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456 WHITE IMAGE SCREENS [CH. XII
to use white linseed oil, raw or boiled. The oil is put on with a soft brush like paint. It is well to make all the brush strokes in one direction, so that the lint or nap on the surface of the cloth will be smoothed down in one direction. After the linseed oil is dry the cloth is painted, preferably with sanitary paint and turpentine, although white lead thinned with turpentine answers well. One coat should be allowed to dry before adding another. It takes from one to two days for each coat to dry. The screen will be white and opaque with three to five coats. Care should be taken to strain the paint as for the walls (§ 623), then there will be no rough spots (§ 62 $a, 62 sb).
If the curtain gets grimy it can be wiped off with soap and water, and if necessary after it is dry, a fresh coat of the paint can be put on.
§ 626. Roller screens. — Cloth screens which have been painted as just described make excellent roller curtains, for the sizing and
§ 62 Sa. Amounts of sizing oil and paint for a cloth screen. — For oil-sizing and painting a muslin screen the following times for drying in the summer, and the following amounts of oil and paint were used to make a perfect screen. For sizing, white raw linseed oil was used, and only one coat was applied.
For this it required 220 cubic centimeters of the linseed oil per square meter of cloth, or about one-tenth of this amount per square foot.
For painting, a preparation of sanitary paint known as "Artists' Scenic White," ready for use on screens was used, two coats were applied. It required no cc. of the paint for each square meter of surface.
It required about 36 hours for the raw oil sizing to dry; 24 hours was sufficient time for a coat of the white paint to dry. The finished screen was flexible and easily rolled.
For a screen 3 meters or 10 feet square it would require for sizing and painting about two quarts of linseed oil and about the same amount of the "Artists' Scenic White" or any other white paint for two coats of the paint.
§ 625b. The cloth may be sized by the use of white shellac. This is thinned about half with denatured alcohol and painted on the surface just as described for the oil size. It gives a good surface to paint on, but does not leave the curtain so flexible.
A hot 15% to 20% solution of white glue in water may also be used as described for the oil or shellac size. This has the advantage of pasting down the nap of the cloth and of giving a very good surface to paint on. It has the disadvantage of expanding and contracting greatly with different conditions of moisture. If the glue size is used the curtain should have at least one coat of paint on the back, so that the glue size cannot be so easily affected by moisture.
§ 625c. The authors wish to express their appreciation for information on paints and the painting of wall and cloth screens for projection, to Mr. A. E. Nash, Superintendent of the Cornell University paint shop.