This section is from the book "Modern Photography In Theory And Practice", by By Henry G. Abbott. Also available from Amazon: Modern photography in theory and practice: A hand book for the amateur.
The early fathers of photography soon discovered, when they worked with glass plates, that if it was necessary to set the camera in a subdued light and point it towards a bright light, the outline surrounding the bright light, instead of being sharp and distinct, was invisible and in its place there was a blurred appearance, a halo, like that sometimes seen around the moon, or the heads of holy personages in the pictures by the old masters. This disagreeable photographic effect is very noticeable in interiors where windows have to be taken and no little trouble and anxiety are sometimes the lot of the professional photographer, who has to make the picture from whatever point of view his client demands. Many a very beautiful photograph of a forest scene too is spoiled by the halation around the leaves of the trees.
Halation is supposed to be produced, when the plate is exposed in the camera, by the rays of light from the window through the trees, or from any highly polished surface, such as silver, passing through the film and being reflected from the back surface of the plate. In all probability the refraction of the light, caused by passing through the glass, plays a very considerable part also. This theory is supported by the fact that very little halation is seen in celluloid negatives, commonly called "films" where the celluloid support for the sensitive film is comparatively thin and the surface of the celluloid is not so bright as that of glass.
*By Geo. J. M. Ashby, Chicago.
It has long been known that halation could be reduced or entirely presented by painting the back of the plate with a suitable preparation, but all the methods recommended were so objectionable, for one reason or another, mostly for their messiness, that few photographers cared to adopt them. The effective preparations for backing plates were found to be asphaltum varnish or a solution of burnt sugar, to which some powdered color, preferably sienna earth, was added. This did not appear to satisfy every one, for so recently as in last year's Photographic Journal Almanac, there was a description of a process for backing plates with asphaltum. The author of the paper vaunted this method as being the best of all and stated that "the backing could be very easily removed with a carpenter's chisel."
The same publication contained a paper by another author recommending a solution of burnt sugar for backing plates without the admixture of sienna earth or any other coloring matter. The directions for preparing the backing solution were formidable, inasmuch as heating the sugar was said to produce first, caramelane, then carameline and lastly, caramelin. The temperatures were given at which the various substances were formed. Caramelane was not wanted as it had too little coloring property and was hygroscopic, while the product of the highest temperature, caramelin, was insoluble, so a separation had to be made by means of alcohol in which the carameline was precipitated. Notwithstanding these and more complications I was tempted to try it, partly because of the very sensible plan being recommended of applying a thin sheet of paper to the back of the plate upon which the caramel solution had been painted.
I tried the backing with carameline and found the result in a negative made with a backed plate so much superior to an unbacked one that I determined to back all plates in future. My first solution was made in strict accordance with the directions, carefully observing the temperatures, separating with alcohol, then adding a proportion of carmelane to prevent the backing becoming too hard, etc., but when the next lot of backing solution had to be prepared I attempted to simplify the operation and succeeded perfectly. The way I now do it is as follows:
A pound of the ordinary crystalized sugar, the kind we sweeten our tea with, is put in a frying pan, which is placed upon the kitchen gas stove. The sugar is stirred with an iron spoon - a stick would do as well - this stir-ing is continued the whole time so that the sugar be-comes heated equally throughout. In a few minutes it melts, then changes color, becoming yellow, light brown, lark brown, then very dark. At this moment I pour a few drops of water in from a jug of boiling water that has been placed ready to hand. The frying operation is continued, a few drops of water being added from time to time to prevent the sugar from becoming too hard, until it is almost black; then, while it is still hot, I add hot water a little at a time, stirring the while, until the solution is of the thickness of ordinary mucilage and a very dark brown or reddish black color. I pour this into a bottle, whereupon it is ready for use. To about six ounces of the solution I add an ounce of wood alcohol, though I am doubtful of the value of it and the next quantity of backing will have the alcohol omitted. The whole operation does not take more than a quarter of an hour and a pound of sugar will make backing solution to coat two gross of 8 x 10 plates.
