BLOCKING OUT, OR STOPPING OUT A method of painting out undesirable details upon a negative, the painted portions appearing white upon the finished print ; parts of lantern slides may also be blocked out, in which case the blocking out appears black upon the screen. Blocking out is extensively used on photographs of machinery, furniture, etc., for reproduction in catalogues, etc. It is generally desired that the article photographed should stand by itself upon a white ground, in which case opaque pigment is used as the medium for stopping out the rest of the picture ; but when a perfectly black back ground is required the best thing to do is to make a transparency and block this out, and from this to make a second negative, on which the blocked-out portions will be clear glass, which will, of course, print black. Whichever method is adopted, the actual work of blocking out is precisely the same. In order to do the work properly, the following materials are necessary: a retouching desk, one or two sable or crow-quill brushes with fine points, a mapping pen, Indian ink, ruler, a bottle of black varnish, and red water-colour or other opaque medium. The negative is placed film side uppermost on the retouching desk and the film worked on with the opaque. A rough print to serve as a guide should he taken from the negative before the work of blocking out is begun. It is advisable also to begin from the centre of the negative and work outwards. A retoucher with a steady hand may be able to do all the necessary work with a brush, but many will need the rule, pen and ink. The pen, if used, is charged with Indian ink and held perfectly vertical to the sur face of the plate ; it should have a smooth and well-rounded point, as otherwise it is apt to cut the film. Any errors made with the pen and ink may be removed by washing away the line by means of a camel-hair brash charged with water ; but when this is done care must be taken to wait until the gelatine film is perfectly dry before going over it again with a pen, otherwise the film will be torn. Having ruled all the necessary lines, the rest of the blocking out may be done with the black varnish diluted with turpentine, or with any other opaque pigment.
Those pigments used with water are perhaps the easiest to use because, should any error be made in the work, they may easily be washed off, or wiped off with a damp sponge, whereas black varnish is difficult to remove even with tur pentine; the varnish, however, is the more durable, and will stand any amount of wear and tear.
All the fine work, if desired, may be done on the film side with a pen and Indian ink, or with a brush charged with opaque or red water-colour, and the bulk of the stopping out on the glass side with black varnish or Brunswick black, taking care that the working on one side over laps the other.
Another method is to take a rough print from the unblocked negative, cut out the part required, and nse the cut print as a mask, which may be pasted on the glass side of the negative. This serves as opaque, and but little fine work may be required on the film side, care, however, being taken to let the working on the film side overlap the paper mask. The latter may, if desired, be wetted and placed on the film side, but it is removed more easily from the glass side of the negative. (See also " Camphor.") Any of the above methods may also be employed for transparency work, but in the case of lantern slides it will be necessary to use a stopping-out mixture which will not crack when subjected to the heat of the lantern illuminant concentrated by the condenser.
The present-day commercial practice is for the photographer to make as good a photograph of the subject as he can, supply a good print, and leave the blocking out to the process worker's artist, the work being done on the print and not on the negative. The photo grapher can often assist matters by seeing that the background is of such a nature that the work of blocking out is facilitated, as the merging of the picture of a machine, for instance, into the background makes it difficult to see where one ends and the other begins. Frequently it is sufficient to run a line of white pigment between the subject and the background, and the process worker then understands that the latter is not to be included. The aerograph is largely used for blocking out on prints, etc.