PRINTING ON IODIZED PAPER.

IT is necessary to give a detailed account of this mode of printing, as the ammonia nitrate paper is not sufficiently sensible to the feeble light of the dull winter months, excepting on very favourable days. With ammonia nitrate paper half a day's printing will hardly be sufficient to produce a strong picture.

With iodized paper the exposure to light will vary from three to fifteen seconds; seldom longer than that. This process requires care in the choice of paper. With some kinds of paper it is hardly possible to obtain good proofs. The best for this purpose is Turner's Talbotype paper. It will be found the strongest, more likely to resist injury during the various washings it must undergo, besides giving more clear and evenly developed detail than any other paper hitherto made. These qualities also give this paper advantages in the production of negative drawings.

First Operation. I may remark, that the same precautions are recommended for the first operation in this mode of printing as were mentioned for the first operation with the ammonia nitrate paper. The bath is of the same strength, only that the former is a solution of chloride of sodium; whilst the latter is a solution of iodide of potassium.

Second Operation. To render the paper sensitive, prepare a solution of nitrate of silver, of the following strength:,

This solution is filtered into a clean, flat-bottomed, porcelain dish, to the depth of one-eighth of an inch. Care must he taken that there is no film or dirt on the surface of the liquid, as it will produce a marbled appearance on the paper, and a dirty drawing will be the result. The sheet of iodized paper is placed with the marked side downwards, upon the surface of the clear liquid, taking care to lift it up once or twice, to remove air bubbles.

This operation should be done with a light hand, to prevent the liquid running over the back of the paper; as, by so doing, it would be liable to produce a darker development on that part of the picture. Each sheet of paper will require to be in contact with the silver bath for three minutes, to become properly saturated. Great care should be taken in the regulation of the light during the operation. The light used should be shaded with yellow glass. Very many of the failures in this mode of printing arise from the want of caution in preventing even a feeble white light striking upon the paper. When dry, or nearly so, the paper can be placed in the pressure frame, the sensitive side in contact with the surface of the negative drawing, and exposed to the light.

No definite time can be stated , generally from three to fifteen seconds are required. A slight colour on the margin of the paper will roughly indicate the necessary exposure. The experience gained after a few trials is, however, the surest guide. On removal from the pressure frame the paper is placed on a flat board, and washed with a saturated solution of gallic acid, with a cotton brush. .Sufficient is applied to wet the paper thoroughly. On applying this solution, I prefer, if possible, to turn up the margin of the paper, so as to form a kind of dish, then spreading the solution with a cotton brush.

This operation must be done lightly, but with sufficient celerity, so that no part of the latent image shall have time to appear previous to the uniform covering of the whole surface. The spreading of the solution, also, with the cotton brush, should be done with a light hand, as the friction of the cotton brush would be likely to disturb the moistened surface of the paper, and injure the delicate lines of the drawing.

If the paper has been exposed the right time, the image will begin to appear in ten or fifteen seconds after the application of gallic acid, and will gradually acquire its full development. Should it not blacken as much and as quickly as may be desired, the addition of a few drops of nitrate of silver solution to the gallic acid after the latter has been on a minute will greatly assist the action. The time occupied in bringing out the image will vary greatly, as it must depend on the blackness of the negative drawing, and the quality and brightness of the light to which it has been exposed in the pressure frame.

When sufficiently developed, dip it into clean water for a few seconds; then place it in the hyposulphite bath, in which it should remain until the yellow tone is removed. This is facilitated by warming the hyposulphite bath, taking care, in the latter case, to watch the picture, so as to remove it immediately the yellow colour is gone; then place the drawing in clean water. The washing required is exactly the same as when printing with the ammonia nitrate paper.