Decoration makes a plain clay piece feel unique, but the time for stamping determines whether the lines remain crisp or become ragged and messy. A stamp pressed into wet clay is often pushed too deep. A line drawn into too-dry clay may drag or break and leave ragged edges. When you use handmade plates, shallow serving trays, spoon rests, and small pinch bowls, there is an exact window for surface decoration between very soft and fairly stiff.
It is often very tempting to start stamping, drawing, and carving right when the work has just been finished. At this time, the clay has a fresh open texture that looks like a perfect surface. In many cases, however, the clay dish is still settling into its final shape. A slab tray has just been placed and the edges may still be shifting or lifting. A pinch bowl has a rim that may move at the touch. A clay surface that is still soft to the touch will be disturbed easily, particularly if pressure is exerted on the top surface without support from below.
Wait until the surface is able to sustain gentle pressure. If the surface can be marked with your stamp or needle tool without the stamp getting stuck, but you are pressing on the walls, the rim of the clay dish should not move easily. For fine carving, this is usually the very moment when clay has turned leather hard, or a little earlier for simple texture when the shape is supported. Each clay body and temperature, combined with clay thickness and amount of water in the clay will affect this moment.
Make a sample tile to save time. Create a small piece of clay in the same manner as the dish you are working on and stamp or draw into that to determine the degree of wetness of your clay. If the stamp sinks in, wait. If the needle tool tears rather than cuts, wait or adjust the angle. If the mark looks pale and disappears when touched, the clay may be too wet or the pressure too light. A separate piece of clay will be easier to test on than your original piece.
When you mark the surface, consider where the stamp or carving will fall on your piece. If you use a deep stamp near a thin area of rim, you can weaken the rim. If you carve into the clay at the base of a small bowl to create a surface motif, the area that is now carved could be a weak point that could lead to breakage. For a first work, it is better to mark the surface with an area of space on the rim and base area and then place a mark on the surface when there is sufficient area of thickness to take detail. In this way, the marks are more controlled than if you attempted to cover the surface with repetitive marks.
Also, if you use a glaze or underglaze, allow time to apply surface design. If the clay is still a gloss surface, it will be difficult to get even coverage with a brush and the color will catch in the finger marks. If the clay is very dry, the brush will not flow well and will create a scratchy surface. A light surface mark will usually best if the clay is stiff enough to handle but not yet dry enough to resist. Keep your brush slightly wet, with no excess water on it. Rubbing back on an area can re-wet the surface.
The question is not, Can I mark it now? It is, Can the piece take the mark? Check the rim, hold the piece, press on the clay, see whether the piece will move more than the design on it can. The moment when the surface mark will be received without the clay changing its shape, is when the marking can be integrated into the shaping.