SECTION III.. _the Sel D'or Process for Toning Positives._

The Sel d'or Process for toning Positives.

This process is somewhat more troublesome than the plan of fixing and toning in one solution, but possesses advantages which will presently be enumerated. The description may be divided into the preparation of the toning Bath, and the manipulatory details.

THE PREPARATION OF THE TONING BATH.

Take of

Chloride of Gold 1 grain. Pure Hyposulphite of Soda 3 grains. Hydrochloric Acid 4 minims. Water, distilled or common 4 fluid ounces.

Dissolve the Gold and Hyposulphite of Soda each in two ounces of the water; then mix quickly by pouring the former solution into the latter, and add the Hydrochloric Acid. If the Chloride of Gold be neutral, the liquid will have a red tinge, but if acid, then the solution may be colourless. The commercial Chloride of Gold, containing usually much free Hydrochloric Acid, will not require any addition of that substance. (See the Vocabulary, Part III.)

In place of making an extemporaneous Hyposulphite of Gold by mixing the Chloride with Hyposulphite of Soda, the Crystallized Sel d'or may be used, adding about half a grain to the ounce of water, acidified as before; but the objection to the employment of this salt is its expense, and also the difficulty of obtaining it in a pure form; some samples containing less than five per cent, of Gold.

It will be found very convenient to keep the two solutions on hand ready for mixing, viz. the Chloride of Gold dissolved in water in the proportion of a grain to the drachm, and the Hyposulphite of Soda, three grains to the drachm. When required for use, measure out a fluid drachm of each, dilute with water to two ounces, and mix.

It is possible that the three-grain solution of Hyposulphite of Soda may by long keeping become decomposed, with precipitation of Sulphur. The effect of this would be to produce a turbidity and deposit of Gold on mixing the ingredients for the Bath, the Chloride of Gold being in excess over the Hyposulphite of Soda (see p. 250).

The Bath of Sel d'or is always most active when recently mixed, but it will keep good for some days if contact with free Nitrate of Silver be avoided. The addition of this substance produces a red deposit in the Bath, containing Gold, and the solution then becomes useless.

DETAILS OF MANIPULATION.

The paper may be prepared by either of the formulæ given in the first Section of this Chapter, according to the tint desired. The pure black tones are obtained most easily with the Ammonio-Nitrate paper, and the purple tints, without gloss, on paper prepared with plain Chloride and Citrate of Soda.

The printing is not carried quite to the usual intensity, as the half-tones are very little dissolved in this process.

On being taken from the frame, the prints are washed thoroughly in common water until it ceases to become milky; that is, until the greater part of the Nitrate of Silver has been removed. The washing must be conducted in a dark place, but it is not necessary to hasten it; the proofs may be thrown into a pan of water covered with a cloth, and allowed to remain until required for tinting.

A trace of free Nitrate of Silver usually escapes the washing; this would cause a yellow deposit on the Print, and also in the toning Bath. It must therefore be removed, either by adding a little common salt to the water during the last washings, or by means of a dilute solution of Ammonia.

For plain paper Prints the former plan will be found the least troublesome; but with Albumen proofs the Ammonia is required, in order to dissolve away a portion of the Albuminate of Silver which has escaped the action of light, before submitting the print to the gold; otherwise the dark tones would nearly disappear in the fixing Bath, the Hyposulphite carrying away the Gold with this superficial layer of silver salt.

The amateur is recommended not to use Albuminized paper in this process until he has become accustomed to the manipulations; the plain paper prints being toned with more ease and certainty.

To prepare the Ammonia Bath, take of

Liquor Ammoniæ 1 drachm. Common Water 1 pint.

The exact quantity is not material; if the liquid smells faintly of Ammonia, it will be sufficient. Place the washed Prints in this Bath, two or three at a time, and allow them to remain until the purple tint gives place to a red tone. The action must be watched, because if the Ammonia Bath be strong, the proof becomes unusually pale and red, and when this is the case a little brilliancy is lost in the after-tinting.

As the Print is comparatively insensitive to light when the excess of Nitrate has been washed away, it is not necessary to darken the room; but a bright light proceeding from an open door or window should be avoided.

After using the salt or the Ammonia, soak the Prints again for a minute or so in common water. Then place them in the toning Bath of Gold and acid; do not put in too many at once, and move them about occasionally, to prevent spots of imperfect action at the point where the sheets touch each other.

The foreign papers, plain salted, colour rapidly in two or three minutes. English papers require five to ten minutes; Albuminized, ten minutes to a quarter of an hour. The tendency of the Gold Bath is to give a blue tone to the image; hence proofs which are light red after using the salt or Ammonia, become, first red-purple, and then violet-purple in the Sel d'or. Albumen Prints assume some shade of brown, or of purple if not too strongly Albuminized. Ammonio-Nitrate papers highly salted, and prepared without Citrate, become first dark purple, and then blue and inky; the Citrate is intended to obviate this inky tint.

When the darkest tones are reached, the Bath produces no further effect, but eventually (more especially if the solution be not shielded from light [?]) there is a little decomposition, producing a cream-coloured deposit upon the lights.

The toning being completed, the Prints are again washed for an instant in water, to remove the excess of gold solution. This washing must not be continued longer than two or three minutes, or there will be danger of yellowness of the whites; this however ought not to happen with proper precautions.

Lastly, the proofs are fixed in a solution of Hyposulphite of Soda, one part to four of water; which may be used many times successively. This Bath alters the tone very little if the deposit of Gold be well fixed on the Print; but the writer has often observed in the case of Albumen paper and paper prepared with Citrate (Formula II.) that if removed too quickly from the Sel d'or, the purple tones change by immersion in the Hyposulphite to a chocolate-brown. Ammonio-Nitrate Prints are less liable to alter in this way.

In order that the fixing may be properly performed, the time of immersion should not be less than ten minutes with a porous paper, plain salted; or fifteen minutes in the case of an English or albuminized paper.

Ammonia may be used for fixing plain paper Prints; about one part of the Liquor Ammoniæ, to four of water. Ten minutes' immersion will usually be sufficient, and the tone is very little affected. This process is a good one, but the pungent smell of the Ammonia is an objection, and the Bath discolours by use. Some care too is required in order to ensure a proper fixing of the prints (see the remarks at page 131).

For directions to wash and mount the proofs, see page 255.

It will sometimes happen in the Sel d'or process, from the toning Bath having but little solvent action on the light shades, that the Prints, after being washed and dried, appear too dark; this may be remedied by laying them for a few minutes in a very dilute solution of Chloride of Gold (five or six drops of the yellow solution of the Chloride to a few ounces of water) and washing for an additional quarter of an hour. Or an over-printed Positive may be saved by toning it with Chloride of Gold instead of Sel d'or. In that case, after proper removal of the free Nitrate of Silver, a few drops of a lemon-yellow solution of Chloride of Gold (with a fragment of Carbonate of Soda added to remove acidity, p. 132), should be poured over the Print, which is to be subsequently fixed in the usual way.

Advantages of toning by Sel d'or.--This process will be found especially useful by those who print large Positives. The solutions may be mixed in a few minutes, and, being very dilute, are economical. It is not even necessary to employ a Bath for toning, but if the Sel d'or solution be prepared of about twice or three times the strength given in the formula, it will be sufficient to pour a few drachms upon the surface of the print. As the Gold solution is always used soon after mixing, a uniform and permanent tint can be obtained; whereas the single fixing and toning Bath of Gold and Hyposulphite loses much of its efficacy by keeping, and over-printing of the proof is required in proportion as the Bath becomes older.