This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
The Blue Print. First, prepare a 5 per cent. solution of thin sheet gelatin, which should be filtered before use. Place the blue film in water at a temperature of 70° Fahr., and after two minutes, place the yellow print in water 65° Fahr., until limp. Lift the latter out onto a folded piece of blotting paper, and pour over it sufficient gelatin solution to cover. Now take the blue film and superimpose over the yellow, carefully rubbing out all air-bells and superfluous gelatin. It is, of course, important that perfect registration of the two prints shall be secured. If the blue print is smaller than the yellow it may be expanded by placing it into hotter water. The combination print must be dried, the celluloid stripped from it, and the surface cleaned exactly as directed for the yellow print.
The Red Print. The green print must now be immersed in a 5 per cent. solution of formalin for one minute, and washed for three minutes in cold water. Having liquefied the gelatin cementing solution (or made up fresh), proceed in the same way as with the blue print, but do not soak the red film - merely dip into the water and immediately bring it into position over the green print. If too small expand by soaking for a minute. When quite dry and the celluloid removed, clean the surface and the print is ready for mounting.
1205. The finished print may be spotted with suitable colored dyes and, if desired, may be given greater brilliancy by varnishing with a very thin celluloid varnish, which should be flowed over as when varnishing a negative; or the surface may be matted before the print is mounted, by squeegeeing to a collodionized matt opal, or ground-glass, in the well-known manner.
1206. When sensitizing the films, should patches appear to be greasy and refuse to take up the solution, rub over quickly with a small wad of cotton-wool.
1207. It is frequently advisable to sensitize the red film with a weaker sensitizing solution than that used for the yellow and blue films.
1208. If no formalin is to be obtained for treating the green print, use a strong solution of chrome alum for the purpose.
1209. Flat and full tri-color prints indicate that the sensitizer used for the films was too strong.
1210. Prints in which the colors of the films employed are clearly defined, there being a lack of correct blending, indicate either that the sensitizer used was too weak or the negatives too strong.
1211. Finished prints should be mounted under cutout mounts, so that the surface may not touch the glass, as any moisture condensing on the latter would spoil the print.