This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
All possible values which can be represented in drawing, lie between the pure whites of paper or pigments and the pure black of pencil, ink, or other pigments. In order to think or speak precisely of the great range of values between black and white, it is necessary that they shall be classified in some way. It is not sufficient to say that a given shadow is light, or medium, or dark in value. Dr. Ross has overcome the difficulty by arranging; a value scale of nine equal intervals, which covers the whole range from pure white to pure black. Each interval has its appropriate designation and a convenient abbreviation. This scale affords a practical working basis for the study of values. It is evident that while the individual scale does not include all possible values, it can readily be enlarged indefinitely by introducing values between those of the scale as described. As a matter of fact, any differences in value that might come between any two intervals of the scale would rarely be represented, as it is the practice in drawing to simplify values as much as possible; that is to consider the general value of a mass, rather than to cut it up into a number of slightly varying tones which are not necessary for expressing anything of importance in the object.
HIGH LIGHT HL.
LOW LIGHT LL.
HIGH DARK HD.
LOW DARK LD.
Fig. 24. Value Scale.
BUILDING FOR THE AMERICAN SCHOOL OF CORRESPONDENCE, CHICAGO, ILL.
Pond & Pond, Architects, Chicago Base and Piers of Purple Continental Paving Brick, Laid with a Dark Joint. Body of Wall of Medium Red Paving Brick, Laid with a White Joint. Trimmings of Bedford Stone
Fig. 24 shows a value scale with the names of the intervals and their abbreviations. In making a value scale the student should work in pencil, confining each interval within a circle three-quarters of an inch in diameter. White will be represented by the white paper with a circle penciled about it, Black (B) and white (W) should be established first, then the middle value (M), light (L) and dark (D); afterward the remaining values, low light (LL), high light (BL), low dark (LD) and high dark (HD).
When the objects to be drawn are neutral in color, that is, are black, white, or gray, the relative values are perceived without special difficulty. When the objects are in color, the draftsman is obliged to translate the color element into terms of light and dark.
In order to determine the value of any surface, it is a help to compare the surface with a piece of white paper held in such a way that it receives the greatest amount of light. It not infrequently happens that two surfaces quite different in color will be of exactly the same value. The student should make a practice of observing the relative values of things about him, even When he is not engaged in drawing.
Place a sheet of white paper in the sunlight as it falls through a window and compare its value with that of white paper further in the room and outside of the sunlight. Try a similar experiment with black. These merely show what everyone may suppose that he knows already-that the less light a surface receives the darker value it appears to have. As a matter of fact, beginners are more ready to accept this truth with regard to color than they are when it relates to black and white.
An instructive way of studying values is to look through a closed window and compare the values of forms outside to the value of the window sash. Even when the sash is painted white, it will often be observed to appear darker than any shadow out of doors.