water for twelve hours. The correct method of working is to weigh a beaker or jar empty, then weigh in the glue and measure the water; at the end of the time pour off as much water as possible and weigh the jar again. Add enough water to make the weight 660 gm and add:
Gelatine 55 g.
Allow to soak for half an hour, then raise the temperature to 500 C. (1200 F.), and stir until all is dissolved, then add:
Alcohol 40 ccm.
To this should be added some emulsion of silver bromide, and this is made as follows:
Gelatine 10 g.
Potassium bromide 6 g.
Water 100 ccm.
Allow the gelatine to soak for ten minutes, melt by heat at 500 C. (1200 F.), and add slowly with constant stirring:
Silver nitrate 5 g.
Water 50 ccm.
This can be made by artificial light, as the silver salt merely plays the part of an inert pigment and prevents too high a relief; its light-sensitiveness is not used. Keep the emulsion at 500 C. (1200 F.), for about fifteen minutes, and then pour the emulsion out into a flat dish to set, and leave all night; if one has a refrigerator, the dish should be put in it so as to thoroughly chill the gelatine. The next morning score the emulsion with a silver fork first lengthwise and then across, so as to cut it up into small nodules. Collect these in a clean cloth, tie the ends of the latter into a bag and suspend in water; change this six times in the course of half an hour, giving the bag a good squeeze each time, so as to express as much water as possible. Then leave the bag to drain for an hour, for preference opening it and spreading the shreds out so as to give them a chance to drain well. Melt at 500 C. (1200 F.), and add 75 ccm alcohol.
This should be stirred into the hot gelatine-glue mixture, and the result should be about 1000 ccm of a milky emulsion, which should be made up to that quantity; if on the other hand it measures a little more it will not matter.
There are now two methods open, the one to sensitize this, the other to coat as it is and sensitize as wanted. The last plan is preferable, as one can thus prepare enough celluloid at one sitting to last a year; whereas if sensitized with bichromate it will not keep more than two or three days. To sensitize, 30 g of ammonium bichromate should be added to the 1000 ccm, and it can be made to keep better by the addition of 10 g of potassium citrate. If the gelatine-coated celluloid is prepared, then the normal bichromate sensitizer should be used as already advised on p. 113.
Printing is effected through the back and, therefore, this must be polished well. The printing frame should preferably be placed at the bottom of a lidless box, about eighteen inches deep, so as to prevent the access of side light as far as possible. The films are very sensitive, and from two to four minutes will be about correct exposure in bright diffused light. An actinometer is hardly required, as the image can be easily seen on the white emulsion. Printing should be carried on until all but the details in the highest lights are visible on the back of the film, that is, on the celluloid side. Care must be taken in examining the progress of printing not to expose the film to too bright a light, or more or less general insolubility will be caused. After exposure, the films are developed in warm water at a temperature not exceeding 450 C. (1130 F.), and the progress of development is easily seen, as the silver bromide acts as a pigment. As soon as the picture is developed, immerse in cold water. One now has a carbon or gelatine picture with the silver bromide as pigment, and as this is now valueless, having done its work, it must be removed, either by using a hypo bath, or what is preferable, hypo and ferricyanide or the reducer given on p. 93. The reason for advising the use of these is that the silver salt may be so far reduced by the exposure as to be actually darkened and as this means metallic silver it darkens the colors. A good wash should follow, and the reliefs may be dyed up, or dried and subsequently dyed.