This section is from the book "Modern Photography In Theory And Practice", by By Henry G. Abbott. Also available from Amazon: Modern photography in theory and practice: A hand book for the amateur.
When a negative has small transparent spots on it caused by dust on the plate or air bubbles in the devel-oper, they may be successfully treated by what is known as the spotting-out process. The negative is placed at an angle of 450 and a piece of card board or white paper is placed back of it in such a way as to reflect the light through the negative, or better still, it is placed on retouching stand. The spotting is usually done with water colors, although India ink is sometimes used. Gum water or the white of an egg should be used with the color in order to make it adhere firmly and it also prevents it running. The gum water is prepared by putting a few pieces of gum Arabic in a small vial and covering it with warm water. Shake the vial occasionally and when the gum is dissolved it is ready for use. One or two sable brushes with very fine points should be secured. A little color should be mixed with the gum water or albumen and care must be taken to match the color of the film on the negative as nearly as possible. The best water colors for the purpose are what are known as moist colors, which are put up in small china pans. Sepia, vandyke brown, burnt umber, black and dark blue are the principal colors used. Mix a little of the color to the right shade by means of a large brush, but be careful not to thin the color too much. Now take your fine pointed brush, wet it in the gum water and draw it to a fine point between the fingers and by turning it around and around on the pallette or saucer Now lift a small quantity of the color on the extreme end of the brush and apply it to the transparent spot You must not attempt to fill in the entire spot with one daub but rather apply the extreme point of the brush to one side of the spot, making a very small stipple, about the size of the point of a pin. Allow it to dry for a minute and then make another stipple and so on, until the spot is entirely filled in. Be careful not to use too dark a color or to apply too much of it, so the spot will be darker than the surrounding film, or when you come to print you will leave a white, instead of a black mark on your paper. If you match the color of the negative carefully and do not apply too much color, when you come to make your print you cannot find the spot because the light has penetrated through it to just the extent that it penetrated the film. The dots need not touch one another exactly and the spot need not be entirely filled in. Very small spaces between the dots do not show on the print.
Negative spotting may also be done by means of a medium hard lead pencil, sharpened to a fine point but when a pencil is used, it is first necessary to apply a small quantity of retouching varnish to the spot or the pencil will not act. Retouching varnish can be purchased at very small expense from any photographic supply house, or the following will answer:
Gum damar........................................................60 grs.
Turpentine.......................................................... 1 oz.
A very small quantity of the varnish should be taken on the end of the forefinger and applied to the spot by rubbing it lightly. Allow the varnish to dry, which it will do in a few minutes and then proceed to stipple in the spot with the extreme point of the pencil, making as it were minute commas. These can lap one over the other until the spot is entirely filled in. Sometimes the photographer prefers to fill in the spot with one or two daubs of black varnish or Gihon's opaque, which will leave a white spot on the print and then spot out the print with water color that matches the color of the print. Spotting out on negatives must not be confounded with spotting out on prints. It sometimes happens that a negative prints too dark in spots or one part prints more rapidly than another. This is often found in landscapes that have a heavy foreground and also sky effects. If we print long enough to bring out the details of the foreground we find we have over-printed the sky. Sometimes we find the reverse, the top of the negative prints slower than the foreground. This can be remedied in several ways. If the dividing line between is straight, we can cover the portion which prints too quickly with tissue paper or matt varnish. Matt varnish is prepared as follows;
Gum Mastic..................... 30 grs.
Ether..................... 7 ozs.
Benzole..................1 1/2 ozs.
Dissolve the sandarac and mastic in the ether and then add the benzole. Filter through muslin before using. This preparation should not be used close to a lamp or gas flame, as it is highly combustible. The matt varnish is never applied to the film, but always to the glass side of the negative. The plate should be held in the left hand in a horizontal position, glass side up and the varnish poured in the center of the plate. Then tilt the plate from one side to the other until the varnish runs to all sides of it but do not allow it to drip over the sides.
When the plate has been covered entirely, turn it on end, in a vertical position, putting one corner of the plate in the mouth of the bottle so that all surplus varnish will run into the bottle. When the plate has been well drained, allow the varnish to dry and it can then be scraped away from that portion of the negative which prints slowly and allowed to cover the back of that portion which prints too rapidly.
It sometimes happens, that an otherwise good negative has a poor sky or that that portion of the negative has met with some accident. In such an event the sky can be blanked out entirely as mentioned in the last chapter in the case of blisters and the sky from another negative printed in. This would be easy to accomplish with a piece of black paper if the sky line was a straight one, but it often happens that trees, houses and church steeples encroach into the sky, thus making the line irregular. To blank out in such a case, use a fine pointed pen (those known as crow quills and sold by artists' supply houses, are best) and black varnish or India ink, mixed with white of egg. The negative should be placed on the retouching stand with the sky downward and the film side towards you. Charge the pen by means of a brush and go carefully around the outlines of the picture where it merges into the sky. Care must be exercised not to run the line over the picture but this can easily be avoided. When the outline has been completed with the pen, take a small, fine-pointed brush and follow this line but not lapping it and you will then have a broad line separating the picture from the sky. You can now use a larger brush and blank out the balance of the sky. When dry the negative is ready to print and when printed the resulting picture will have a perfectly plain white sky, on which can be printed any sky effect desired with another negative.