Too often in art education the grading criteria for an assignment or art project is determined by the teacher after the project is completed by the student. Almost as an afterthought once the teacher realizes that there must be an objective criteria to justify the grade a student earns. When we make lesson plans we set objectives. Those objectives must be measurable. When designing an art project for students to demonstrate what they know a grading rubric must also be designed. Students need to know the expectations and how they will be assessed. When a student knows how they will be evaluated then they are better able to meet the goals set out at the beginning of a unit of instruction.
I must admit that I used to be one of those assessment afterthought teachers. I was so focused on what the final artwork would look like that I didn't always think through what elements (other than it looks good) that would need to be in the final artwork until it was all done. Through my learning in the graduate art ed program at the University of Florida I have gotten much better at creating objective grading rubrics for student artwork.
I have approached grading rubrics, and there implementation different ways. I have given the rubric to students at the end of an art project and had them self evaluate. I have also given students the grading rubric at the beginning of a project then still had them use it to self assess when they complete their work. I've found that students are more successful at meeting the expectations of the project and creating a final project they are happy with when they get the grading rubric before they start an artwork.
Creating grading rubrics at the onset of a unit also helps the teacher focus their instruction so that students will succeed in their learning. It just all goes back to when you know the end point you are better able to map out the best path to get there.
The most recent rubric I've created is for the letter relief sculpture project my students are starting now. The short description of the project is that students are using their first and last initial to create a relief sculpture which illustrate their personal identity. They will utilize low and high relief, symbolic color and image transfer techniques for their surface design. To view the grading rubric visit the student/teacher lesson resource page my website.
A great resource for the role of assessments in art education and how to make an effective rubric is Stewart and Walker's Rethinking Curriculum in Art from Davis Publications.