Prints to be toned by this process must be _thoroughly_ free from hypo or stains will be the inevitable result. The toning bath should be made up as follows, and it must be used at once as it will not keep after mixing A and B:--
A.
Pota.s.sium ferricyanide 20 grains Water 20 ounces Glacial acetic acid 1 ounce
When quite dissolved add
B.
Uranium nitrate 20 grains Water 1 ounce
Immerse the print and keep the solution in motion until the desired colour is produced, then wash the print for half an hour in several changes of water acidulated (1 dram in 30 ounces) with acetic acid.
Weak, under-developed prints are much improved by this method of toning.
At the end of half an hour, if the whites are at all yellow they may be cleared by immersing the print for a minute or two in the following bath:--
Ammonium sulphocyanide 20 grains Water 10 ounces
After immersion, rinse the print for five minutes and dry.
_Intensification._--It sometimes happens (especially when too little light has been used to properly judge development) that one acquires a collection of prints that, owing to under or over-development, are useless; let us see how they may be rendered serviceable.
An under-developed print, though weakly looking and "washed out,"
simply needs intensification to give it the requisite pluck. The foregoing uranium bath acts as an intensifier while conferring a ruddy tone on the deposit. A black deposit can be obtained by intensifying the well-washed print with mercury. The print must first be immersed in a saturated solution of mercuric chloride until the image disappears; it must then be again thoroughly washed to remove all traces of free mercury and may then be redeveloped by flowing over it an old ferrous-oxalate developer. If ferrous oxalate is not at hand, an old metol developer may be subst.i.tuted, but the former is the more reliable.
When the image is sufficiently intense, the print must once more be thoroughly washed. All the toning and intensifying operations may be conducted by daylight.
_Reduction of Density._--Over dense prints can be made fit for many purposes by means of a "reducer" capable of dissolving part of the deposit. The best for the purpose and the one least liable to cause stains is know as the Belitzski"s; it is prepared thus:--
Water 60 ounces Pota.s.sium ferric oxalate 3 "
Sodium sulphite 3 "
Dissolve and add to the red solution so obtained.
Oxalic acid 1 ounce
Shake until the solution turns green and then immediately pour off the solution from any crystals remaining undissolved. To this solution add
Hyposulphite of soda 15 ounces
and shake until dissolved, when it is ready for use.
The print to be reduced need not be free from hypo, but should be rinsed for a few minutes after fixing (or soaked until limp, if previously dried) and may then be placed in a tray and flooded with the reducer. The tray must be well rocked and the print, when sufficiently reduced, must be removed without delay and rapidly washed in running water.
_Some Cheap and Useful Trays._--If large-sized prints are made, the cost of suitable trays becomes a very serious item. The expense of these may be reduced to a mere nothing, without loss of effectiveness, by the subst.i.tution of home-made ones. All that is required to make a tray of any size is a thin wooden confectionery box (or the bottom part of a larger case) lined with the shiny white marbled oilcloth known as "American moleskin." This is fitted inside the box (the corners being turned under) and secured by a row of tacks around the top edge. No further lining or preparation is required and the tray will stand all sorts of ill-treatment. As for durability: I had three such trays made out of old herring-boxes picked up at Calgary and lined with moleskin that had already seen service as cover to a wash-handstand and chest of drawers in a Canadian boardinghouse. For upwards of a year those trays were used daily and travelled many hundreds of miles by mule and dog train, and were not worn out when I returned home. My porcelain trays were smashed by a fall from a refractory mule, but the rough and ready makeshifts were a priceless boon.
It seems to me that by practising economy of this kind and in various similar ways (_i.e._, where economy is necessary as, unfortunately, it sometimes is) the cost of practising our pet recreation is very materially reduced.
_W. Ethelbert Henry, C.E._
_The Gum-Bichromate Process._
[Ill.u.s.tration]
Pictorial photography is answerable for the revival of this, one of the almost forgotten methods of printing. Results unacceptable to bygone requirements have been reintroduced with advantage, where suggestive individuality and artistic effect have been desired.
The gum process has an unlimited range of possibilities, it would be impossible to describe them all. The minutest details, or the broadest diffusion together with the power of working from the highest to the lowest keys of _chiaroscuro_ are values that can only be realized when the infatuation consequent on successfully working the process is experienced.
