Letter stamps press words and initials into soft clay, and clay-specific sets differ from metal-stamping alphabets in a way that matters: they are designed to press cleanly into a soft surface rather than to strike a hard one.
Clay stamps against metal-stamping sets
Alphabet sets sold for metal or leather stamping are struck with a hammer and have deep, narrow characters. Clay sets are pressed by hand and have shapes that release from sticky clay without tearing it. Using a metal-stamping set on clay works after a fashion and tends to drag the surface, which is why clay-specific sets exist.
Getting a straight line
Free-hand lettering into a curved pot wanders, and the usual fixes are a guide line scored lightly into the clay first, or a holder that keeps several letters in a row. Working on a flat area, such as the base or an applied tag, is far easier than lettering a curved wall, and applied tags are how most makers put words on round forms.
Depth and glaze
A shallow impression fills with glaze and disappears. Press deeper than looks necessary, and where the lettering must read clearly under a glaze, consider leaving the area unglazed or rubbing an oxide into the impression and wiping the surface so the letters read dark.
Making a tag rather than stamping a pot
Rolling a small slab, stamping it flat where the letters go in cleanly, and applying it to the pot with slip is easier than stamping a thrown wall and gives a better result. It also lets you discard a badly stamped tag rather than a whole pot, which is the practical reason most production makers do it that way.
Questions people ask about letter stamps for clay
Can I use metal alphabet stamps on clay?
They work poorly. They are made to be struck into hard material and tend to drag soft clay. Clay-specific sets release cleanly.
How do I keep lettering straight?
Score a light guide line first, or stamp a flat applied tag rather than a curved wall.
Why can I not read my lettering after glazing?
The impression was too shallow and the glaze filled it. Press deeper, or rub an oxide in and wipe the surface.