Scratching photographic negatives is a type of photo manipulation that started in the 19th century. It seeks to transform photography from being just the capturing of a moment in time to an art form in which the developer of the photograph uses artistic techniques to beautify the image or to make a statement.
Negative scratching, along with other techniques of image manipulation such as printing several images together, employing soft focus or using chemicals to alter the final appearance of a photograph, began with photographers later known as Pictorialists. To such photographers, photography was less about reproducing reality and more about using photographic and artistic techniques to create photographs with more emotional impact than simple snapshots.
Scratches on the emulsion side of a black-and-white film negative will show up in the photographic prints produced from such a negative as black lines. This is because such scratches remove the emulsion from the negative. The emulsion is a mixture of silver halide crystals (which react to light and make film photography possible,) and a gelatin which holds them in place. Scratches on the non-emulsion side of a photographic negative generally appear as white lines in the prints made from such negatives.
When negative scratching is performed on colour negatives, the results aren't always as predictable as they are when applied to black-and-white film. Scratches made on colour negatives may appear as white lines or black lines on the photographs produced from the negative, or they may come out multicoloured. It's a good idea to practice on negatives that you are willing to destroy to see how things work out before moving on to those you wish to turn into artwork.
While negative scratches may appear small to a person's naked eye, they will be enlarged many times on prints so it is important to use as small a tool as possible if you don't want them to take all the attention away from the other features of the photograph. Scratches that appear straight or smoothly curved on a negative may be crooked or jagged on the finished photograph. Using a magnifying glass when scratching the negatives can help ensure you get the final result you want.