Any kind of negative may be used for bromide printing, but by far the best results are obtained from negatives specially made for the process. When daylight is used for printing we are not quite so dependent on the quality of the negative, as the light is more penetrating. With artificial light the negative should essentially be a thin one, but of full gradation and detail; and here let me give a word of advice as to the best developers to use. With regard to the production of density and detail in a negative, all developers are alike, time being the only interfering factor; but, with regard to the stage in development at which these same make their appearance, there is a wide difference between the various developers. Roughly speaking, some produce density first, the detail slowly following; others produce detail first, the density slowly accumulating later. It is from the latter kind we must choose, the reason for which is obvious.
Since it is essential that the negative must be fully gradated, clear and brilliant, without fogging or clogging of the shadows, and at the same time possess all detail, it follows that it will be what is technically known as " thin." A developer which produces the density first and compels us to wait for the detail, obviously cannot give us the above essentials. But where we can get the full scale of gradation and detail almost immediately, and control the density at will, such a developer must be used. And let me here add that, in my experience, I have found that the best developer for the paper is the best for the negativeŚwith modifications, of course.
There are two developers most suitable for the work. One is a combination of metol and hydroquinone, and the formula given is the result of a series of experiments I carried out expressly for the purpose:Ś Normal Developer.
Ten per cent, bromide of potassium to be added as required.
The keeping quality is practically indefinite, and it may be used at full strength or diluted.
The other is an old favourite of mine, of the qualities of which I cannot speak too highly for this work.
Ten per cent, bromide of potassium to be added as required.
Using this, the negative should be developed rather farther than with the first formula, as there is a considerable loss of apparent density in the fixing bath.
Before giving the details of the process I wish to emphasise one very pertinent fact. In this, as in other printing processes, there can be little hope of successful work if no more is known of the chemical and physical cotiditions under which the process works than is given in the manufacturer's instructions which accompany each packet of paper. I will therefore give a brief account of the cause and effect of bromide printing. It may be as well to remark here, that the details of the process apply equally to that part of bromide work known as " contact printing".
Bromide paper is made by coating paper with a gelatino bromide of silver emulsion, almost precisely similar in Metol.....ioo grains or 10 grammes.
Hydroquinone . . . . 50 ,,5
Sodium sulphite . . .3 ounces ,,130 Potassium carbonate. . . l\ ,, ,,65 ,, Water.....40 ,, ,, 1,750 c.c.
Enlarging composition to that of a dry plate, but much less rapid. The process necessary for the production of a graphic deposit of silver is precisely the same as for a negative, although the manipulation is required to be rather more delicate.
The simple chemistry is briefly this.
When light acts on the paper it effects a change in the emulsion, which change is only rendered visible on development. The extent of this change has a two-way limit, technically known as under-exposure and over-exposure. The actinicity or power of the light, or the period of its duration, acting on the paper after passing through the negative, may have been insufficient to effect the necessary change in the emulsion in all and various parts, to yield a correspondingly correct positive or development. This is the under-exposure limit. On the other hand the exposure may have been too prolonged in all or some parts, in which case the change effected in the emulsion has passed the limit and commenced to decompose it, or render it beyond control for the production of a graphic image. This is the limit of over-exposure. By this we see that under-exposure is irremediable; but with over-exposure we have both a chemical and physical control, providing it be not excessive. The actio7i of light on an emulsion may effect a change to such an extent that the image is developable to a limited or fixed extent.
I emphasise this statement because so many failures are due to ignorance of the fact.
Development is simply the reduction 01 the light-affected bromide of silver salt to the metallic state.
The rapidity of reduction is governed by the strength of the reducing agent, but over-exposure develops more rapidly than under-exposure with a developer of similar strength.
Providing there be sufficient of the reducing agentprese?it a weak, or dilute, developer will produce precisely the same effect as a strong one, time being the only consideration ; and since the effect of the exposure is only visible on development it were better to have the reduction under control a?id the possibility Oj rectifying faults, both selectively and generally.
The perfect reproduction in the positive of a negative, is wholly dependent on the actinicity of the printing light, hence we may never be able to get the correct exposure for a certain negative; but practically, the scale of gradation in the negative can be reproduced inversely in the positive. In enlarging, this possibility is somewhat restricted, since we cannot alter the distance of the printing light; therefore, generally speaking, when once a negative with a particular scale of gradation has been found to be most effective with the light used, it should be kept as a sort of standard guide. While under-exposure is irremediable, we can both chemically and physically exercise a considerable amount of control over excessive exposure, providing the emulsion has not been affected to a state of decomposition. Physical control is rarely necessary, except in the case of selective repression or accentuation, when glycerine may be added to the developer to restrain development. But if the method of weak development be employed, the same control may be exerted, and for the same purpose.