This method of printing, as with the so-called "carbon process," is dependent upon the characteristic behaviour of the chromic salts when in combination with organic substances, such as gelatine, gums of various kinds, starch, etc.
When any of these mixtures are submitted to the action of actinic light, they become more or less insoluble.
This property was partially discovered as far back as 1798, by Vauquelin. Professor Sucrow, Mungo Ponton, Beauregard and others advanced its application to photography up to about 1840, but it was not until some ten years later that its great value as a photographic agent was definitely established.
Hunt, Fox Talbot and Poitevin, each worked indefatigably to bring the application of the chromic process to a successful issue; but to Poitevin must be accredited the honour of being the original inventor of the chromated pigment or carbon process. This brings us up to about 1855.
None of these investigators appear to have been remarkably successful, beyond having established definite, but valuable facts of the changes produced.
This want of success may possibly be accounted for by the general employment of gelatine and direct printing. It was not until Pouncey and others, about 1859, employed gum as the colloid medium, that any great advance was made.
About this time an important commission of inquiry decided that to Pouncey, Gamier and Salmon, and Beauregard the honour of producing permanent prints must be equally credited, and accordingly divided the Duc de Luyue"s prize between them, giving to Poitevin the credit of the priority of invention.
Pouncey appears to have followed up the process with some considerable success, as some of his existing examples are excellent; it is much to be regretted that we have not more detailed particulars of his methods of working; but he evidently was before his time and met with but little encouragement.
To Alfred Maskell and M. Demachy must be accredited the revival of this long neglected process, and during the last three years much advancement has been made towards perfecting it.
Serious workers, both at home and abroad, are industriously exhausting the possibilities of the process, and crude as some of the earlier examples of this revival have been, improvements and simplicity of working are giving us productions of every description, of such excellent quality that it may soon be expected to satisfy even the caustic criticism that has so persistently opposed its re-introduction.
Dexterity in the various stages of practical manipulation is necessary before skilful efficiency can be secured, and in order to arrive at this, due consideration must be given to the selection of the paper the colour most suitable to the subject and the effect desired.
Almost any kind of paper will be found workable, if it be of fairly good quality. Those that are thickly coated with soluble sizing media are unsuitable, for although they may give clear whites they sometimes produce harsh prints, the half-tones are also liable to be lost in development unless very deeply printed. Several of the continental kinds are well adapted to the process and work in an excellent manner, giving soft and even results; of course, it will be understood that for definition and fine detail the finer grained descriptions are the best, but where diffusion is desired those of a coa.r.s.er texture may be advantageously used, they give a granulation that tends materially to secure the peculiarities of gradation characteristic of this process.
A few of the continental papers that will be found to work with ease to the beginner, are as follows:--
Michallet paper is rather coa.r.s.e, but takes the gum coating easily, it has a series of lines running in both directions, which are rather objectionable for some subjects; but it is an excellent paper for first experiments.
Ingres, is also a paper of similar character, and can be worked with equal facility. Lallane is another paper of the same cla.s.s, but much finer.
Allonge paper is entirely free from the markings peculiar to those previously mentioned. This paper is best worked on the reverse side, which can be distinguished by examining the name marked in one corner.
Among the English papers the ordinary cartridge, Whatman"s drawing papers and many others are adaptable, but it must be borne in mind that those with a toothed or grained surface are preferable.
There are two methods of working, and results of equal excellence have been produced by either. Some of the most proficient workers of the process adopt the easier one of coating the paper, without previous preparation, with a mixture of gum, bichromate of pota.s.s and pigment. Others adopt the precaution of first saturating the paper with a strong solution of bichromate, and when dry coating it with a mixture containing only gum and pigment.
Experience is in favour of the previous saturation of the paper, this is recommended especially for beginners, as there are several kinds of paper that will not work efficiently by the first method; but when skill and practical knowledge of the special behaviour of the materials employed is acquired, either method can be adopted.
We may presume that the advantage of the previous saturation of the paper with the chromic salt is, that should there be any inequality in its structural character, or should it be unequally sized, the bichromate appears to act as a kind of resist to the penetration of the pigment, thereby securing an increased range of tone and a corresponding purity of the whites.