It is impossible to give totally hard-and-fast indications about grading policies where paper-writing is concerned, because the act of writing a paper is creative. It is designed to develop your thoughts in unexpected directions, test them out, and produce conclusions. No one can predict every possible way of writing a paper, so no single grading policy can be imposed rigidly. The comments below are guidelines only.
Remember that your paper is trying to demonstrate that you have come to grips with the subject and that you are in control of the information at your disposal (including the bits you may think don't matter, like spelling). To establish all this, there are some things that every paper should contain. I have assigned them to various grade bands, to emphasize how they work together to build your paper, and how they affect your final grade. Let us assume that your paper is anyway worth 50 points. Now:
Obviously, it is not enough merely to do these things. You need to do them successfully and you
need to do them well. Also obviously, if you have an A band quality of understanding the
question, but your organization, presentation and accuracy are out - you won't get an A. Each
grade is also about having got the ones before it right. It is hard to get an A: we
normally expect less than 10% of any class to achieve one. Be proud of a B.
In the end, what you have to keep in mind is that papers in Art History deal mainly with non-factual material. Non-factual material is slippery stuff, so it has to be handled carefully. Your competence has to be proved in every possible way, as outlined above, so that your reader trusts your judgement about the rest.
If you need them, get the notes on:
Paper preparation (the research end)
Paper writing (organization to best effect)
Paper Presentation Requirements (to give yourself the edge)
Note taking (the course contains unique material - you won't be able to read up on it all)