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paper, solution, silver, bath, sheet, light and dry

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SENSITIZED PAPER.—This term properly includes any kind of prepared paper sensitive to light, the action being either visible or latent and made visible by development. It is more generally applied, however, to albumenized or plain paper treated with a chloride and afterwards floated on nitrate of silver. See Sensitizing.

Sensitized paper unless specially prepared will not keep for any length of time. That sold commercially is all prepared with acids as preservatives. The addition of io drops of perchloric acid to every ounce of the sensitizing solution is often recommended.

Ashman's method is as follows. The paper, when surface dry, is floated on one of the fol lowing solutions properly made up and filtered : No. t.

Picked white gum arabic oz.

Rochelle salts " Distilled water 20 ' No. 2.

Picked white gum arabic I oz.

Tartaric acid it " Distilled water 20 After the paper is dry it should be stored away in sheets of sodaic blotting paper (q.v.) No. i will preserve it for about a fortnight and No. 2 for several months.

SENSITIZER.—Any substance added or applied to a photographic material to increase or alter its sensitiveness to light.

SENSITIZING.—The operation of causing the albumenized paper to become sensitive to light. This is done by floating the surface of the paper already prepared with albumen contain ing a chloride upon a solution of silver nitrate. (See Silver Bath.) The albumen soaks up a portion of the silver solution, which combines with the chloride and albumen to form silver chloride and silver albuminate, these compounds, being sensitive to light, darkening rapidly by its action.

The operation of sensitizing must be performed either by gas-light or during the day in a room lighted with windows covered with yellow paper or fabric. The room should be kept scrupulously clean and free from dust, and be fitted up with a convenient arrangement for hang ing the paper lines ; or string stretched across the room with American clips strung along them will be found very convenient. The dish containing the sensitizing solution should be con veniently larger than the sheets of paper to be sensitized. It should be placed on the table as near to the window as possible to get plenty of light to work with. Over the bath should be fitted

a few American clips to hold the paper by one corner, and at such a height from the bath that the bottom corner of the sheet is nearly touching the liquid in the bath ; this prevents splashing in drawing, and also the formation of air-bubbles.

The bath is placed lengthways before the operator. and the silver solution poured in. With a thin strip of blotting paper the surface of the liquid is skimmed, removing any scum or air bubbles.

The paper to be sensitized should neither be too hard and dry, nor too moist. In very hot and dry weather it is advisable to place the sheets in a cool damp cellar for some little time before floating.

The paper has now to be floated, prepared side downwards, upon the silver solution. This is done by first turning up a small portion of each corner of the sheet for convenience in examin ing and lifting it from the bath. Two diagonally opposite corners are now held with the thumb and first finger of each hand, and the sheet curved slightly inward. That part of the sheet near est the left hand is lowered gently on to the center of the solution, and the sheet slowly drawn on to the solution, steadily lowering the right hand, until the whole sheet is floating in contact with the solution, with the exception of the four upturned corners.

It this method be followed no air-bells will form between the paper and the solution. After a little time the corners are gently raised and lowered again one at a time to enable the operator to examine the progress of the sensitizing operation. Any air bubbles visible should be removed before again lowering the corner on to the solution. If any of the solution should by accident get on to the back of the paper it should be blotted up at once with a piece of blotting paper. It often happens that the paper after floating will curl up, the corners and sides leaving the solution. The cause of this is that the paper is too dry; by breathing hard on to it the paper can be made to resume its position on the solution.

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