The next point is the coating or backing of the plates. I like to buy my plates by the gross, then I know that, so long as they last, there is no trouble with variation in the speed of the emulsion. The most convenient time to back plates is when all the family has gone to bed and the gas can be turned out or the electric light switched off everywhere. On a large table I set out four plate-drying racks, these will hold six dozen or more plates. In front of me is the ruby lamp and a saucer with backing solution and in this is a flat varnish brush, two inches wide. On my right is a pile of black or brown tissue paper, cut to the size for covering the back of a plate nearly to the edge; if I am backing 8 x 10 plates my paper is 7 1/2 x 9 1/2 and this size cut into four is right for 4 x 5 plates. All lights being out, except the ruby lamp, a box is opened and a pair of plates taken out and held just as they are packed with their films together, the back of one plate is painted with the solution of caramel, upon this a piece of the tissue paper is laid. The pair is then turned over and the other plate is painted, a piece of tissue paper is spread upon this and the pair of plates is put in the rack to dry. As the two plates are not separated the film surface is not exposed at any time. Another couple of plates is taken out and treated in a similar manner, until all are done, whereupon another box of plates is opened. It will be found that before six dozen plates are backed the first dozen will be found to be dry enough to put back again in the carton or plate box. Even when the backing was not absolutely dry before returning them to the box I have not found any harm result to the plates, though I do not recommend repacking in a moist state. The next thing is the removal of the backing when developing and this is the simplest operation of all; no carpenter's chisel is needed for this. The removal may be said to be automatic, for as the caramel has no effect upon the developer, except, perhaps, that of making its action slightly slower, the plate, with its backing, can be dropped into the developer without further ceremony and in a few seconds the paper is detached from the plate and the caramel is dissolved. As my favorite developer is Tolidol which I use over and over again until it is exhausted, I prefer to remove the backing and the paper before putting the plate in the developer. This is done with the greatest ease by a few strokes with a small wet sponge.
The power that caramel has of preventing halation is very remarkable. It occurred to me that a solution of Spanish licorice, being of much the same color as caramel, might answer the purpose as well and dispense with the use of the family fry-pan. I tried this, putting black or brown tissue paper on the back of the plate as in the other case, but the result was a complete disappointment; in fact the halation seemed rather worse in a plate backed with licorice than with a plain unbacked plate.
The two illustrations are of an interior from backed and unbacked plates that received exactly similar treatment, both in exposing and developing. The test was sufficiently severe, as the sun was shining upon the window at 4 in the afternoon of a fine, clear day, early in September.
The subject of halation should not be treated without a reference to the non-halation plates invented, about eight or nine years ago, by Dr. Sandell, in which halation is prevented by the constitution of the film and not by backing. I want to say here that I think it would be a graceful act on the part of our plate manufacturers who use this invention to, at least, give credit to the inventor.
Taken on an Ordinary Plate
Taken on the Same Brand of Plates Backed.
The Sandell non-halation plate is coated with two or three films of different degrees of sensitiveness, the one film being much more sensitive than another. It is probable that where the surfaces of two films unite they form gradations equivalent to having a number of films, each of a different degree of sensitiveness. This very largely increases the latitude of exposure, for if one of the sensitive planes is over and another is under exposed there will be one in between them that will be right; at the same time - in the triple coated plate - halation is effectually prevented.
The triple coated plate has remarkable properties and a print from a negative, that a friend sent me from the other side, shows this. The photograph is of a powerful flash light or magnesium torch in front of the lens. There is such a complete absence of halation that all the details of the background are distinctly seen and a man sitting near the magnesium light (I think my friend said this is Dr. Sandell) is perfectly distinct.
Before I took to backing plates I used the double coated plates for about two years, but found they were not always to be relied upon and halation would frequently appear. There is much less pleasure in developing a double or triple coated plate as the details are not so distinctly seen. These plates should he left at least four times as long in the fixing bath. I may say here, that in an acid, chrome-alum and sulphite fixing bath, a plate may remain for a long while without suffering. I once overlooked one of a set of 8 x 10 negatives, leaving it in the fixing box for twelve days and it was not in the least degree the worse; it is not possible now to pick out this plate from the others of the set. It should be noted that the plate was on its edge in a grooved fixing box, the only way in which fixing should be done.
The triple-coated plate is too expensive for ordinary use and I do not consider the double-coated plate quite so good as that with a carmalene backing. Another advantage the photographer has in doing his own backing, in addition to the excellent result and the low cost, is that he can use any plate on the market; for my part I always use one of the low-priced plates and shall continue to do so until some one else does better work on the expensive plates than I can on the cheaper ones.