Mere experience will give you that knowledge more or less; but there are ways in which you can help yourself.
When you first begin to work out-doors try to find a good solid shade in which to place your easel, and then try to paint up to the full key, even at the risk of a little crudeness of color. Use colors that seem rather pure than otherwise. You may be sure that the color will "come down" a little anyhow, so keep the pitch well up. Then, if the shade has been pretty even, and your canvas has had a fair light, you will get a fairly good color-key.
Predetermined Pitch.—Another way is to determine the pitch of the painting in some way before you take the canvas out-of-doors. There are various ways of doing this. The most practical is, perhaps, to know the relative value, in the house and out-doors, of the priming of your canvas. Have a definite knowledge of how near to the highest light you will want that priming is. Then, when you put on the light paint, if you keep it light with reference to the known pitch of the priming, you will keep the whole painting light.
Discouragement.—We all get discouraged sometimes, but it is something to know that the case is not hopeless because we are. That what we are trying to do does not get done easily is no reason that it may not get done eventually. Often the discouragement is not even a sign that what we are doing is not going well. The discouragement may be one way that fatigue shows itself, and we may feel discouraged after a particularly successful day's work—in consequence of it very probably. Make it a rule not to judge of a day's work at the end of that day. Wait till next morning, when fresh and rested, and you will have a much more just notion of what you have done.
When you begin to get blue about your work is the time to stop and rest. If the blues are the result of tire, working longer will only make your picture worse. A tired brain and eye never improved a piece of painting. And in the same spirit rest often while you are painting. If your model rests, it is as well that you rest also. Turn away from your work, and when you get to work again you will look at it with a fresh eye.
Change Your Work Often.—Too continued and concentrated work on the same picture also will lead to discouragement. Change your work, keep several things going at the same time, and when you are tired of one you may work with fresh perceptions and interest on another.
Stop often to walk away from your work. Lay down your palette and brushes, and put the canvas at the other end of the room. Straighten your back and look at the picture at a distance. You get an impression of the thing as a whole. What you have been doing will be judged of less by itself and more in relation to the rest of the picture, and so more justly.
When things are going wrong, stop work for the day. Take a rest. Then, before you begin again on it to-morrow, take plenty of time to look the picture over—consider it, compare it with nature, and make up your mind just what it lacks, just what it needs, just what you will do first to make it as it should be. It is marvellous how it drives off the blues to know just what you are going to do next.
