Constructive drawing of a skull of a human head: Aerial perspective. Lighting. Partial tone introduction. 
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Aerial perspective. Lighting. Partial tone introduction.
After constructive construction of the shape of the skull of a human head, a drawing may consist of many lines associated with constructive construction, compositional search. Move the “order” in the linear drawing o Remove unnecessary lines, leave only those associated with the design of the form, prepare the lines for aesthetic perception.
Apply the laws of aerial perspective to the linear pattern of the skull of a human head. The aerial perspective method is familiar to you, it is the “fog effect”, you always used it in constructive drawings of a still life of geometric bodies and household items. The darkest line is on the zygomatic part of the skull, it will correspond to the boundary of the rotation of the form in space when drawing the skull in three quarters. This is the closest part of the zygomatic arch of the skull to you.
Pay attention to the nature of the lines in the picture: you have two kinds of lines in the picture. This is a contour line that closes a form or a set of forms and separates them from space, and a line of borders or faces of a form, giving an idea of the three-dimensionality of the form and the position of its sides in space.
Consider lines that correspond to the boundaries of shapes in space. The cube has sharp edges between surfaces and a sharp contrast between light and shadow, enhancing this effect with chiaroscuro. As you can see, chiaroscuro almost always coincides with the boundary of the sides of the form in space. The borders of the sides of the skull shapes have soft transitions and different radii, in contrast to the cube shape. The larger the radius, the less pronounced the boundary and the more difficult it is to determine; source of light helps; chiaroscuro helps.
Translate the lines of the edges of the form into a soft, blurry state. To do this, you will take only chiaroscuro from light and shadow, but be sure to use an aerial perspective! Technically, this can be done with sharpened gum, working it across the line in shape. The constructive drawing will turn out linear, spatial.
Thus, all the possibilities of a linear pattern for transmitting the volume of a form in space are exhausted.
Take advantage of the light that will help you enhance the sense of volume of the form.Most design drawings are performed without special lighting. If you understand the design of the form, then it is very easy to cope with this task. You will choose the position of the light source that will help you to convey the volume in the drawing as much as possible, without even having to resort to the light source itself, that is, simply representing it.
Imagine this source, or sketch a small drawing of a cube next to your drawing, which is illuminated by a certain light source and is located in space in the same way as the skull in your drawing. So, one surface of the cube is lit, light glides along the other surface, and it is lit less, and the third surface of the cube is in shadow.
Look at your drawing of the skull of a human head and tell yourself that all surfaces of the shape of the skull that correspond to the light surface of the cube will be light, and all surfaces of the skull corresponding to the shadow surface of the cube will be in shadow. All anterior areas, such as the anterior and upper areas of the brain, will be light, and areas such as the lateral surface of the front will have a slight shadow.
The chiaroscuro in the picture is already there. Enter a partial tone; do not touch light surfaces, they should be clean, like paper! Own shadows become lighter as you go deeper.