A study is less of a picture than a sketch is. For unity of effect is vital to both a sketch and a picture. But this quality is of no essential value in a study--unless it be a study of unity. For you can make a _study_ of anything, from a foreground weed to a detailed interior, from a bit of pebble to a cavalry charge.
But in a study of one thing you concentrate on that thing, you deliberately and carefully study everything in it, while in a sketch you work only for general effect. The study is the storehouse of facts to the painter. By it he assures himself of the literal truths he needs, collecting them as material in color or black and white, and as mental material by his mental understanding of them, only to be gained in this way.
In making a study you may work as long as you please, timing yourself by the difficulty and size of the thing you are studying. A study of an interior or a landscape may occupy a week or two; one of a simple object for some detail in a picture may be a matter of only a few hours. But in any work of this kind you should be deliberate, and remember that what you are doing is neither a sketch nor a picture, but the gathering of material which is to be useful, but which can be useful only so far as it is accurate.
In making studies, don't try for surface finish; get the facts, and leave all other qualities for the picture. Don't glaze and scumble, but work as directly as you can. Study the structure and texture of whatever you are doing. Understand it thoroughly as you go on, and search out whatever is not clear to you. This is no place for effects; nor for slighting or shirking. If you do not do work of this kind thoroughly, you might as well not do it at all--better; for you are at least not training yourself to be careless.
There are places where you may be careless, but the making of a study is not that place.
Take plenty of trouble with preliminaries. Get all your foundation work true. Have a good drawing, get the groundwork well laid in, and then build your superstructure of careful study.
Don't be afraid of over-exactness, nor of hardness and edginess here. All that is only an excess of precision, and it is just as well to have it. You can leave it out if you want to in your picture, but a groundwork of exactness is not to be despised.