Potter's needles are one of the most basic and versatile tools we have available to us. They have many uses; I am always finding more uses for mine.
One of the primary uses of the potter's needle is to get an accurate measure of how thick a slab or the floor of a pot is. Simply hold the needle completely vertical then push through the clay. Pinch the shaft of the needle were it meets the clay. Pull the needle straight out, keeping your fingers still on the shaft. The length of the needle's shaft from point to your fingertips is how thick the clay is at that point.
The small hole made is easy to seal. If you are working on the wheel, compress the floor of the pot a few times. If you are hand building, gently push the clay back together.
