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CH. XII] WHITE IMAGE SCREENS 455
In spreading the paint on the wall one should use a soft brush and apply only the tip of the brush. This will give a smooth finish and if one uses plenty of paint there will be no joints, but the whole will appear like one uniform coat. Practical painters call this "flowing on the paint."
After one coat is well dried another can be put on until the wall is perfectly white. If plenty of turpentine is used the surface will be dull. It should not be glossy or shiny.
Whenever the surface becomes dirty it can be washed off with soap and water. If it is not up to standard whiteness after the washing and drying, put on another coat of the paint.
Sometimes hot glue, 15% to 20% in water, is used for sizing the wall. This answers well if the wall is perfectly dry and not subject to moisture. In general it is safer to use the linseed oil sizing.
In our experiments several white paints were used, but the pure white lead (sometimes called "flake white") and the non-lead containing paint called "sanitary paint" were found most satisfactory. The latter has the advantage over white lead that it does not yellow with age, and gives a very opaque and white surface which stands washing with soap and water very well.
§ 624. Whitewashed wall screens. — A smoothly plastered wall that has been carefully whitewashed with milk of lime gives a good, dull white surface for a projection screen. It rubs off rather easily and cannot be cleaned. Of course a fresh coat of whitewash will renew the screen. It. is cheap as well as good. One should take pains to strain the whitewash, and to apply it smoothly so that a uniform surface will be produced.
We did not find a kalsomined wall satisfactory for projection. It is, or soon becomes, too yellow.
§ 625. Painted cloth screens. — A good screen can be made by stretching some smoothly woven, strong cotton cloth (strong muslin) upon a frame and painting it as for the wall (§ 623). The frame must be strong and the cloth stretched tight so that there will be no wrinkles, and it must not rest against anything.
One could paint directly on the cloth, but it is more satisfactory to size the cloth in some way first. One of the best methods is